A Plea for Innovative Research in Accelerator Science
As discussed in its summary report to P5 (November 2, 2013), the Snowmass Accelerator
Capabilities study finds that the advance of accelerator science is being handicapped by
an excessive focus by the agencies on project-driven R&D. There is no longer much
“free energy” for innovation or for broad, foundational research such as accelerator
theory. The few exceptions to this trend are the areas of superconducting magnet
development and laser- and beam-plasma wakefield accelerator research, both of which
have become designated programs in their own right.
The dwindling opportunities for innovative, advanced accelerator research have deep
implications for the education of the next generation of accelerator physicists and
engineers.1 The present funding regime puts four of the historically top six producers of
U.S. accelerator PhDs in jeopardy of drastic reductions in their capacity to accept and
train students.2 Fortunately two of the
programs remain in good financial health.
We are gravely concerned that the
accelerator research program that emerges
from the P5 process in the present highly
constrained financial environment will
leave inadequate room for the kind of
innovative research that has largely been
driven by the OHEP Washington-managed,
university-based program in advanced
3
accelerator research (AARD) . The much larger general accelerator research (GARD)
program is ever more tightly focused on short- and mid-term efforts in support of
proposed laboratory projects or to improve services to the users of accelerator facilities.
Even the expanded “stewardship program” in OHEP appears to be directed more at
servicing technology transfer than revitalizing fundamental work in accelerator science
and technology. This paper describes the rationale for expanding mid-to-long-range
accelerator research plus strengthening university-based programs.
Examples of prior innovative R&D that have already made a large impact
The development of very high field superconducting (SC) magnets since 1982, built on a
combination of advanced materials R&D and advanced magnet development not related
to a specific project, has been particularly important in developing accelerator quality
magnets using Nb3Sn. Through its advanced work on Nb3Sn, the U.S. has provided an
enabling technology for the up-coming luminosity upgrades of the Large Hadron Collider
(LHC) at CERN.
A review of the present status of programs of accelerator education worldwide can be found at “Educating
and Training Accelerator Scientists and Technologists for Tomorrow,” W.A. Barletta, S. Chattopadhyay,
A. Seryi, Reviews of Accelerator Science and Technology, Vol. 5 (2012) 313–331.
2
U. Maryland, Cornell, Indiana U., Michigan State, UCLA, and Wisconsin account for more than 70% of
all U.S. PhDs in accelerator physics. The figure is from the reference of footnote 1.
3
The origins of that program are described in the Appendix.
1
1
Basic device physics related to RF superconductivity (SCRF) has made great strides,
enabling the CEBAF linac, the cold RF option for the International Linear Collider (ILC),
the ATLAS and FRIB heavy ion accelerators, and the possibility of energy recovery
linacs (ERL) for ultra-bright electron sources for both photon science and for electron-ion
colliders. As in the case of SC magnets, fundamental research into SCRF has been
substantially incremented by near-term development in support of specific projects.
Increasing beam brightness with photocathodes has made practical the development of
the free electron laser as the principal tool in 4th generation light sources, such as the Xray FEL. Likewise photocathode research is important for use in the proposed linear
collider and in recirculating linacs for nuclear physics. Similarly, high quality, high
average current ion sources are essential to future high intensity cyclotrons and linacs.
Accelerator theory has seen spectacular progress in areas of beam-related nonlinear
dynamics, electron beam interaction with short-wavelength coherent radiation, advanced
beam-cooling schemes, physics of space charge dominated beams, and collective beam
instabilities. Continuing advances in the methods for computational analysis4 of beam
dynamics, electromagnetic structures, collective effects such as the free electron laser,
and plasma dynamics now enable the “end-to-end beam” simulations of high-energy
colliders, storage rings, light sources and high intensity linear accelerators. Such
simulations are among the essential, highly cost-effective means of evaluating the
feasibility and practicality of any proposed accelerator-based project.
