Proposal to the Max-Planck Society
Next-Generation Low-Energy Storage Rings
for
Antiprotons, Molecules, and Atomic Ions in
Extreme Charge States
J. Ullrich, A. Wolf, R. von Hahn, M. Grieser, J. Crespo López-Urrutia,
D. A. Orlov, C. D. Schröter, D. Schwalm, C. P. Welsch
Max-Planck Institute for Nuclear Physics, Heidelberg
D. Zajfman
The Weizmann Institute of Science, Rehovot, Israel and
Max-Planck Institute for Nuclear Physics, Heidelberg
X. Urbain
Université Catholique de Louvain, Louvain-La-Neuve, Belgium
Executive Summary
In recognition of the increasing demand on quantum state preparation and kinematical
control in atomic and molecular reaction studies, options for enhancing the quality of
low-energy ion beams have been studied at the Max-Planck Institute for Nuclear Physics,
which now have converged to the design of a unique next-generation electrostatic ring for
storing low-energy beams of essentially all ion species – molecular ions, macro- and
biomolecules, clusters, atomic ions at extreme charge states, and even antiprotons.
We propose to realise an ultra-cold version of this device at the MPI-K, in which all
parts facing the ions can be kept at a temperature as low as ~2 Kelvin (CSR – Cryogenic
Storage Ring). At unequalled low radiation and matter densities (absence of thermal
blackbody radiation and ultra low rest gas pressures) this storage ring offers unique and
unprecedented possibilities to study the dynamics of ionic quantum systems under welldefined conditions and in hitherto not achieved precision: For molecular ion species, the
2 Kelvin environment together with ultimate storage times allows to freeze out vibrational, and, for the first time, even rotational degrees of freedom, thus ideally simulating
interstellar space conditions. Equipped with a cold merged electron beam, with laser
beams and co-propagating as well as transverse atomic beams, and with fore-front imaging technologies, the CSR will enable fundamental, quantum-state selective studies on
photo-excitation, -association and -ionisation, on ion-electron interactions, electron transfer, atom exchange and many other reactions. For exotic atomic ions, like H-, He- or Lilike uranium as present in super-hot plasmas (as e.g. in supernovae), extraordinarily long
storage times at unsurpassed low velocities will allow cutting-edge precision measurements on quantum electrodynamics, few-electron correlation and relativistic contributions. Femtosecond few-electron transfer dynamics can be explored in collisions of stored
ions with atoms and molecules and possibly controlled using intense, ultra-short lasers.
Thus, state-of-the-art quantum dynamics calculations can be benchmarked for well defined initial and final quantum states in temperature regimes not accessible so far, indispensable for further promoting our basic understanding of atomic and molecular structure
and dynamics and that of fundamental processes in astrophysics, plasma physics, planetary science, and up to radiation biophysics.
For storage and cooling of very low-energetic antiprotons within the FLAIR project at
GSI (see attached Letter of Intent), a second version of this novel storage device is foreseen to be built at a later time (USR – Ultra-low energy Storage Ring), which can be operated at room temperature but which does rely on the successful technological development of electron cooling, deceleration and pulsing techniques at the CSR. The USR will
allow unique experiments on the dynamics of matter–antimatter interactions, on the for-
mation and spectroscopy of anti-protonic atoms and of anti-hydrogen with unprecedented
precision, capable of providing the most sensitive test of the CPT-symmetry in the baryonic sector and, thus, shedding light on the origin of the striking matter–antimatter
asymmetry in our observable Universe.
The total investment and infrastructure costs for building the CSR are estimated to be
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Test Storage Ring TSR
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Contents
1. Introduction and Objective
2. The Physics Case
2.1 Quantum Reaction Dynamics of Cold Molecular Ions
2.2 Atomic Ions in Extreme Charge States
2.3 Physics with Antiprotons and Antimatter
3. Technical Layout
3.1 The Cryogenic Storage Ring CSR
3.2 The Ultra-Low Energy Storage Ring USR
4. Cost Estimates and Requested Funding
4.1 Cost Estimate
4.2 Requested Funding
5. Organisational Structure, Timetable and Milestones
5.1 Organisational Structure
5.2 Timetable
5.3 Technical Milestones and Milestone Experiments
Appendices
A.1 Embedment into MPI-K “Future Scientific Directions”
A.2 Existing Situation
1 Introduction and Objectives
The technical concept to store, cool and manipulate beams of rare ionic species, first developed for antiprotons at CERN (“first generation”) and then quickly expanded in the
late 80s and early 90s (“second generation”) to the storage of highly charged ions, radioactive nuclei, and ionic molecules (Aarhus, Darmstadt, Heidelberg, Jülich, Stockholm,
Tokyo), has proven extraordinarily fruitful, marking a break-through for a broad variety
of fields such as atomic, molecular, and nuclear physics. With one of the first and most
successful second-generation rings designed, developed and operated at the MPI-K in
Heidelberg, with major new developments especially in electron and laser cooling techniques, in the operation and design of in-ring targets, detection and imaging devices,
MPI-K groups have played a generic role in technological and methodical developments
of this storage technique to solve pertinent issues in atomic many particle systems since
nearly two decades (see e.g. “The Test Storage Ring TSR: Ten years of operation”, Heidelberg, 1999). In particular, the MPI-K groups have performed, in collaboration with D.
Zajfman's group from the Weizmann Institute of Science, Rehovot, Israel, pioneering
state-resolved investigations of the interaction of light molecular ions with electrons and
photons, relevant for promoting our theoretical understanding of molecular structure and
dynamics and that of fundamental processes in astrophysics, plasma physics, and planetary science.
In recognition of the increasing demand on quantum state preparation and kinematical
control in modern atomic and molecular reaction studies, an inter-divisional project team
(D. Schwalm, J. Ullrich, in collaboration with D. Zajfman, Rehovot, X. Urbain, Louvainla-Neuve, and GSI) was formed by the end of 2003 to explore the prospects and novel
concepts for third-generation storage devices to further enhance the properties of lowenergetic ion beams. The new devices should allow questions on the dynamics of matter–
antimatter interactions to be addressed by the use of slow cooled antiproton beams, and
should integrate at the same time the needs for novel and unique experiments with ultracold molecular ions, ionized clusters, biomolecules and aerosols, and highly charged
atomic ions. The projects, which are embedded and central to one of the major interdisciplinary research-lines pursued at the MPI-K in the future, i.e., Many-Body Dynamics of
Atoms and Molecules (see Sec. A.1), take advantage of the world-wide unique expertise
accumulated at the MPI-K during the past years in manipulating, storing and cooling ion
beams for atomic and molecular physics experiments (A. Wolf), and the complementing
unique expertise in atomic and molecular beam as well as in target fragment imaging
techniques for in-ring experiments (“Reaction Microscopes”) brought to the MPI-K by
the appointment of J. Ullrich in 2001.
7
The physics questions to be addressed with the new storage device and outlined in
more detail in Sec. 2 require new features well beyond those of the second-generation
facilities like the TSR. Despite the pioneering role of these storage devices in promoting
our understanding of basic atomic and molecular processes, some serious limitations have
been reached and further progress towards the ultimate goal of state-resolved experiments
of ionic systems requires new and dedicated technological approaches:
•
The planned atomic and molecular interaction studies mostly depend on long-time
storage, and partly on strong phase space cooling, of ion beams with low energies
from a few keV up to ~300 keV. As described below, this holds for target fragment
imaging experiments on atomic matter-antimatter interactions using antiproton
beams, for ions in extreme charge states (precision x-ray spectroscopy), and for highresolution cold reaction studies with merged atomic and molecular ion beams. Unlike
the MeV energy range for which the magnetic storage rings were designed, efficient
long-time storage and phase space cooling is largely unexplored for keV ion beams.
•
For state selective, high resolution experiments on molecular reaction dynamics, and
also to reach useful storage times for ions in extreme charge states, a cryogenic storage environment is of paramount importance. Keeping all parts facing the ions at temperatures of a few Kelvin, thermal radiation can be eliminated and ultimately low rest
gas pressures, ensuring long storage times, can be reached. For most molecular ions
this freezes even the rotational motion, thus adding a decisive step to the control of
vibrational excitation so far achieved at room temperature with second generation
storage rings like the TSR.
•
A direct transfer of magnetic storage ring techniques will not be adequate. Electrostatic containment of the ions appears more economic and will allow a much wider
spectrum of molecular species to become accessible for long-time storage and highresolution reaction dynamics experiments. An all-electrostatic design appears preferable also for the containment of very slow antiproton beams.
In response to these demanding requirements we are presently designing a unique
next-generation electrostatic ring capable of storing and cooling low-energy beams of essentially all kind of ion species, ranging from antiprotons and atomic ions in extreme
charge states via molecular ions and clusters up to macro- and bio-molecules, and to proceed in realising our ideas in two steps:
The CSR-initiative: We propose to first construct an ultra-cold version of this device
at the Max Planck Institute for Nuclear Physics (i) to open up new opportunities for energy and state resolved reaction studies with molecular and atomic ions by exploring the
storage and cooling of low-energy ion beams in a low-temperature, virtually perturbationfree environment, and (ii) to develop and demonstrate the ambitious methods and techniques required for producing brilliant low-energy beams of antiprotons within the
FLAIR-project at GSI.
8
In the proposed Cryogenic Storage Ring (CSR) ion beams will be stored at temperatures down to 2 Kelvin under extreme vacuum conditions with corresponding, extraordinarily long storage times, so far only realised in small-scale cryogenic ion traps. It will
be possible to store slow beams of molecular ions in a wide mass range (up to clusters
and bio-molecules) and atomic ions. For stored molecular ion beams, complete rotational
relaxation can be achieved for the first time through the reduced level of blackbody radiation in connection with the long (up to 1 hour) storage times required for thermalisation.
For atomic ions, it will become possible to store slow beams of ions in the highest charge
states, available in our laboratory from the EBIT facility, and to increase their phasespace density by electron cooling, at velocities much lower than accessible so far. The
stored ion beams will be used in event-by-event reaction studies, starting from single,
well defined initial quantum states and analysing the final-state kinematics by imaging
detection of the reaction products. The primary tool for interaction studies will be a cold
merged electron beam based on a cryogenic photocathode electron source, to be used for
electron collisions on state-selected molecular ions and for ion beam cooling. Additionally, it will be possible for the first time to perform reaction studies with a fast neutral
atom beam velocity-matched with the stored, state selected molecular ion beam. Similarly, a crossed atomic or molecular beam target equipped with an electron and ion momentum imaging system will be operated with a high flux of stored, low-energy ions for
studying atomic multi-electron reaction dynamics. This combination of techniques will
yield a unique spectrum of unprecedented experimental opportunities.
The USR-initiative: Triggered by the MPI-K (J. Ullrich et al.), a proposal has recently been put forward by an international collaboration to advance low-energy antimatter physics to the next generation of experiments by consequently exploiting and further
developing cutting-edge storage and cooling techniques; a Facility for Low-energy Antiproton and Ion Research (FLAIR) is proposed to be built at the approved future GSI accelerator and storage ring complex, making optimum use of the unprecedented antiproton
flux delivered by the GSI antiproton factory. With two consecutive rings, a conventional
medium-size magnetic storage ring followed by a novel electrostatic Ultra-low energy
Storage Ring (USR), and possibly supplemented by a low-energy positron ring for merging beam experiments, a series of new fundamental investigations will be possible, well
beyond the present activities of different groups at the CERN antiproton decelerator (AD)
and actually combining them.
Within the FLAIR proposal the USR, expected to supply a brilliant, electron cooled
antiproton beam with variable energies between ~300 keV and ~20 keV, is of central importance for essentially all experiments exploring antimatter–matter interactions, for inring experiments at an internal gas jet target, as well as for all experiments employing
traps as they can be efficiently filled using the decelerated and cooled antiproton beam.