Future R&D directions
Although it is difficult to predict specific benefits of advanced accelerator research,
several directions promise considerable benefit. The development of SCRF cavities with
ultra-high Q can greatly reduce operating costs and therefore substantially extend the
utility of CW accelerators for nuclear physics and photon science. Advanced computer
modeling of beams including development of novel capabilities such as boosted-frame
simulation of coherent synchrotron radiation and scalable, spectral electromagnetic
solvers can enable the full simulation of future accelerators that will require exploiting
the computing power of exascale computers. Novel accelerator configurations, such nonlinear lattices5 in high intensity circular machines, have the potential to introduce a large
betatron tune spread to suppress instabilities and to mitigate the effects of space-charge
and magnetic field errors; both experimental and theoretical investigations are needed.
New engineering superconductors with capabilities beyond those of Nb3Sn may permit
compact, iron-free cyclotrons and affordable, energy frontier proton colliders with
energies higher than are possible with present engineering materials. These materials may
also make practical very high field, short-period, long undulators both as beam damping
systems and as radiation sources. Realizing the fullest potential of the free electron laser
for multi-dimensional spectroscopy and cine-imaging of molecular dynamics with
chemical specificity will need investigation into the means of extending full phasecoherence of the radiation to wavelength at least 10x shorter than 4 nm – the best
presently achieved performance. This list is very far from exhaustive.
4
Examples include symplectic integrators, integrators, micro-manipulations in beam phase space, and
precision calculation of electromagnetic modes in 3-D structures
5
S. Nagaitsev and V. Danilov, arxiv.org/pdf/1207.3813
2
What difference this makes to educating the next generation
Educating and training a new generation of accelerator physicists and technologists
requires that our field retain a vibrant level of intellectual excitement in an atmosphere of
inquiry unconstrained by the time-pressures of construction projects or the limitations
imposed by user-support programs. As it has in the past, broad foundational research
attracts the highest caliber of students, who become leaders of our field, and it also
produces science that can grow into focused programs in their own right. The early
efforts related to plasma-based accelerators, high-frequency accelerators, and muon
accelerators are examples of both of these benefits.
Of particular importance is the health of university programs in accelerator physics where
the predominant focus is on advanced accelerator research and where the majority of U.S.
accelerator physicists have been trained during the last twenty years. In addition,
increased opportunities for experimental study on-campus also attract high quality,
motivated undergraduates to our field. Restricting the funding of accelerator research to
work with immediate relevance to the high-energy physics is both shortsighted and
inconsistent with the long-range, accelerator stewardship mission of OHEP.
We are concerned that the Congressionally mandated stewardship program to bridge the
“Valley of Death” (that prevents the movement of accelerator technology from the
research laboratories to the marketplace) is being coupled with broad spectrum,
innovative accelerator research in a way that can only be detrimental to advanced
accelerator R&D, particularly in the universities. Such a short-term “stewardship”
program requires a very different management and funding strategy from advanced
accelerator research that provides broad and fundamental benefit for accelerator-based
science. The former is of necessity a highly focused, national-laboratory-centered
activity. The latter is strongly university-centric, comprising both theoretical studies and
experiments either on campus or at national laboratory based user facilities, as
appropriate, and is an essential part of the education of future accelerator scientists and
engineers. While both types of research are useful and needed, it is essential that the
university-based advanced research activities not fall victim to the new short- to mediumterm stewardship activity.
Recently, the NSF announced a new `initiative focused only on transformative
accelerator research conducted at universities. That program also aims to broaden the
opportunities for new, tenure-track faculty and post-doctoral researchers who will pursue
accelerator science. Although the program is starting at modest funding levels, the strong
interest expressed by the NSF in initiating this program is very encouraging.
Recommendation
We urge P5 to recommend strengthening the NSF initiative and to recommend a similar
initiative in the OHEP within an expanded advanced accelerator research program with a
strong university-based component.