Compared to present techniques the proposed scheme will provide an increased antiproton intensity of more than a factor of hundred for trap experiments and a five-order-ofmagnitude increase in the luminosity of matter–antimatter experiments using in-ring reac9
tion microscopes. For the USR to fulfil its key role in the FLAIR project, the development of novel and challenging methods and technologies is required: Combination of the
electrostatic storage mode with a deceleration of the antiprotons from 300 keV to 20 keV
(as needed for in-ring experiments as well as for efficient injection into traps), electron
cooling at all energies for the phase-space optimisation of the stored beam, RF bunching
for in-ring experiments with the reaction microscope and for the deceleration process, as
well as incorporation of an internal gas jet and an in-ring reaction microscope for collision experiments. As already pointed out, these key-characteristics exactly meet the requirements on a molecular ion storage ring with two major exceptions: vacuum requirements are considerably relaxed since antiprotons cannot capture electrons from rest-gas
atoms or molecules, and no cryogenically cooled environment is necessary, both effectively reducing the costs for the USR by about one half as compared to the CSR.
In view of the schedule for the new GSI facility and in order to gain enough time to
demonstrate that the technological challenges of the FLAIR project can be met, the final
decision on the project will be taken in 2008, the MPI-K group being responsible for developing the ultra-low energy storage ring USR. At that time one should also reconsider
the idea to use the FLAIR complex for investigating very heavy, highly charged atomic
ions and, in particular, radioactive ions available at the New Experimental Storage Ring
NESR of the new GSI facility beside antiprotons. Nuclear properties of isotopes far-offstability could be investigated with unprecedented precision by laser spectroscopy of the
hyperfine transition in one- or few-electron systems or by dielectronic resonances since
uncertainties due to many-electron contributions are avoided. Collisions of highly
charged ions with atoms involve few to many-electron transfer reactions proceeding
within the collision time between a few femtoseconds down to a few attoseconds, thus
being ideal prototype reactions to explore ultra-fast, correlated electron dynamics. Such a
programme would require, however, that also the USR is operated at a few degrees Kelvin to ensure vacuum conditions better than 10-13 mbar required for adequate beam life
times.
As the two initiatives combine in a unique way the technical expertise accumulated in
almost 15 years of storage ring operation, decades of accelerator and RF development (v.
Hahn, Grieser), electron cooling (Grieser, Wolf), pioneering work in the development of
beam- (Zajfman, Wolf) and target-imaging techniques (Ullrich, Moshammer), experience
in cryogenic techniques and extreme vacuum (Crespo López-Urrutia, Orlov, Schröter,
Ullrich) and in electrostatic storage ring techniques (Welsch), as well as in merged
atomic beam techniques (Urbain), we feel well prepared to reach the ambitious technological goals needed to advance the physics with antiprotons, molecular as well as highly
charged ions in a unique way, and to accomplish a forefront experimental research program in the area of atomic and molecular quantum dynamics.
10
2 The Physics Case
The physics questions to be addressed in all three fields of research based on the CSR
technology will be summarized in the sections below:
•
Physics with molecular ions in the CSR,
•
Physics with highly charged ions in the CSR,
•
Physics with antiprotons in the USR within the proposed FLAIR facility at GSI.
Regarding the physics case for the USR a summary of the GSI projects as set out in
the FLAIR [1] and SPARC [2] Letters of Intent will be included. These Letters of Intent
also include more on the overall physics case, while here the emphasis is on those experiments which MPI-K groups are especially interested in.
2.1 Quantum Reaction Dynamics of Cold Molecular Ions
Cold molecular ions are of large importance in nature and of fundamental interest. In nature, cold ionic species play a key role in driving chemical reactions at low densities and
low temperatures which determine, for example, the chemical composition of interstellar
clouds, cold plasmas and planetary atmospheres. At the same time, ionic systems can be
prepared in well-defined kinematical and quantum states, thus playing a generic role in
the investigation and understanding of quantum pathways in molecular and atomic reaction dynamics.
It is one of the purposes of the CSR to store molecular ion beams and to study their
properties when the black body radiation is strongly reduced compared to normal laboratory conditions. So far, heavy-ion storage rings have been used for storing molecular
ions, where they can cool to their vibrational ground state, through radiative cooling [3].
However, the finite temperature of the ring enclosure of ~300 Kelvin imposes a limit to
the rotational cooling. With the CSR, experiments at variable temperatures (between 2 to
300 Kelvin) can be performed, where the internal degrees of freedom of the stored molecular ions are in equilibrium with the black body radiation field. A short description of
the molecular ion physics which will be explored under these special conditions is given
in the following.
A. Electron impact on rotationally cold molecular ions
Evidence for the strong dependence of the reaction rate coefficient over the vibrational
distribution has been clearly demonstrated using the heavy-ion storage rings, especially
for the dissociative recombination (DR) process [4]. New results, based on the room tem11
perature storage ring technique [5] are now showing that the rotational distribution also
influences the recombination rate. It is very difficult, if not impossible, to control such a
distribution in the large magnetic storage rings.
Using the electron target installed in the CSR, a detailed study of this dependence, for
well defined initial rotational temperatures of the stored molecular ions, can be carried
out. Moreover, very high energy resolution can be expected in the merged electron–ion
beam configuration by using the ultra-cold electron source with a GaAs photocathode
developed [6] for the TSR. Identification and study of single Rydberg resonances will be
possible, for the first time, for rotational-ground-state molecular ions. Starting with the
simplest molecular system (HD+) these data will provide a wealth of information on the
structure of the highly excited neutral species, as well as on the dynamics of the dissociation process. These results will allow for direct comparison with existing theoretical calculations based, for example, on Molecular Quantum Defect Theory. It is important to
point out that the low beam energy in the CSR will also improve the resolution of the
three-dimensional imaging technique we have developed for the TSR [7]. Thus, imaging
of multiple final electronic or even vibrational states (in case molecular fragments are
produced) will be possible. Additionally, experiments related to the electron impact rotational and vibrational excitation can be carried out under well defined conditions. This
process, which is central in the understanding of molecular interstellar clouds, has never
been measured directly. Numerous calculations have produced data showing that the rate
coefficients are quite large, but there has been no experimental confirmation so far.
B. Cold Atom Exchange Reactions
The concept that a simple model based on angular momentum barriers would be sufficient to predict the rates of exothermic ion-atom collisions has been undisputed for many
years. Since within this framework, the reactive cross section is inversely proportional to
the relative velocity, the statistical rate coefficient is supposed to be independent of the
temperature T. This feature is the essence of the Langevin model, which attributes the
attraction between reactions partners to charge induced dipole potentials (polarization)
[8]. Such a universal behavior is generally accepted for ion-neutral reactions involving
nonpolar molecules at low collision energies (below 1 eV).
However, recently, deviations from the general Langevin dependence have been observed, especially when high energy resolution is achieved for cold collisions (<50 meV
relative energy [9]). These high-resolution experiments are usually performed when a
neutral beam of molecules collide with an atomic ion beam. The reactions where molecular ions are involved (in collisions with neutral atoms) are more difficult to realize, as
the molecular ions are usually ro-vibrationally excited at the production stage. It can be
suspected that a rich resonance structure will become evident at sufficient resolution in
collision energies and with state-selected molecular ions.
We hence propose to perform high resolution, merged beam experiments where stored
12
and rotationally cold beams of molecular ions are merged with nearly velocity matched
fast atomic beams (such as H, C or O) and atom exchange reactions are observed at very
low relative energy. A model reaction is for example H3+ + O
OH+ + H2 (or H2O+ +
H). Studies of this type of reaction have mostly been carried out using flowing afterglow
techniques, which do not allow precise measurement of the reaction cross section as a
function of the relative energy. Of equal importance for these reactions is the effect of
deuteration which, because of the very small energy barrier, can have dramatic effects on
the reaction rates. Performing such experiments with the high resolution which will be
achievable using the CSR (~5 meV) and the available temperature control will allow us
to study these reactions in unprecedented detail and observe deviations from the typical
Langevin dependence. These deviations, which are present in the form of structures and
resonances in the energy dependent cross sections, represent today a real challenge for
quantum chemistry, as their description requires full 3D potential surfaces, including the
nonadiabiatic coupling matrix elements between the entrance channels and the vibrational
states of the intermediate complex [10]. Moreover, these results will have a direct impact
on the modeling of the interstellar medium, where this type of reactions occurs at very
low temperatures.
Additionally, the merging between cold molecular ions and Rydberg atoms should be
of great interest: Rydberg atoms can be thought as targets with quasi free electrons with
well defined energy, and could allow dissociative recombination studies to be extended to
electrons initially bound on an atomic collision partner.
C. Laser Interaction with Temperature Controlled Molecular Ions
As demonstrated at the TSR (for CH+ and OH+) [11,12], the possibility to photodissociate molecular ions that were previously stored and thermalised with roomtemperature walls (see Figure 1) greatly enhances both the quality of the data and the resulting analysis. This is mainly because (i) the number of initial states is reduced, and (ii)
the time dependent population of the excited states adds an additional parameter for the
assignment and understanding. On the other hand, the room-temperature limit imposed at
the TSR does not allow such experiments to be performed for rotationally cold molecular
ions. Using the cold environment which will be available at the CSR, experiments of this
type would allow theoretical predictions to be tested not only regarding the resonance positions, but also for their strengths, as the initial rotational populations would be known
(and variable). Additionally, photodissociation can be used as a practical tool for the time
dependent diagnostic of the population of the stored molecular ions. Monitoring of the
internal excitation, which can be manipulated with the cold electron beam [5], permits to
directly measure electron impact excitation or cooling.
D. State Control using Infrared Lasers
The manipulation of molecules using laser fields usually requires molecular beams in a
well defined initial rovibrational state. Among the simplest cases, which would be unique
13
Figure 1: Near-threshold laser-photodissociation spectrum of stored CH+ ions (yielding C+ + H fragments) measured at the TSR [11] for three storage time (ts) intervals as
shown. The spectrum for the storage time interval of 15–30 s (lowest frame) shows
new structure [11] assigned to resonances from single rotational levels J = 0–5 of the
CH+ vibrational ground state, with populations thermalised at 300 K, as imposed by
the TSR storage ring walls
for the CSR system, would be the possibility to use infrared lasers (CO or CO2) to directly change the population of the stored molecular ions [13]. The most exciting example for the use of this technique would be to measure the reaction rates with cold electrons or atoms for individual, excited rovibrational states. This can be carried out for the
most typical and important small molecular ions such as HD+ and H3+. As it is known that
the change of one quantum of vibration can change, for example, the dissociative recombination rate coefficient with electrons by a huge factor (between one to two orders of
magnitude), even minute changes of the internal populations become measurable through
such processes.
Additionally, more complex schemes for the cooling of molecules have been proposed recently [14], where infrared lasers pump the molecular system and the blackbody
radiation is used to cool it back to “dark” states. The CSR produces the perfect environment for performing such experiments. Control of the degree of excitation will be performed with the use of the in-ring supersonic jet, using recoil-ion momentum spectros-
14
copy in coincidence with the imaging of the neutral dissociation products.
The CSR will offer many additional new opportunities on top of those discussed
above. For larger systems, such as ionic clusters, the low temperature environment will
allow to study weakly bound systems (such as hydrogen clusters) and to measure their
properties and cooling mechanism. Evaporation and fragmentation processes induced either by low energy electrons, photo-excitation or strong femtosecond lasers will be studied in detail, providing new insight in the many-body dissociation dynamics of large systems. The possibility to store large molecular ions of biological relevance in the cryogenic electrostatic storage ring will give access to some of their gas phase properties,
yielding complementary information to studies in the gas-phase, liquid phase or true biological environment. Thus, comparing the interaction of these bio-molecules with low
energy electrons to that with photons will allow us to probe the difference in bond breaking by these two types of interaction, as well as the so called collision-activated dissociation. Considering ongoing detector developments, new superconductor detection techniques may become available by which it would be possible to analyze all (charged and
neutral) dissociation products of very heavy molecules.
2.2 Atomic Ions in Extreme Charge States
Here we illuminate in some detail the physics with highly charged ions stored at low to
ultra-low energies in the CSR at the MPI-K, where highly charged heavy ions can be injected from the Electron Beam Ion Trap (EBIT). As of now, He-like Hg78+ has been created with the goal to trap even hydrogen-like uranium in sizable amounts within 2004.
A. Precision spectroscopy of highly charged ions
The ultimate testing ground of bound-state non-perturbative quantum electrodynamics
(QED) is the Lamb shift of the 1s ground-state in hydrogen-like heavy ions up to U91+.
Here, the expansion parameter grows to Z⋅α 1 (Z: nuclear charge; α 1/137: finestructure constant). Whereas the treatment of QED contributions in weak fields appears to
be understood at levels not achieved in any other physical theory [15-17], recent theoretical and experimental results indicate that two-loop corrections converge only poorly in
powers of Z⋅α (see e.g. [18-20] and references therein). These contributions were expected to reach the 1 eV level in U91+ [21,22], where a perturbative QED treatment is definitively not acceptable and calculations must be complete in all orders of Z⋅α. Such
studies have been started [21-24] with some early confusing results (see the discussion in
[25]), and completed recently predicting a 1.57 eV ± 0.31 eV contribution [25]. Theoretical uncertainties due to nuclear size (0.38 eV [25]) and virtual nuclear excitations (0.1 eV
[25]) are expected to contribute on a similar level (see also [26,27]. On a long run, using
different isotopes or double-magic nuclei like lead, these limitations might even be
strongly reduced.
Experimentally, the 1s ground-state Lamb shift in hydrogen-like uranium has recently
15
been determined at the high-energy experimental storage ring (ESR) at GSI, Darmstadt,
to 468 eV ± 13 eV [28]. The technique applied was to investigate Lyman-α radiation of
U91+ using segmented solid-state germanium detectors (500 eV resolution at 122 keV)
after capture of one (or more, which is not clear and causes problems if present) electron
into cooled, decelerated bare uranium. Deceleration and cooling of 358 MeV/u U92+, the
optimum production energy by stripping of uranium in foils, to 49 MeV/u was decisive to
reach the 13 eV resolution since the main source of error at high energies, the correction
for the Doppler shift, could be substantially reduced. The authors state: “By using decelerated beams, further progress towards an absolute accuracy of 1 eV may be anticipated.
The deceleration mode not only reduces the uncertainties in the Doppler corrections but,
in particular, it provides a very efficient production of characteristic projectile radiation.”
Therefore, an ultra-low energy cryogenic storage ring, equipped with an internal super-sonic jet target, a recoil-ion momentum spectrometer and large-area germanium detectors or bolometers will be the ultimate tool to perform precision measurements of the
Lamb shift in strong fields for hydrogen-like ions with Z ≥ 54 and to reach the 1 eV accuracy for the uranium 1s Lamb shift as the final goal. Similarly, He-, Li-, or Be-like systems might be explored in the ring in order to explore few electron QED, electron correlation and relativistic effects. The method shall be developed and pioneered at the CSR
by injecting highly charged ions from the Heidelberg EBIT [29]. As a first experiment,
the feasibility of such an approach shall be demonstrated by reaching a 10-4 resolution in
the Lyman-α transition in hydrogen-like xenon.
The MPI-K EBIT produces about 104 bare xenon ions (Z = 54) per second “at rest”.
Precision QED experiments have been demonstrated there very recently [30]. The upgrade of the electron beam energy from 100 to 350 keV is underway and the production
of about 103 bare uranium ions every 10 seconds is anticipated from current experience as
well as from the first production of U92+ in an EBIT at Livermore National Laboratory
[31], since the Heidelberg EBIT has achieved a twofold beam intensity compared to the
earlier devices. The ions will be injected into the cryogenic storage ring at a total energy
of around 20 keV, making the Doppler shift completely negligible. The lifetime of the
beam will be about 10 sec at an electron capture cross section from the residual gas of
around σ ≈ 10-14 cm2 and 10-13 mbar vacuum. Providing a supersonic low line-density
internal helium gas-jet target of 109 cm-2, 99 % of the stored ions will undergo electron
capture in the jet at a rate of 104 per second. Using a recoil-ion momentum spectrometer
to be implemented into the ring (for an overview see [32]), one-electron capture into
well-controlled Rydberg states in U91+ can be ensured by measuring the recoil-ion charge
and longitudinal momentum with a 4π solid angle (see e.g. [33]). Due to the negligible
Doppler shift, large area germanium detectors can be placed very close to the cooled
beam reaching an x-ray detection solid angle of ∆Ω/4π ≈5 % to detect the photons emitted by the deexcitation of those levels. Up to 50 characteristic Lyman-α x-ray photons
per second are expected for hydrogen-like xenon Xe53+ and 0.5 s-1 for uranium U91+ under
16
Figure 2: Design sketch and photograph of the in-ring reaction microscope to be operated in 2005 in the ESR
such conditions. Thus, a statistical accuracy at the level of 1 eV for the 1s Lamb shift of
hydrogen-like uranium might be expected within about one week of beam-time. An energy calibration of the germanium detectors with appropriate accuracy will be carried out
by using neutron-activated radioactive sources. Preliminary experiments with ions
trapped in an EBIT [34] have shown the potential of this method. However, there was
uncertainty about the number of electrons captured and possible energy shifts caused by
them. In storage ring experiments, the number of electrons captured and their states prior
to decay are measured and can be used in coincidence with the photon signal.
B. Femtosecond Electron transfer: Spectroscopy and Dynamics
In slow collisions (a few keV beam energy) of highly-charged stored ions with atoms of
the gas jet target, one or more electrons might be transferred with large cross sections
within the collision time of typically less than a femtosecond into high-lying quantum
states. In-ring high-resolution reaction microscopes will enable the precision spectroscopy of these exotic singly or multiply excited states, the investigation of electron rearrangement and deexcitation dynamics if multiple electron transfer had occurred and, if
combined with intense lasers, the electron transfer might be manipulated or even controlled on a sub-femtosecond time-scale. Moreover, at higher energies of up to 300 keV
as anticipated for the CSR, single or multiple electron promotion into the continuum
might be explored in unprecedented detail and completeness.
In-ring reaction microscopes, suitable for operation in extremely high vacuum (XHV)
and imaging all target reaction products with nearly 100% efficiency, have been developed by MPI-K groups (see Figure 2), will be tested in 2005 in the Experimental Storage
Ring (ESR) at GSI and, thus, be available for CSR and USR.
17
Figure 3: Longitudinal recoil ion momentum for single electron capture in Ne7+ on He
collisions [33]
1. Precision spectroscopy of exotic excited states
At low velocities corresponding to keV energies as realized in the CSR, the dominant reaction channel in ion-atom collisions is electron transfer from the atom to the ion with
negligible contributions of ionization, i.e. electron promotion into the continuum. Thus,
in the initial as well as final state there are only two heavy particles. Then, the inelasticity
of the reaction (so-called Q value), i.e. the difference in binding energy of the electronic
states between the final and initial channels, can be determined by measuring the momentum change of one collision partner, in our case of the recoiling target ion. As shown in
detail in Ref. [32], under certain conditions which are usually well fulfilled in ion-atom
collisions, the Q-value is in good approximation determined by the longitudinal momentum change of the recoiling target along the beam direction. Determination of this momentum component after capture of one or more electrons, thus, provides exact spectroscopic information on the final states of transferred electrons since the initial ground state
electronic energies are well-known.
This is illustrated in Figure 3, were the final longitudinal momentum pfR || is plotted
for single electron capture of Ne7+ projectiles from a helium target. A large variety of
populated states due to capture into 4l or 3l states is observed in addition to doubly excited states, where the electron was captured into 3l states and the 2s electron of the projectile was simultaneously excited. Reaching a 0.7 eV resolution in the line width and,
18
thus, up to 3 meV accuracy in the determination of the energy of the excited states, this
method challenges precisions usually obtained using conventional spectroscopic tools.
Moreover, all states which were populated are determined simultaneously irrespective of
their decay channels, and the dynamics, i.e. the impact parameter dependence of the
process is investigated simultaneously by inspecting the recoil-ion momentum component transverse to the beam directions, which corresponds to the projectile scattering angle.
2. Sub-femtosecond few electron transfer and rearrangement
While single electron capture at ultra-low keV energies can be explored reasonably well
by extracting highly-charged ion beams directly from the EBIT into a reaction microscope, reachable beam intensities of about 104 ions per second for highly charged ions are
not sufficient to investigate double or multiple transfer and rearrangement reactions.
Those are of paramount generic interest for the understanding and description of correlated ultra-short few electron dynamics. Since the impact parameter can be controlled via
the transverse momentum measurement, the transfer of several electrons from one potential well to the other within several tenth of attoseconds, i.e. within typical orbiting times
of outer-shell bound electrons, can be explored under ultimately controlled conditions
providing benchmark data for theoretical descriptions. Questions on the importance of
electron correlation during the transfer, the coupling to the nuclear motion, on the “swapping” of the electron density between the target and the projectile as well as on the final
many electron states populated in the projectile as well as in the target can be addressed.
Moreover, since the reaction microscope allows one to measure the momenta of several electrons emitted during the collision, the rearrangement and deexcitation processes
in exotic highly excited states of “hollow atoms” can be experimentally explored for the
first time.
3. Sub-femtosecond control of one or many electron transfer reactions
Recently, it was predicted theoretically [35], that a strong laser pulse being present during
the collision might strongly influence the electron transfer probability as a function of the
impact parameter and of the relative phase between the distance of closed approach and
the laser field. It seems feasible to monitor the relative phase on a 100-attosecond level
by high-resolution recoil-ion momentum spectroscopy and, thus, to explore for the first
time controlled electron manipulation on these time scales. From another point of view,
this situation with an electron drain (the highly charged ion), a source (the target atom)
and the switch (the laser) represents a prototype of an ultra-fast (ultimate velocity!) subnanometer (ultimate packing density!) optical transistor. The understanding of controlled
electron motion in such a situation might help to design, understand and control future
transistors reaching the nanometer scale and Terahertz switching frequencies.
4. Single and multiple ionization
At increased projectile ion velocities corresponding to a few ten to few hundred keV
19
beam energy, electrons are not only transferred from the target atom to the ion but, with
increasing probability promoted into the continuum. There are only few measurements
for single electron ionization displaying surprisingly structured final-state electron momentum distributions, strongly varying with the impact parameter of the collision, the
collision velocity and the projectile ion species (for a recent summary see [32]). While
single electron transfer is reasonably well understood, there is until today no consistent
theoretical description of single ionisation in slow ion-atom collisions. The high flux
reached in a low-energy storage ring would, for the first time, allow investigating in kinematically complete experiments two or many electron transfers into the continuum.
C. Radioactive ion option for the FLAIR facility
The whole FLAIR facility as described in Sec. 2.3.1 below can be used as well to store,
cool, slow down and trap any radioactive ions that are produced in the GSI accelerator
complex and brought to the NESR. In particular the spectroscopic studies described
above can also be considered for very heavy, radioactive ions as long as their lifetimes
allow sufficient storage times, once the USR is integrated into the FLAIR facility. As
mentioned before, the physics case for this field of research is described in more detail in
the SPARC Letter of Intent [2].