Signed:
William A. Barletta
Ilan Ben-Zvi
Alex Chao
Kwang-je Kim
Claudio Pellegrini
Andrew M. Sessler
Maury Tigner
Jonathan Wurtele
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Appendix: Origins and Status of the OHEP AARD program
In 1982 the DOE Office of High Energy Physics (OHEP) initiated a formal program in
advanced accelerator R&D that was heavily university-focused. The basis for this new
direction was the June, 1980, report of a HEPAP subpanel charged with examining the
state of OHEP funded accelerator science and technology from the perspective of new
facilities that it was anticipated would be needed to maintain a robust future research
program in support of elementary particle physics. The 50-year history of accelerator
development up to 1980 was notable for dramatic improvements in cost per unit of beam
energy. However, known technologies appeared to have been pushed to practical limits,
implying an unacceptable escalation of cost for future facilities. The program of
advanced accelerator research (AARD) which begun in 19826, has proved to be quite
successful. This appendix describes the breadth to which the program evolved and
provides examples of fundamental advances and their impact originating from that
program.
The 1980 HEPAP Subpanel – the Charge
In 1979 the High Energy Physics Advisory Panel (HEPAP) to the U.S. DOE’s Office of
Energy Research (now the Office of Science) charged a subpanel, chaired by Professor
Maury Tigner of Cornell University, to review the overall scope and quality of the
accelerator R&D effort in the U.S. High Energy Physics Program. The review was not
limited to just the accelerator R&D supported by the DOE Office of High Energy Physics
and was specifically to include:
1. “An examination of the existing accelerator and detector R&D effort in the
U.S.
2. “A comparison of the U.S. effort with efforts abroad.
3. “Specific recommendations concerning the U.S. accelerator R&D effort, with
particular emphasis on:
(a) breadth and depth of the R&D effort;
(b) balance among short-term, mid-term and long-term R&D;
(c) priorities within the U.S. accelerator R&D effort;
(d) appropriate funding levels.”
The charge stipulated that the review was to be broad, and general in approach, and not
address specific concerns about potential construction projects. More specifically, the
review was to address the issue of whether the then planned program level-of-effort and
content for accelerator R&D would provide an adequate scientific and technological base
for improvement of the then existing accelerators and the design of future accelerators
that might be needed by the high-energy physics research community.
HEPAP had requested a similar examination of the status of both accelerator and detector
R&D, but the committee decided that to include detector R&D made for a task that could
not be done in a reasonable time. The subpanel further decided that it was most
6
This was the first year in which significant funds could be made available in the two-year budget cycle
following the subpanel’s recommendation.
4
important to focus on questions connected with the long-term future of high-energy
physics research and so accordingly should devote the bulk of the attention to the longrange, advanced component of accelerator R&D. Both HEPAP and DOE accepted these
proposed modifications to the original charge.
The 1980 HEPAP Subpanel – the Recommendations
The Tigner sub-panel report was delivered to HEPAP in June of 1980 and was forwarded
almost immediately to the DOE by HEPAP Chairman Sidney Drell with the unanimous
endorsement of the panel.
The report included the following specific recommendations:
1. OHEP should significantly increase its financial support for advanced accelerator
R&D (AARD) from the then-current level of about 1% of current operating-levels [note
that this excludes capital equipment and construction funds] to about 4%.
2. The subpanel identified certain technical areas to be emphasized, noting that the list
was not presumed to be complete, exclusive, or indicative of relative priorities:
a) Development of very high field superconducting magnets, to be carried out as
a collaboration among the major laboratories having relevant capability;
b) Development of highly efficient and reliable liquid helium refrigeration
systems;
c) Theoretical and experimental exploration of the limits of acceleration
gradients in microwave linacs and of the related peak power limits in the S- to
X-band regions;
d) Basic physics and device development in superconducting RF accelerators;
e) Theoretical and experimental studies of basic accelerator phenomenon, for
example, the beam-beam interaction and other performance limiting
phenomena;
f) Search for and preliminary development of new acceleration schemes with
high performance-potential, such as laser accelerators or other high peak
power devices;
g) New techniques and devices for manipulating very high power and /or very
high peak energy beams;
h) The general problem of increasing the brightness of particle beams with
emphasis on [phase space] cooling of high-energy beams.
i) Development of new beam diagnostic techniques and devices.