2.3 Physics with Antiprotons and Antimatter
2.3.1 FLAIR and USR – a Project Summary
The future accelerator and storage ring facility for beams of ions and antiprotons at GSI
in Darmstadt will produce the highest flux of antiprotons in the world. Within the planned
complex of storage rings, it is possible to decelerate the antiprotons to a kinetic energy of
about 30 MeV in the NESR (see Figure 4). Further deceleration in dedicated machines
will open the unique possibility to provide cooled, high-quality beams of low to ultra-low
energy antiprotons at intensities exceeding those achieved presently at the Antiproton
Decelerator (AD) of CERN, Switzerland, by factors of ten to hundred.
In the FLAIR Letter of Intent [1] a facility is described consisting of two storage
rings, a magnetic (LSR) and an electrostatic one (USR) providing stored as well as fast
and slow extracted cooled beams at energies between 20 MeV and 300 keV (LSR), between 300 keV and 20 keV (USR) and cooled particles at rest or at ultra-low eV energies
in the various traps. This will allow a large variety of new experiments to be performed
as will be described in some detail in Sec. 2.3.2. A possible, important synergy aspect is
that the whole facility can also be used to study the unique range of nuclides available
from the new radioactive ion facility at GSI: radioisotopes with sufficient natural lifetimes can be stored and cooled in the LSR and USR machines for precision measurements at low velocity or even in traps like in HITRAP or in the new Cave A.
As illustrated in Figure 4, the planned LSR is a typical magnetic storage ring such as
CRYRING in Stockholm or the TSR of MPI-K in Heidelberg. Interest has been an20
Figure 4: Layout of the Facility for Low-energy Antiproton and Ion Research
(FLAIR). NESR: New Experimental Storage Ring (part of the approved GSI proposal). LSR: Low-energy Storage Ring, decelerates antiprotons to 300 keV with electron cooling and internal gas jet (reaction microscope). USR: Ultra-low energy Storage Ring with electrostatic operation, electron cooling and internal gas jet target (reaction microscope).
announced to move the CRYRING machine to GSI, in which case only the USR needs to
be newly constructed. As stated in the FLAIR Letter of Intent, MPI-K presently designs
21
such a next-generation low-energy ring for molecules and highly charged ions and intends, in parallel, to take over the responsibility for the development of key technologies
needed for the USR, provided that funding within the present proposal can be secured.
2.3.2 Low-energy antiproton physics
The case for low-energy antiproton physics has been made in numerous project studies of
present CERN collaborations as well as in the recent FLAIR Letter of Intent [1] at GSI,
which is includes more detailed descriptions of the planned experiments. Present and future research goals can be subdivided in five groups and are shortly characterized below.
MPI-K is mainly involved in experiments described in the group 2.
1. Fundamental interactions
Among those experiments are tests of the CPT symmetry as well as of QED via (laser-)
precision spectroscopy of antiprotonic atoms or of antihydrogen, via Ramsey type experiments on the hyperfine splitting of antihydrogen in its ground state as well as g-factor
measurements of the antiproton in a continuous Stern-Gerlach experiment. Until 2010
initial results on spectroscopy are expected from the AD, but the ultimate goal of reaching accuracies similar to those obtained in hydrogen requires the trapping and lasercooling of antihydrogen atoms, and presumably, the high antiproton fluxes to be aimed at
with the LSR/USR facility. Once trapped and laser cooled, other challenging experiments
in this class will become possible, like the direct measurement of the gravitational force
on antimatter, a long-standing question that has never been answered experimentally.
2. Few-particle, sub-femtosecond quantum dynamics and matter-antimatter collisions
At energies below about 500 keV down to 1 keV the interaction time between an antiproton passing atoms or molecules is on the order of 70 attoseconds (as) up to 1 femtosecond
(fs) and, thus, comparable to the revolution time of outer-shell electrons. Moreover, the
antiprotons’s electric field is so strong that any perturbative theoretical approach must
fail. Therefore, slow antiprotons provide an unsurpassed, precise and irreplaceable tool to
study many-electron dynamics in the strongly correlated, non-linear, sub-femtosecond
regime, the most interesting and, at the same time, most challenging domain for theory.
In-ring experiments at the USR using reaction microscopes will allow, for the first time,
kinematically complete experiments and, thus, provide benchmark data for the development of theory. Moreover, the capture of antiprotons into atoms and molecules, so-called
antiprotonic atom formation, can be investigated under single collision conditions, at well
defined velocity, determining distributions of captured antiprotons over hydrogenic states
n, l under unprecedented clean conditions in kinematically complete in-ring experiments.
If a positron beam will be merged (as sketched in Figure 4) by implementation of a
positron storage ring (Low Energy Particle Toroidal Accumulator, LEPTA [36]), recombination experiments of antiprotons with positrons will become accessible, laser-assisted
recombination might be explored and a beam of antihydrogen at well defined energy can
22
be formed to be used in further experiments: Among them are matter-antimatter collisions, formation of intermediate H H molecules, the creation of H + and of other exotic
systems as a result of collisions, as well as Ramsey experiments on the hyperfine splitting
of antihydrogen.
Most of the experiments in this group, exploring fundamental few-particle quantum
dynamics, are not feasible at all at the present AD CERN facility and will decisively
benefit from storage techniques: For in-ring experiments, the effective flux of antiprotons
is increased by a factor or 105 due to the 10 s revolution time in the ring.
3. Nuclear physics
Here, the antiproton is used as a hadronic probe to study the nuclear structure. X-ray
spectroscopy of the low-lying states of pp or other light atoms gives important information on the nucleon-antinucleon interaction in the low-energy limit, where scattering experiments cannot provide precise data. Such results are vital for the improvement of QCD
calculations in the low-energy (hence non-perturbative) regime. X-ray spectroscopy of
heavy antiprotonic atoms can be used to obtain information about the density ratio of
neutrons to protons at the nuclear periphery, i.e. to investigate neutron halo or skin effects. The application of this technique, pioneered at LEAR, to unstable radioactive ions
available at FLAIR via the SuperFRS (FRagment Separator) will generate important contributions to the study of the structure of nuclei far from stability.
4. Particle Physics
Concerning the particle physics point of view, the elementary antinucleon-nucleon interaction processes still present a substantial number of unclear aspects, in spite of the
wealth of results produced at LEAR. In particular, some anomalous effects have been observed close to the threshold which are likely related to the interplay between quark and
antiquark degrees of freedom – that could be, for instance, responsible for the existence
of quasi-nuclear sub-threshold nucleon-antinucleon bound states, long searched for at
LEAR without large success. FLAIR might provide the possibility to use antineutrons as
probes, an effective way to create a more transparent environment, free from Coulomb
contributions.
5. Applications
Recently, interest has been shown in the potential medical application of antiprotons for
tumor therapy. The reason is an expected strongly enhanced biological effect close to the
end of the track since, firstly, antiprotons do not capture electrons such that their effective
charge remains equal to unity until very shortly before coming to rest and, secondly, the
annihilation at rest then produces residual nuclear fragments of high charge and low energy, depositing a large biological dose in the immediate surrounding of the p stopping
points. Since the cooled low-emittance antiproton beams delivered from the NESR/LSR
complex can be stopped in well-defined regions of space, the presumably large energy
23
deposited locally makes them a suitable tool for tumor therapy. A test experiment is under way at the AD of CERN.
References to Section 2:
1. FLAIR Letter of Intent (copy attached)
2. SPARC of Intent (copy attached)
3. M. Larsson, in: Advanced Series in Physical Chemistry, Vol. 10: Photoionization
and Photodetachment, ed. by C.-Y. Ng (World Scientific, Singapore, 2000), p. 693
4. Z. Amitay et al., Science 281, 75 (1998).
5. L. Lammich et al., Phys. Rev. Lett. 91, 143201 (2003).
6. D. A. Orlov et al., Nuclear Instruments and Methods A, in press.
7. D. Strasser et al., Rev. Sci. Instruments 71, 3092 (2000)
8. D. P. Stevenson and D. O. Schissler, J. Chem. Phys. 29, 282 (1958)
9. P. Tosi et al., Phys. Rev. Lett. 67, 1254 (1991)
10. P. Tosi et al., J. Chem. Phys. 99, 985 (1993)
11. U. Hechtfischer et al., Phys. Rev. Lett. 80, 2809 (1998)
12. J. Levin et al., Hyperfine Interactions 127, 267 (2000)
13. W. H. Wing et al., Phys. Rev. Lett. 36, 1488 (1976)
14. S. Vogelius et al., Phys. Rev. Lett. 89, 173003 (2002)
15. B. de Beauvoir et al., Phys. Rev. Lett. 78, 440 (1997)
16. Th. Udem et al., Phys. Rev. Lett. 79, 2646 (1997)
17. M. Niering et al., Phys. Rev. Lett. 84, 5496 (2000)
18. K. Pachucki, Phys. Rev. Lett. 72, 3154 (1994)
19. M. Eides and V. Shelyuto, Phys Rev. A 52, 954 (1995)
20. K. Pachucki, Phys. Rev. A 63, 042503 (2001)
21. A. Mitrushenkov et al., Phys. Lett. A 200, 51 (1995)
22. S. Mallampalli and J. Saphirstein, Phys. Rev. Lett. 80, 5297 (1998)
23. I. Goidenko et al., Phys. Rev. Lett. 83, 2312 (1999)
24. V. Yerokin, Phys. Rev. A 62, 012508 (2000)
25. V. Yerokin, Phys. Rev. Lett. 86, 1990 (2001)
26. T. Beier et al., Phys. Lett. A 236, 329 (1997)
27. P. J. Mohr, G. Plunien, and G. Soff, Phys. Rep. 293, 227, (1998)
28. Th. Stöhlker et al., Phys. Rev. Lett. 85, 3109 (2000)
29. J. R. Crespo López-Urrutia et al., Physica Scripta T92, 110 (2001)
30. I. Dragani et al., Phys. Rev. Lett. 91, 183001 (2003)
31. R. E. Marrs, S. R. Elliott, and D. A. Knapp, Phys. Rev. Lett. 72, 4082 (1994)
24
32. J. Ullrich et al., Rep. Prog. Phys. 66, 1463 (2003)
33. D. Fischer et al., J. Phys. B 35, 1369 (2002)
34. P. Beiersdorfer et al., Physica Scripta T80, 121 (1999)
35. T. Kirchner, Phys. Rev. Lett. 89, 093203 (2002)
36. I. Meshkov et al., Nucl. Instrum. Methods in Physics Research A 441, 145 (2000)
25
3 Technical Layout
3.1 The Cryogenic Storage Ring CSR
For ion beams of low kinetic energies (~300 keV), electrostatic storage rings (bending
field strength ~10 kV/cm, bending radius ~1 m) outperform magnetic storage rings with
bending powers of the order of 0.1 Tm. At a fixed setting of the confinement fields, electrostatic storage rings offer mass-independent storage of ion beams with a given kinetic
energy, while for magnetic storage rings the beam energy decreases inversely proportional to the ion mass. This property makes it attractive to apply electrostatic storage as
essentially a single technical concept to the entire physics program outlined in Sec. 2, involving a wide range of ion species, from antiprotons and light molecular ions to heavier
polyatomic molecular species, bio molecules, cluster ions at any size and, looking even
beyond, dust particles or aerosols. Limited experience is available on the international
scene and electrostatic storage rings in Denmark [1] and Japan [2] could already prove
the benefits of these machines for some of the above mentioned research areas. Various
geometrical shapes have been under discussion [3], where the size of the machine is
mainly determined by the size of the electron cooler and the experimental areas.