3. and 4. Laboratory and university managements, aided by the agencies, should take
specific measures to increase participation of particle theorists and
experimenters in accelerator R&D and to facilitate cross-fertilization from
other fields such as plasma physics, lasers and materials science and other
accelerator-related activities.
Recommendations 3 and 4 were included lieu of a clear recommendation to establish a
formal, university-based program in accelerator physics, because some committee
members thought such a specific recommendation unnecessary. Nevertheless, the idea
was planted.
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Response to the Recommendations
The following snapshot addresses the recommendations item by item as they were listed
above; some important areas of progress may have been omitted:
1. Beginning in 1982, the OHEP increased the funding for AARD above the 1% of
operating level; the highest that it ever reached was about 3.5% of operating. This
number is somewhat debatable in very recent years because not all enumerators call the
same work AARD, but it is roughly true.
2.a) The development of very high field superconducting magnets since 1982 has been an
outstanding success story of the OHEP accelerator R&D program. It built on a
combination of advanced materials research and advanced magnet development not
related to a specific project – this characteristic has been particularly important in
developing accelerator quality magnets using Nb3Sn and the new high-temperature
superconductors. It also benefitted from the huge amount of project R&D needed for the
SSC and the LHC. Through its advanced work on Nb3Sn, the U.S. has provided an
enabling technology for the upgrades of the LHC accelerator.7
2.b) Helium refrigeration was not developed in AARD, because the progress in the
refrigeration industry, based on market-pull, has met the needs of high-energy physics.
2.c) The push to very high gradients in warm S- to X-band RF structures has produced
accelerator structures capable of reaching 100 MV/m reliably. This area – adopted by the
efforts aimed at the warm option for the international linear collider (ILC) – was mostly
short- to mid-term R&D needed for a proposed construction project. High gradient R&D
continues in the U.S. National High Gradient collaboration and is essential to the CLIC
linear collider concept.
2.d) The area of fundamental device development for RF superconductivity has made
great progress, driven by the demands of the cold-RF option adopted in 2003 for the ILC.
There has been particularly good progress on suppressing multipactoring and impuritybased voltage breakdown through better understanding of the surface physics and
improved structure geometries. As with the warm-RF structures, much of the work has
been short- to mid-term R&D in support of a proposed project.
2.e) Accelerator theory has seen spectacular progress. Here, the university programs
have made crucial contributions. The work was also motivated by issues uncovered by
the designs of the SSC and LHC. Areas developed after 1980 include beam-related
nonlinear dynamics, advanced beam-cooling schemes, deeper understanding of the
physics of space charge dominated beams (partially driven by issues related to heavy ion
fusion), increased understanding of beam instabilities, and the analysis and modeling of
beam-loading effects.
7
The development of practical superconducting materials, an activity involving materials scientists,
magnet builders and the SC wire industry, was greatly facilitated by the initiation of Professor David
Larbalestier, then at the U. of Wisconsin, of the Low Temperature Superconducting Wire workshop
(LTSW). Run in a Gordon Conference format, it annually brings together the stakeholders in improved
superconducting wire and the manufacture of new and advanced materials. This forum has contributed
most strongly to the remarkable increase in critical current density of NbTi, Nb 3Sn and most high
temperature superconductors. It remains an active forum.
6
A most important part of the theory effort, not mentioned in the Tigner sub-panel
recommendations, but now almost a discipline of its own, is advanced methods for beam
simulation. This work addresses the computational study of advanced beam dynamics,
plasma accelerators and space charge: it includes nearly all aspects of the field of
accelerator physics. End-to-end beam simulations are now are now possible and are
considered essential to any proposed accelerator project.