The requirements on the stored beam and on the machine itself, requested by the
broad physics program to be covered at the CSR and USR, differ substantially from those
of all existing machines (see also Appendix A.2) and make new and challenging developments necessary, some of them being at the very limit of state-of-the-art technology.
Thus, in spite of a recent proof-of-principle demonstration of electron cooling at low energies in an electrostatic ring [4], an efficient improvement of the stored-ion beam quality
by phase-space cooling is not in general available. Exploiting the properties of newly
available cryogenic electron sources pioneered in Heidelberg for their use in coolers, reasonably short cooling times can be estimated for electron cooling of slow ion beams, allowing the high beam quality required for many of the planned projects to be achieved.
In-ring experiments with the reaction microscope need in addition a nanosecond bunch
structure that has never been realized in any ring and is especially demanding in an electrostatic device due to the coupling between transversal and longitudinal phase space in
the cylindrical deflectors [5]. Furthermore, none of the existing machines varies the beam
energy so that radio-frequency de- or acceleration cavities have to be newly designed,
developed and tested.
Since highly-charged ions have to be stored and molecular ions shall be cooled into
their rotational ground states, demanding vacuum conditions of about 10-13 mbar and 2
26
Kelvin wall temperatures have to be achieved, respectively. The essential design features
are summarized below.
Final design features of the CSR
•
The ring temperature should be controllable between 2 K and 300 K.
•
Base pressure at 2 K should be 1 x 10-13 mbar or below.
•
The beam energy should be variable from ~20 keV up to ~300 keV.
•
The beam should have a nanosecond bunch structure required by experiments with
the reaction microscope.
•
An electron target/cooler should be located in a straight section of the ring, realising
electron cooling at beam energies down to 20 keV.
•
A gas target along with a reaction microscope should be located in a straight section
of the ring.
•
A neutral atomic beam should be merged with the stored ion beam in a straight section of the ring in the final stage. (This feature shall not be implemented until 2008.)
•
Laser beams should be merged with the ion beam along the straight sections of the
ring.
•
The ring should be connected to a low energy, singly charged ion source for positive
and negative ions, including a cluster (supersonic expansion) source and a cryogenic
ion source.
•
The ring should be connected to an EBIT for injection of highly charged ions.
•
Space for particle imaging detectors observing charge-changing processes and molecular dissociation should be accessible in two of the ring corners, after the straight
sections for the electron cooler and the reaction microscope. Three-dimensional largearea imaging detectors shall be installed.
•
Beam diagnostic (Schottky noise pickup, beam current probes) should be implemented.
•
Two dimensional neutral beam imaging shall be implemented for beam diagnostics.
Design goals for 2008 and for the final stage
Trying to reach all of the above characteristics until 2008 seems too ambitious in view of
the available man-power and resources. We have therefore defined intermediate goals to
be reached in the initial construction period until 2008. Nevertheless, the overall design
should be such that the final values can be reached without modifications of the general
concept. The resulting design goals are listed in Table 1.
27
Table 1: CSR design goals to be reached over different periods of time
Feature
Ring temperature
2008
final
variable, <10 K–300 K
variable, <3 K–300 K
-13
Base pressure (2 K)
5⋅10
mbar
1⋅10-13 mbar
Beam energy
20–300 keV
20–300 keV
<3 ns
<1 ns
Electron cooler
x
x
Transverse jet target
x
x
Reaction microscope
ion and electron imaging
ion and electron imaging
Neutral merged beam
-
x
Laser merged beam
-
x
singly charged ions, EBIT
singly charged ions, EBIT,
cluster ion source, cryo source
Beam imaging detectors
x
x
Schottky detectors
x
x
Neutral beam imaging
-
x
Bunch time-structure
Ion sources
Present concept (subject to changes during the design period until summer 2004)
1. Ring design
In Figure 5, the preliminary layout of the CSR is illustrated, with integrated electron
cooler and reaction microscope, which both are scheduled to be operational until 2008.
Major constraints on the ring design are set by the demand to implement efficient, largesolid-angle imaging detection systems for neutral as well as charged-fragments in the
corners behind the cooler and the gas-jet target, respectively. Electron-volt dissociation
energies require large apertures for all optical elements on the one hand and, for efficient,
large-angle neutral beam extraction, deflectors before and after the corner bending elements, making the vacuum tube diameters as well as the total length of the ring quite
large. At present, the diameter of the inner tubes is between 10 cm and 40 cm and the anticipated circumference is about 35 m. The four-sided ring lattice is symmetric and each
of the four sectors consist of a 78° deflector, two 6° bending elements along with two sets
of quadrupole lenses and a straight section for the implementation of insertion devices.
2. The cryogenic and vacuum system
A requested base pressure of less than 10-13 mbar all over the ring along with wall temperatures below 2 K represent major challenges. Nevertheless, with state-of-the-art vacuum and cryogenic technologies, the design goals can be reached. As of now, among a
series of different concepts two different solutions turned out to be most promising and
are presently pursued in parallel, both with advantages and disadvantages. A final deci28
Figure 5: Preliminary layout of the CSR facility with the electron cooler, ion injector
and experimental devices (reaction microscope, merged neutral atom beam, lasers); fastbeam fragment counting and imaging detectors will be placed in the encircled “detection
areas”
sion for one or the other to be implemented will be made during summer 2005 after exhaustive tests of respective prototype ring segments. For historical reasons the two
technological strategies are named the “closed” and “mixed” solution, respectively.
Basic principles for building cryogenic systems will be followed. A cryostat at room
temperature is equipped with a vacuum pumping system in order to reduce the heat
conduction via gas flow. A super insulation (SI) system suppresses the heat radiation and
a few intermediate shields at fixed temperatures below the one of the SI guarantee a low
heat radiation input to the cold surface. Each thermal insulation component helps to re-
29
Figure 6: Outline of the “closed solution” (see text). Dimensions are given in mm
duce the required cooling power which enters about linearly into the costs of the cryogenic plant.
Accordingly, in both technical solutions the outer 300-K enclosure with a diameter of
about 500–600 mm, i.e. the cryostat, consists of O-Ring sealed stainless steel chambers
sustaining a 10-6 mbar insulation vacuum in order to minimize gas flow heat conductance.
Second, a system of super insulation will be used to shield the innermost 2-K shell from
the 400 W/m2 main heat input by radiation. As mentioned before, the storage ring shall be
used at all temperatures between 2 K and room temperature (RT). Whereas superior vacuum conditions are automatically achieved when the walls are cooled below 5 K, a base
pressure below 10-11 mbar is maintained at RT by highly efficient volume getter pumps
(NEG) [6] distributed in the innermost shell directly surrounding the beam. These pumps
must be backed up by external pumps connected via pumping domes (spacing ~2 m along
the ring). In addition, achieving the given base pressure for RT operation requires baking
of the inner ring, including the SI system, to temperatures above 150oC. These temperatures being too high for standard SI materials, a special technique with thin aluminium
foils and spacers in-between has been developed.
The difference between the two “solutions” presently discussed essentially lies in the
design of the inner shells. The closed solution, as illustrated in Figure 6, consists of a
stainless steel cryostat at RT. The next shell is a 1–2 mm copper or aluminium 40 K
30
Figure 7: Outline of the “mixed solution” (see text). Dimensions are given in mm
shield which carries the super insulation system (not seen in the figure). Finally, the cold
inner part is a CF-sealed 2-K shell, cooled via high-thermal-conductivity copper strips
mounted at a spacing of about ~20 to 30 cm and refrigerated by a 2-K helium tube below
the 40-K shield. This design, being based on and exploiting extensive experience in super
conducting accelerator technology, focuses on a simple and easy-to-construct system
guaranteeing high reliability at favourable maintenance conditions. Its construction shall
be optimized using as few feed-throughs, moving elements, flanges and bellows as possible. Special effort in this solution is required in the design of the connections to the external pumps, linked directly to the 2-K shell. The estimated heat input from conductance
onto the 2-K shell will be tolerable, but represent a major contribution in spite of the use
of thin-walled bellows and chevron radiation shields, thermally anchored at 40 K.
The mixed solution, as illustrated in Figure 7, is also based on a stainless steel cryostat. The next inner shell consists of CF-sealed stainless steel chambers which act as a
40-K heat shield and are surrounded by SI (not shown in the figure). Here, the external
pumps are connected to the 40-K stage. In contrast to the “closed” solution the innermost,
third shell is designed as an open system of cryogenic walls, also carrying all the electrodes, which can be cooled down to 2 K. This inner shell is cooled via copper heat-pipes
leading outside the 40-K vacuum chamber. There the heat pipes are refrigerated by separately shielded liquid He tubes, staying in the outer, thermal insulation vacuum to avoid
the risk of helium leaks in the inner storage ring vacuum system. This solution entails
more surfaces in the inner vacuum system and additional effort regarding the heat pipes,
31
Figure 8: Lattice functions calculated for the planned CSR layout with the OptiM
code. The horizontal and vertical focusing functions H(s) and V(s), respectively, and
the (horizontal) dispersion function D(s) are shown as functions of the longitudinal
coordinate s along one ring quadrant. The electrostatic optical ring elements, horizontally (QH) and vertically (QV) focusing quadrupoles and the 6˚ (B6) and 78˚ (B78)
bends, are indicated
but minimizes the solid-conductance heat input to the 2-K shell, since all external vacuum pumps are connected to the 40-K shield. At the present stage, both cryogenic solutions are being pursued. Measurements of the heat load, cooling-down time and final
pressure are planned on prototypes for both solutions in order to optimize the final design.
3. Imaging detectors
As mentioned before, an efficient 4π detection for neutral as well as charged fragments
after the electron cooler and the reaction microscope (in the same arm it is planned to
later implement the merged atomic beam) is one of the major design goals of the ring. In
order to achieve this goal, we propose to use the “all optical” design for the detectors, as
it was developed by D. Zajfman and coworkers at the Weizmann Institute of Science. In
such a detector, only a micro-channel plate (MCP) detector, with a phosphor screen serving as anode is located inside the vacuum chamber. The phosphor screen is imaged by
two CCD cameras, from which both position and time information is extracted. Subsequent data analysis allows the complete three-dimensional image to be recovered. An additional advantage of this system is its zero dead-time, allowing events with multi-particle
impact to be recorded. The system has already been used on a test beam, and is currently
being installed on the TSR.
32
4. Lattice Description
Different stable working points exist for the geometry presented above and, depending on
the experimental requirements, different modes of operation can be chosen. The focusing
and dispersion functions yielding approximately round ion beams in the straight (experimental) sections are shown in Figure 8. These working conditions also imply relatively
small horizontal beta functions (and thus horizontal beam sizes) in the electrostatic bends,
B(6) and B(78) in Figure 8, so that the mechanical aperture in these critical regions can
be kept small. For the working point shown, the voltages on the 6° and 78° bending elements are ±18 kV and the voltages on the electrostatic quadrupoles are ±3.3 kV and ±2.8
kV, respectively, for a 300 keV beam.
5. Electron cooling
Electron cooling reduces the diameter and the divergence of the stored ion beam and, in
connection with rf bunching of the stored beam, can produce short ion pulses as desirable
for measurements with the in-beam reaction microscope. The electron cooling technique
employs an electron beam moving at the same average velocity as the ion beam over a
part of the closed ion orbit to damp the ion motion in the storage ring. At the 300-keV
maximum ion energy of the CSR, electron cooling of a stored proton beam requires an
electron beam energy of about 165 eV; for heavier stored ions of mass number A the electron energy scales as 1/A, reaching ~5 eV for A ~ 30 (see Table 2). Similarly low electron
energies will be required for cooling antiproton beams in the USR at the desired lower
energy limit (<20 keV). Establishing efficient phase-space cooling with electron beams of
few-eV energies is one of the challenges we plan to take up in the CSR/USR project. In
contrast to electron cooling in the magnetic storage rings operating at higher beam veloci-
Table 2: Estimated parameters for electron cooling at the CSR. The cooling times are
calculated from the thermal equilibration time of a two-component plasma for an electron temperature of kT = 0.44 meV (cathode temperature kTc = 11 meV and adiabatic
expansion by a factor of 25; Coulomb logarithm 3.3; singly charged ions; ion velocity
spread neglected). Listed are 1/e cooling times for the transverse ion beam divergence
or the ion beam size (4 times the temperature equilibration times) assuming that the
electron beam fills 2.8% of the storage ring circumference (space-charge limited electron current for a perveance of 2 AV–3/2). The column “hot beam” accounts for an
additional geometrical overlap factor of 10, assuming that the electron beam of 10 mm
diameter covers only 10% of the ion beam cross section.
Ion
Ion
Electron Electron Electron
mass energy energy current
density
(amu) (keV)
(eV)
(mA) (106 cm-3 )
1
1
3
32
100
300
10
300
300
300
165
5.5
54
5.1
1.6
4.2
0.02
0.8
0.02
0.004
44
1.3
15
1.3
0.44
33
Cooling time
(cold beam)
(s)
Cooling time
(hot beam)
(s)
0.0025
0.084
0.025
2.5
25
0.025
0.84
0.25
25
250
ties, the electrical power transported by the electron beam will be extremely low (ranging
from ~1 W at the highest electron energy down to <1mW at 5 eV); moreover, only a low
magnetic guiding field (<10 mT) is required. Both aspects facilitate the setup of the cooling device and make its realization feasible following the general cryogenic design ideas
of the CSR. The electron beam will be generated and collected outside the main cryogenic system and merged with the ion beam using toroidal sections of ~0.4 m radius; the
length of the straight beam-overlap section will be ~1 m. We expect to reduce the fraction
of the transported electron current that will end on the cryogenically cooled walls surrounding the ion beam to ~10-5, which will keep the effects of electron impact desorption
due to stray electrons tolerable even at the 10-13 mbar level of envisaged operating pressures.
The most critical parameters for the achieved electron cooling time are the electron
temperature (lower temperatures reducing the cooling time with a scaling to a power
slightly lower than 3/2) and the electron density (higher densities reducing the cooling
time roughly to power –1). The generation of intense (~mA) electron beams from GaAs
photocathodes was demonstrated [7] for the TSR; moreover, with photocathodes cooled
by liquid nitrogen (operating temperature ~95 K) the production of beams with electron
temperatures corresponding to the GaAs bulk was demonstrated [8] at effective (coldelectron) quantum yields exceeding 1% (Ref. [6] of Sec. 2). This photocathode technique
will be applied to the CSR; the resulting lower electron temperature is estimated to accelerate electron cooling by at least one order of magnitude as compared to the use of
thermionic cathodes operated at >1200 K. The photocathode operation intrinsically requires ultrahigh-vacuum and low-temperature conditions and, thus, also under this aspect
is ideally adapted to the CSR vacuum requirements.
The expected operating parameters and cooling times for electron cooling at the CSR
are listed in Table 2. Operation down to electron beam energies of ~5 eV is envisaged
and the extension to even lower energies will be experimentally explored. Useful cooling
times appear feasible even at electron beam energies down to ~1.6 eV. Considering the
kinematical transformation into the co-moving frame of the velocity-matched electron
and ion beams, the required uniformity of the electron energy along the overlap region is
estimated to, e.g., <0.06 eV at 1.6 eV beam energy, which appears to be attainable carefully avoiding stray potentials on the surfaces surrounding the beams. With an electron
temperature of <1 meV the electron cooler will also be well suited as a cold electron target for the merged beam electron-ion collision experiments described in Sec. 2.1.
34
3.2 The Ultra-Low Energy Storage Ring USR
In the USR, antiprotons shall be slowed down from 300 keV to 20 keV for various inring experiments as well as for their efficient injection into ion traps. In this energy range
– especially taking into account the option to realize a real multi-purpose facility with not
only antiprotons but also various highly charged stable and radioactive ions to be stored
and investigated – electrostatic concepts have clear advantages compared to their magnetic counterparts. Moreover, the phase-space cooling of ultra low energy ion beams to
be established at the CSR would be invaluable in case one envisions even approaching
the eV range for the stored antiproton beams, which would be highly desirable for some
of the most interesting experiments (formation of a slow antihydrogen beams for hyperfine structure measurements and precision spectroscopy, formation of antiprotonic atoms
in collisions with atoms, etc.). Hence, we presently consider for the USR a concept basically identical to the CSR as the most natural and economic choice.
Aiming to be a true multi-user facility, the ring should provide an antiproton beam
that can be used by in-ring experiments and external experiments “at the same time”, i.e.,
from bunch to bunch the different experiments may be served at different energies, intensities and beam characteristics (bunched, slowly extracted, quasi-dc operation, etc.). High
luminosity, low emittance and low momentum spread are some of the main characteristics of the electron-cooled antiproton beam that shall be achieved. Thus, while the same
general concept is realized for the USR and CSR, practical requirements might differ to
Table 3: Design goals to be reached for different USR specifications
Feature
Ring temperature
antiprotons
highly charged ions
300 K
<10 K–300 K
Base pressure
1⋅10
mbar
1⋅10-13 mbar
Beam energy
20–300 keV
20–300 keV
<1 ns
<1 ns
Electron cooler
x
x
Transverse jet target
x
x
Reaction microscope
ion and electron imaging
ion and electron imaging
Laser merged beam
x
x
LSR
LSR
Charge change detectors
2
2
Schottky detectors
3
3
Neutral beam imaging
x
x
Bunch time-structure
Ion sources
-11
35
Figure 9: Preliminary layout of the USR with the electron cooler, rf device, diagnostic
elements and reaction microscope. The possible position of the LEPTA-type merged
positron beam is indicated (see text)
some extent, but can be easily accounted for without serious hardware changes.
If only antiprotons shall be stored and decelerated, relaxed vacuum conditions of only
10 mbar have to be maintained, essentially halving the costs and efforts for ensuring
antiproton storage times on the order of one minute. This is by far long enough for electron cooling, deceleration and extraction to the traps (estimated to require less than 5 s)
and sufficient for in-ring experiments with the reaction microscope. For the present proposal, antiproton storage only is assumed, as suggested in the FLAIR proposal. If decisions will be made until 2008 (by the SPARC collaboration or by the GSI atomic physics
groups for example) that highly charged ions shall be stored as well, the CSR vacuum
and cryogenic system can be easily copied maintaining, however, the simpler USR lattice
structure. Accordingly, design goals as listed in Table 3 shall be realized, depending on
the CSR test results and accumulated expertise.
-11
Present concept (subject to changes until finishing the technical report end of 2004)
1. Ring design
The USR lattice will be considerably simpler in its overall structure than the CSR one,
since no complicated many-particle beam imaging detectors are needed (no molecular
beams shall be stored), resulting in a ring circumference reduced to ~25 m. In Figure 9,
the preliminary layout of the USR is shown, with integrated electron cooler, reaction microscope, radio-frequency system for deceleration and various diagnostic elements. Simi36
lar to the CSR the four-sided ring is symmetric, however, with simple 90° bending devices at the corners (1 m bending radius at a gap voltage of ± 19 kV for the 300 keV antiproton beam and 6 cm gap width). The linear lattice consists of eight electrostatic quadrupole doublets, four 90° deflectors in the corner sections and two additional parallel
plate deflectors for independent closed orbit correction in the vertical and horizontal
transverse phase space. Single turn injection and fast extraction are implemented in one
straight section of the ring, leaving space for a merged positron beam to be implemented
in the fourth straight section (see Figure 9) as envisioned in the FLAIR proposal.
2. The vacuum system
As of now, the USR vacuum requirements are relaxed and meet the conditions realized in
all existing storage rings such that no special developments are needed.
3. Imaging detectors
Extraction of neutral antihydrogen beams that emerge due to radiative recombination in
an envisioned positron merged beams section is straightforward; negligibly small momentum is transferred during the neutralization reaction such that a low-emittance
H beam can escape through a small opening in the subsequent bending device. Imaging is
only needed for emittance control and might be realized by a simple position sensitive
multi-channel plate detector.
In case of antiprotonic atoms formed by antiproton capture in the supersonic gas jet
target of the reaction microscope, fragments with various charge-to-mass ratios will occur
at unknown momentum transfers and with a range of time delays (Auger emitted electrons). In inverse kinematics, the reaction might be seen such, that the antiproton picks up
part of the target atom An+ and n electrons are emitted simultaneously. The full momentum vectors of several “left over” electrons can be monitored in the reaction microscope
and their sum momentum should give precise information on the initial pA n + (n,l) distribution as well as on the reaction dynamics in kinematically complete experiments. Thus,
complicated imaging of the antiprotonic atoms does not seem necessary at the moment.
4. Lattice description
The transverse particle motion in the USR is comparable to the motion in the CSR.
Again, the smallest diameter of the beam is attained in the corner sections, where also the
mechanical aperture is smallest.
5. Electron cooling
The layout of the electron cooler, cooling times and cooling forces are identical to those
in the CSR. However, since the walls are not cryogenically cooled for exclusive antiproton operation in the USR, its hardware implementation is much simpler and the vacuum
requirements are relaxed such that efforts and costs are reduced substantially.
37
References to Section 3
1. S. P. Møller, in Proc. European Particle Accelerator Conference (EPAC), Stockholm, Sweden, 1998 (http://accelconf.web.cern.ch/AccelConf/e98/contents.html), p.
73
2. T. Tanabe et al., “An Electrostatic Storage Ring for Atomic and Molecular Science”, KEK Preprint 2001-44, KEK. Japan
3. C. P. Welsch et al., “Design Studies for an Electrostatic Storage Ring“, in Proc. Particle
Accelerator
Conference,
PAC,
Portland,
OR,
USA,
2003
(http://accelconf.web.cern.ch/AccelConf/p03/), p. 1622
4. T. Tanabe, report at Int. Workshop on Beam Cooling and Related Topics
(ECOOL2003), Yamanashi, Japan, May 19-23, 2003 (to be published in Nucl. Instrum. Methods A)
5. C. P. Welsch, et al., “Analysis of Field Perturbation due to Field Errors in an Electrostatic Storage Ring”, European Particle Accelerator Conference, EPAC, Paris,
France, 2002 (http://accelconf.web.cern.ch/AccelConf/e02/), p. 858
6. C. Benvenuti, Physica Scripta T22, 48 (1988)
7. S. Pastuszka et al., J. Appl. Phys. 88, 6788 (2000)
8. D. Orlov et al., Appl. Phys. Lett. 78, 2721 (2001)
38
4 Cost Estimates and Requested Funding
4.1 Cost Estimates
The cost estimate for the CSR is based on the realisation of the design goals to be reached
until 2008 as specified in Table 1 of Section 3.1. They have been defined on the one hand
on the basis of available scientific and technical manpower, the availability of working
time in the mechanical and electronic shops as well as construction time from the construction office. These estimates are conservative since they are only based on the capacity of the MPI-K alone, whereas the collaborating groups at the Weizmann Institute and
at Louvain-La-Neuve have secured support. The goals have, on the other hand, been defined taking into account the risks connected to the development of cutting-edge technology. Since the whole development project as it stands is limited to a total duration of four
years, with well defined goals and milestones to be reached, the overall risk is strongly
minimized and well defined. Should design goals not be reached until 2008, new decisions have to be taken depending on the actual situation. If they are reached, we are in a
unique, very competitive situation such that the realization of the antiproton USR as a
key-element of the FLAIR proposal – as well as the implementation of advanced instrumentation for the CSR, like precision and high-power lasers, other ion sources, the
merged atomic beam and the full imaging detection system – should proceed in a
straightforward manner.
Total costs for the CSR ring including only investments and infrastructure are estimated to be 4.1 Mio Table 4 along with the anticipated funding
distribution. Not listed in this estimate is the manpower (scientific, machine shops, construction bureau) which is separately indicated in Table 5 as far as the additionally required personnel is concerned. Altogether, there are 4 additional postdocs/technicians, 4
PhD Students and two diploma students needed to pursue the project until 2008.
A request of funding from the MPG for the USR of FLAIR after 2009 is detailed in
the present proposal in Table 6. Funding for advanced CSR instrumentation will be addressed separately after 2008 on the basis of successful operation of the CSR. It will
strongly depend on the success of both intermediate initiatives as described in Appendix
A.1, on the scientific goals as well as on the appointment strategy of the Institute in 2008.
39
Table 4: CSR investments and infrastructure
CSR
Estimated
costs
Element
k
Funding distribution
MPIK
MPG
k
k
Third
parties
k
Ring
outer (300 K) enclosure
inner (40 K) enclosure
inner (2 K) shield + He lines
outer (40/300 K) shields
vacuum instrumentation
He refrigerator
ion optics
bakeout system
beam diagnostic
electron cooler
radio-frequency
liquid nitrogen system
supports etc.
power supplies
sum
352
474
500
25
367
750
20
50
50
720
25
20
10
72
3,435
25
20
10
12
212
sum
10
5
15
5
35
35
sum
50
20
10
70
150
150
100
100
300
100
400
300 1
100
400
92
260
474
500
260
750
107
340
380
60
1,502
1,721
1,502
1,721
25
20
50
50
Injector
source
beam line
vacuum
ion optics
Control
temperature
vacuum
liquid nitrogen
ion optics
Infrastructure
Exp. Instrum.
Reaction microscope
Imaging detectors
sum
4,120
Sum
1
Startup funds (Ullrich)
40
897
Table 5: CSR additional postdocs/technicians, PhD and diploma students needed.
Position
GSI
1
1
MPIK
3
1
4
2
1
1
2
2
Cost/year /k
360
195
120
45
Sum / k
1,080
585 1
360
135
Postdoc
Technician
PhD Student
Diploma
1
including 90 k
MPG
Third
parties
Total
2
Table 6: Estimated investment costs for the USR
USR
Element
Estimated costs
Total / k
Ring
vacuum chambers
vacuum equipment
ion optics
bakeout system
beam diagnostics
electron cooler
radio-frequency
supports etc.
power supplies
control system
400
300
80
300
50
500
25
50
72
140
1,917
Sum
4.2 Requested Funding
The cost estimate for the CSR initiative, including additional personnel, is 5.2 Mio three years starting from 2005. A summary (investment plus additional personnel) of
funds requested from the different sources is listed below:
MPIK institutional funding
1.48 Mio
MPG central funds
1.51 Mio
Third party sources
2.10 Mio
GSI
0.14 Mio
Starting to secure the third-party contribution, a pre-proposal to the NEST (New Emerging Science and Technology) funding activity of the European Union has been launched
in April 2004 by the CSR collaboration (A. Wolf, MPI-K, D. Zajfman, Rehovot, X. Urbain, Louvain-la-Neuve). In case of positive pre-evaluation, the final proposal can be
submitted in September 2004 such that a definite answer can be expected by the begin41
ning of 2005, the anticipated official starting point of the project. As the average funding
probability for EU-NEST proposals is as low as 7%, we are presently investigating additional possibilities to apply for substantial third party funding in order to secure the
unique scientific program made possible by the CSR concept.
Since the USR represents a key-component of FLAIR, a project shouldered by a large
collaboration, backed by GSI through a positive evaluation of the Letter of Intent, providing a long-term perspective (starting in 2011) and considerable international visibility for
the MPG, our funding strategy should be such that the success of the FLAIR project is
secured with highest priority. Independent of further third party funding for the CSR we
believe to be able to secure the required developments for the FLAIR project on the basis
of the central MPG and MPI-K institutional funds alone.
Therefore, within the present proposal we apply for central MPG funds of 1.51 Mio
over three years to realize the CSR project.
In case of a successful commissioning of the CSR in 2008 and the demonstration of key
techniques necessary for the USR, we would technologically be able to realize the USR
as a key component of the FLAIR collaboration in 2009/10. Since FLAIR has to convincingly demonstrate a funding strategy until October 2004, the conditional approval of 1.2
Mio. of central MPG funds (linked to the demonstration of decisive technologies at the
CSR until 2008) would be of paramount importance for the successful promotion of
FLAIR. We therefore apply, within the present proposal, in addition for central MPG
funds of 1.2 Mio. to realise in 2009/10 the USR project as our contribution to the
FLAIR collaboration, under the condition that the key technologies mentioned above
have been successfully demonstrated at the CSR. At a total cost estimate for the USR of
1.92 Mio. , we expect the missing 0.8 Mio. to be covered either through third-party
funds by the collaboration partners or to some extent through MPI-K institutional contributions.
42
5 Organizational Structure, Timetable and Milestones
5.1 Organizational Structure
As illustrated in Figure 10, Daniel Zajfman from the Weizmann Insitut in Rehovot, Israel,
will be the scientific leader of the CSR project. He is external scientific member of the
Institute and will take over the responsibility of the division of Dirk Schwalm after his
retirement in early 2005 until 2008, when a successor will be appointed. Andreas Wolf
will substitute him and, at the same time, will be the project coordinator. Joachim Ullrich
is responsible for the long-term USR project which is anticipated to start in 2011 and will
presumably proceed for about 10 years, well within his term of office until 2021.
The technical project leader will be Robert von Hahn, an experienced accelerator scientist who is presently responsible for the MPI-K post-accelerator as well as for the highcurrent injector. Von Hahn has successfully managed several projects of similar size, like
the high-current injector project at the MPI-K or the MPI-K contribution to the REXISOLDE project at CERN. Manfred Grieser will be his substitute and, at the same time,
will be responsible for the CSR/USR ring design, their lattice structure and the layout of
the ion optical elements. He has extended experience in ring design and operation, was
Figure 10: Organizational structure and management of the CSR/USR project.
43
involved into the TSR development since its beginning and presently is responsible for
the TSR operation. Carsten Welsch, an experienced postdoc supported by GSI, will assist
him and, moreover, is responsible for the USR design and the preparation of the USR
contribution of the FLAIR technical report due by the end of 2004.
Andreas Wolf and Dimitri Orlov are responsible for the electron cooler. They both
have long-time experience in cooler design, assembling and operation as becomes obvious from the fact that they have successfully implemented the new electron target into the
TSR. C. D. Schröter has successfully designed and commissioned the new TESLA-FEL
ultra-high vacuum, cryogenic reaction microscope operating at a 10-13 mbar and, thus, is
the ideal person for the design, assembling and test of the CSR/USR reaction microscope.
The unique merged beams arrangement between stored molecular ions and a fast neutral atomic beam in the CSR will be realized under the responsibility of Xavier Urbain,
UCL (Louvain-la-Neuve). He is a world expert in merged beams ion-atom collision studies employing low collision energies and fragment imaging [F. Brouillard and X. Urbain,
Physica Scripta T96, 86 (2002)], and contributes expertise regarding the production of
fast atomic beams by laser-photodetachment of negative ions.
5.2 Timetable
A detailed time schedule concerning the CSR project is presented in Table 7. In order to
realize this project until the end of 2007, intense work started already with the formation
of a project working group in November 2003 after the positive response by our longterm steering commission, the “Stammkommission”, on the Institute’s intermediate strategy plans. Moreover, the scientific case has been worked out in detail at the beginning of
2004, the FLAIR Letter of Intent was submitted, a contribution to the SPARC Letter of
Intent was finished and two proposals for funding, the present one as well as the EU
NEST proposal, were launched.
In 2004, a major amount of work will be necessary concerning the mechanical design
of the ring, including the vacuum, cryogenic and electron-cooling systems, and preparations of the control system. Moreover, the setup of a He refrigerator system and a prototype extremely high vacuum (XHV) chamber with cold inner walls should be started,
which would allow pressures <10-13 mbar to be measured via the ion lifetime in a small
ion trap (denoted as “trap” in Table 7). The design and conceptual work urgently required
in 2004 has been secured by redirecting some of the permanent staff members’ activities
in the division of Dirk Schwalm and the division of Joachim Ullrich, by support from
GSI (postdoc) and by the Weizmann Institute (Daniel Zajfman).
At the beginning of 2005, provided that funding is secured, we will then be in the
situation to immediately enter the production phase for deflectors as well as quadrupoles
and to start with the assembly of the prototype. In parallel, the design of the injection
beam line and the beam diagnostics will be started.
44
Table 7: CSR project schedule
2004
2005
2006
2007
2008
Design Ring
design 1/4 of the ring for 2 concepts
decision for concept
production deflectors and quads
design whole ring
design prototype
assembly prototype
design trap
assembly trap
measurement trap
measurement prototype
Ecool
design ecool
ordering ecool
assembly ecool
tests ecool
mounting ecool
Mounting ring
ordering vacuum chambers
assembly vacuum chambers
ordering He refrigerator system
delivery and assembly He refrigerator
assembly ring
Platform and ion source
design beamline
assembly beamline
assembly ion source
Infrastructure
water cooling
pressurized air
ventilation/air cooling
crane
dismantling experiments
dismantling concrete blocks
available
available
Reaction microscope
Control system
Beam diagnostics
In the middle of 2005 the intense phase for the design of the electron cooler will begin, measurements on the prototype chamber will be performed, all parts of the ring will
be ordered, the injection beam-line will be assembled and the infrastructure will be provided. In parallel, design work on the reaction microscope will be initiated which is
probably straightforward since a similar microscope has already been built for in-ring experiments at the ESR at GSI and will presumably have been commissioned until then.
Moreover, another microscope for the TESLA-FEL using cryogenic techniques to
achieve a 10-13 mbar vacuum has been commissioned already in 2003.
End of 2005 to beginning of 2006 we plan to start ordering parts for the electron
cooler and of the reaction microscope and begin the assembly of ring vacuum chambers.
At the same time the ion source of the injector will be assembled. Middle of 2006 we expect to start assembling the ring vacuum chambers and the reaction microscope.
By the beginning of 2007 the assembly of the ring as a whole will be started. The
electron cooler will be mounted and tested, while the reaction microscope will be tested
45
separately using highly charged ions from the EBIT. Around the middle of 2007 we plan
to implement the electron cooler, the reaction microscope and all diagnostics meanwhile
prepared in parallel, to implement the fully developed control system and to start commissioning the machine by the end of 2007.
A milestone experiment as described below is planned to be performed in 2008 in
parallel to ring commissioning and optimization experiments. In parallel to the work described, the development of the atomic beam line will proceed, starting immediately, at
the collaborators in Louvain-la-Neuve, to be available for implementation at the CSR in
2008.
5.2 Technical Milestones and Milestone Experiments
The proposal contains a number of technical specifications which have never been realized in a storage ring and are at the cutting edge of present technology, most of them having been extrapolated from existing devices (traps, high-energy storage rings). Thus, the
decisive milestones to be achieved by the end of 2007 and in the course of 2008 are
mainly of technical nature and it is far from obvious that they all can in fact be reached.
Reaching these goals, listed below for 2007, would be a major break-through by itself on
the international scene. Nevertheless, as detailed below, we expect to perform already a
first experiment on photodissociation of a small molecule near threshold, which, at the
same time, will demonstrate the benefits of our approach for the physics with molecular
ions.
Technical Milestones
By the end of 2007 the following goals shall be reached:
•
The CSR is completely assembled with a simple ion source, electron cooler, reaction
microscope, simple imaging detector system and beam diagnostics implemented
•
Cryogenic cooling works and a temperature at the inner wall of 20 K can be reached
•
Base pressure is around 10–12 mbar
•
Electron cooling is demonstrated at 300 keV
•
Supersonic in-ring gas jet works and recoil-ion momentum imaging is demonstrated
•
A 10 ns bunch structure is demonstrated
By the end of 2008 the following goals shall be reached:
•
Base pressure is 5⋅10-13 mbar
•
Temperature at the inner wall is 10 K
•
Electron cooling is demonstrated at 20 keV
•
A <3 ns bunch structure is demonstrated
46
•
The EBIT is connected to the ring and storage of highly charged ions is demonstrated
•
The full projectile imaging system is implemented
•
Electron imaging is implemented at the reaction microscope
Rotational Cooling of Molecular Ions
In order to demonstrate the attainment of cryogenic rotational temperatures for molecular
ions in the CSR it is intended to perform a first experiment on near-threshold photodissociation of CH+ once the CSR can be operated at ~10 K, from which the rotational temperature of these ions can be extracted. A similar experiment has been performed in the
TSR (see Ref. [11] of Sec. 2) where we demonstrated that we can cool down to 300 K.
Rotational cooling time to 300 K was about 20 s, the TSR pressure was 3 x 10-11 mbar,
the beam energy 7.2 MeV, and the lifetime about 10 s. The main question is the effect of
the residual gas collisions on the final rotational temperature. Thus, such an experiment
would allow obtaining a direct estimate of the final equilibrium temperature of the stored
molecular ions. In the CSR, at a beam energy of ~20 keV, and a pressure of 5 x 10-13
mbar we can expect a lifetime of ~2000 s. An estimation of the lifetime of the very last
rotational transition (J',J'' ) = (0,1) is 1620 s [Ornellas and Machado, J. Chem. Phys. 84
(1986) 1296], so that we can expect to be sensitive to low temperature and measure the
real rotational equilibrium temperature in the CSR. To perform such an experiment, a
CH+ beam needs to be stored in the ring, and a pulsed laser (300–330 nm), 10 ns, 1
mJ/pulse needs to be merged with the beam. In the photodissociation process CH+ + h
C+ + H either the C+ fragment will be detected after the electrostatic deflector using a
standard MCP detector, or the hydrogen fragment will be detected on an MCP detector
located on a zero-degree exit with a detector having a central hole to let the laser pass
through.
47
Appendices
A.1 Embedment into MPI-K “Future Scientific Directions”
In 1999, the Institute worked out a long-term strategy for its future scientific development
and mission. Following its strongly interdisciplinary philosophy, trying to make optimum
use of MPI-K key technologies, expertise and infrastructure and taking into account an
enforced reduction in personnel until 2007, it was proposed to concentrate future research
onto two interdisciplinary fields with high scientific potential. The two fields, each with
three divisions, are:
1. Many-Body Dynamics of Atoms and Molecules, for advancing fundamental issues pertinent to correlated many-body quantum systems.
2. Cross Roads of Particle Physics and Astrophysics, where experimental techniques of nuclear physics and particle physics and in-house theoretical developments in
astrophysics are used to investigate basic problems common to both fields.
Having received enthusiastic support from the Max-Planck-Society, the proposal was
immediately put into practice by installing two independent junior research groups in
2000, one in each field, and by appointing two new directors working in Many-Body Dynamics of Atoms and Molecules, one in experiment (2001) and one in theory (2004).
In early 2003, the Max-Planck-Society had to implement a major financial consolidation programme in response to external pressures. Concerning the MPI-K, along with
other cuts, the Institute will loose as a result one of its divisions and the reappointment of
one of the directors will be shifted from 2005 to 2008. As a reaction, the Institute developed a revised mid-term strategy until 2008 driven by the strong desire to ensure its mission as outlined in “Future Scientific Directions”, the urgent need to maintain and further
develop technical key-expertise at the Institute, and the demand to keep a competitive
status in emerging fields of research even under the pressure of consolidation. Within the
revised strategy two intermediate initiatives are proposed to be launched lasting until
2008.
The revised strategy received unrestricted consent by the long-term steering committee (“Stammkommission”) of the Institute during its meeting in fall 2003 as well as by
the strategy forum (“Perspektivenkommission”) of the Chemical-Physical-Technical Section (CPTS) of the MPG in January 2004. As outlined in the MPI-K Report to the Institute’s external Review Panel (Fachbeirat), the two initiatives are:
1. The “ββ
ββ-Initiative”
aiming to complete long-running solar neutrino experiments
ββ
48
(GNO) and the developing phase of novel activities (LENS), to await the further evolution of BOREXINO potentially starting data-taking in 2005, to confirm or reject the evidence from the Heidelberg-Moscow experiment on neutrino-less double-beta decay, to
explore the future perspectives of reactor neutrino experiments and, last but not least, to
trace out perspectives for and possibly initiate a large international collaboration for a
large next-generation double-beta and dark-matter experiment along the lines indicated
by the GENIUS proposal. The Institute holds the key-technology, namely its worldleadership in low-level technology, the essential prerequisite for all of these projects. Scientists as well as technicians keeping this expertise will be available until 2008.
2. The “CSR-Initiative” aiming to consequently advance ultra-cold molecular physics – having emerged from the unique expertise at the TSR – by designing and commissioning a dedicated next-generation low-energy (300 - 20 keV) electrostatic ring with
wall temperatures below 10 K, a background vacuum below 10-13 Torr, electron cooling,
and, in the further future, with an internal jet-target, an atomic beam and novel imaging
detectors. At the same time, the CSR (intended to be developed until 2008 at MPIK)
serves as test-facility for the “USR”, the Ultra-low energy Storage Ring, which will be
the central machine of the future “Facility for Low-energy Antiproton and Ion Research”
(FLAIR; http://www-linux.gsi.de/~flair) proposed recently by a large international collaboration at GSI under the generic guidance of MPI-K groups.
Both initiatives definitely explore innovative, cutting edge technologies of the next
generation, with considerable future potential in their fields, exploiting and developing
the unique MPI-K in-house expertise. If successful, they will bring MPI-K into worldleading positions in high-energy gamma astronomy, in double-beta and dark-matter research, in the physics of ultra-cold molecular ions, of highly-charged heavy atomic ions
and last, but not least, in a pole position for low-energy antiproton and antimatter physics
which is of considerable international visibility.
A.2 Existing Situation
Low-energy antiproton physics is currently done at the CERN Antiproton Decelerator
(AD). Since the intensity is limited to about 105 p /s and only pulsed extraction is available, the physics program so far is limited to the spectroscopy of antiprotonic atoms and
antihydrogen formation in charged particle traps. Furthermore, the AD’s output energy of
5 MeV is still significantly higher than the <100 keV energy best suited for this type of
experiments. An electrostatic storage ring as a true multi-user, multi-purpose facility,
where the intensities for in-ring experiments are several orders of magnitude higher and
different experiments can be accommodated at the same time, could clearly overcome
these limitations.
Within the FLAIR, an alternative concept for antiproton deceleration is pursued applying the HITRAP scheme. It uses a radiofrequency quadrupole structure (RFQ) for the
49
ion deceleration. This deceleration scheme does not provide phase space cooling below
the lower energy limit of the magnetic (LSR) machine of 300 keV. In contrast, electron
cooling down to energies of ~20 keV as planned for the USR will provide much lower
emittance and momentum spread for antiproton deceleration. Moreover, experiments with
colliding beams can be performed at much higher luminosities in a low-energy storage
ring than with conventional decelerated beams, making use the ion recirculation and the
continuous phase-space cooling in the ring.
Several experimental methods exist today to study low temperature (few Kelvin) reactions where molecular ions are involved. Among them, the low temperature RF traps [D.
Gerlich, Phys. Scripta T59, 256 (1995)] and cold beams produced by supersonic expansion (Ref. [9] of Sec. 2). These techniques have been successful in measuring some reactions such as atom exchange [W. J. Knott et al., J. Chem. Phys, 102, 214 (1995)] and
even merged beam experiments have been carried out (Ref. [9] of Sec. 2). However, there
are severe limitations in these methods. First, because of the low energies (~eV) the neutral product cannot be detected, and interaction studies with electrons at low relative energies (<eV) are basically impossible due to the difficulty to produce cold electron beams
of these energies. The detection techniques available for >keV ions as well as the possibility to have pure, mass selected species in a collision free environment is unique to the
proposed cryogenic storage ring technique.
The heavy ion storage ring technique based on magnetic structures has succeeded during the last 15 years to produce important insight into the structure and dynamic of simple, vibrationally cold molecular ions. However, only very light molecular ions can be
stored, this technique is generally limited to room temperature, and the stored molecular
ions, reaching equilibrium with the black body radiation of the ring walls, are still in a
Table A.1: Plans for and properties of 3rd generation electrostatic storage rings. T/Mode: Wall
temperature / method of cooling (LN: liquid nitrogen, CR: cryogenic), P: Pressure, EC: Electron
cooler, E: Energy of stored beam; RF: Radio-frequency cavity for deceleration and acceleration;
NB: Neutral beams of atoms or molecules in crossed or merged beams arrangements; IB: Ion
beam in merged beams arrangement, PI: Projectile imaging, TI: Target Imaging
Status
Mode
Aarhus
running
electric 80 K/LN
-
<10-11
static
-
KEK
running
electric
x
<5.10-11
static
Tokyo
comm.
electric 80 K/LN
-
<10-11
approved electric <10 K/CR
-
Stockholm
T/Mode EC p/mbar E/keV
300 K
RF NB
IB
PI
TI
-
-
-
-
-
-
-
-
-
static
-
x
-
-
-
<10-13
static
-
-
x
x
-
-11
static
-
-
-
x
x
proposed electric
300 K
-
<10
CSR/MPI-K
proposal electric
3 K/CR
x
<10-13
20-300
x
x
-
x
x
pbar-USR
proposal electric
300 K
x
~10-11
20-300
x
x
-
-
x
Frankfurt
50
broad distribution of rotational states. New electrostatic storage rings (see also Table
A.1), which can work at liquid nitrogen temperatures have been developed (and are in the
process of being built) which allow to reduce the influence of the black body radiation on
the stored molecules. However, none of these rings allows for a storage time (up to 1
hour) which is required to achieve full rotational cooling, and only one of them is
equipped with an electron merged beam device (electron cooler), which works at the required low energies but with a rather limited resolution. A double electrostatic storage
ring device, which will work at temperatures of ~15 Kelvin is under development in
Stockholm; this ring will allow neutralisation processes through collisions between positive and negative ions, but will not be equipped with an electron target and a neutral
merged beam set-up. In Germany an electrostatic ring operating at room temperature and
capable of storing singly charged ions with energies up to 50 keV has been proposed at
Frankfurt University and some initial development work has been performed. With the
study of the dynamics of complex organic matter, this machine aims at different open
questions and in particular is constructed under relaxed demands on the vacuum system,
envisaging shorter storage times, as well as under relaxed demands on the beam quality.
As a summary, the main features of existing, commissioned or proposed 3rd generation low-energy electrostatic storage rings are listed in Table A.1. Included is the present
proposal to build a cryogenically cooled Storage Ring, the CSR, at the MPI-K and the
Ultra-low energy Storage Ring USR at GSI.
51