2.f) The dedicated search for new acceleration schemes was initiated in the laser
accelerator workshop at Los Alamos (1982). OHEP now supports active facilities in
laser-driven plasma acceleration (an international effort led by U.S. research at LBNL’s
BELLA facility, U. of Texas at Austin, U. of Maryland, the Naval research Laboratory
and BNL’s ATF) and the particle-driven wakefield acceleration (led in the U.S. by work
at the SLAC FACET facility and which includes research in dielectric wakefield
acceleration.) The Muon Accelerator Program – a formal national collaboration led by
Fermilab8 – is also a product of the U.S. AARD effort.
2.g) Development in techniques for handling high microwave power was driven by the
anticipated needs of the ILC designs, particularly the warm-RF option; most of this effort
is short- to mid-term R&D.
There are few new means for manipulating very high average power beams. Considerable
work on high power targets has been done for high intensity muon and neutrino beams
and for spallation neutron sources.
2.h) The general problem of increasing beam brightness has received significant
development. The application of stochastic cooling to proton-antiproton colliders
resulted in a Nobel prize for its inventor, S. van der Meer, and enabled the successful
operation of the Tevatron. The success of high-energy electron cooling and the
development of ionization cooling, essential for the muon collider and storage rings, are
later successful efforts of advanced accelerator R&D. Ionization cooling remains in the
proof-of-principal stage.
Radiation cooling was well known and understood in 1980, but its technology reached a
new level of sophistication through the introduction of undulators and wigglers into
electron storage rings. An outstanding AARD result has been the development of free
electron laser concepts (and in particular the SASE concept) as the principal tool in 4th
generation light sources, such as the X-ray FEL – LCLS at SLAC.
Increasing beam brightness with photocathodes in RF guns has made practical the
development of the free electron laser as the principal tool in 4th generation light sources,
such as the X-ray FEL. Similarly, very bright, photocathode guns are important for use
in the proposed linear collider and in recirculating linacs for nuclear physics.
8
A workshop dedicated to communication among those working in advanced accelerator concepts (AAC)
was initiated on a biennial basis. The AAC workshops continue to draw wide attendance and interest. The
published proceedings of the AAC workshops (APS Conference series) provide a detailed history of
technical progress in advanced accelerator concepts and a wealth of information about acceleration
schemes looked at but judged to be not promising.
7
2.i) The development of beam diagnostic techniques and devices for conventional
accelerators is largely an engineering effort and, with the exception of the development of
transition radiation methods, has not seen radically new technology breakthroughs.
Although beam instrumentation has advanced in accuracy and cost-effectiveness, it is
largely driven by needs centering on existing or planned facilities and so tends to be
short- to mid- term. In contrast, instrumentation supporting plasma accelerators has been
developed as an essential aspect of the long-range plasma acceleration research.
Contributions of Universities
A few examples illustrate where the university program in accelerator physics has made
contributions that might not have occurred or at least taken much longer to happen.
An early university grant was made in 1983 to Professor Alex Dragt at the University of
Maryland to study the application of Lie Algebras to nonlinear-dynamics modeling in
accelerators. A notable long-term result was the development of the IMPACT code for
simulation of space-charge-dominated beams in linacs. The most dramatic impact of
Professor Dragt’s work occurred in a more fundamental area. In the early 80’s the many
codes written to simulate the life cycle of beams in storage rings showed the beams
blowing up in ~20,000 turns – contrary to observation. Dragt showed the necessity of
preserving the symplectic condition9 when doing numerical beam simulations. Indeed,
some commonly-used numerical integrators, including the Runge-Kutta technique, do not
preserve phase space area. In the next 6 months almost every long-term beam simulation
in the U.S. was rewritten.
The work of materials researcher Professor David Larbalestier, funded in 1982, addressed
the issue of critical current density limits in the commonly used, ductile superconductor,
NbTi. In the form of Rutherford cable, NbTi was the backbone of the magnet
development for the Fermilab Energy Saver-Doubler (the Tevatron), the first large-scale
use of SC magnets in an accelerator/storage ring. The material then being produced was
limited to a critical current density of ~ 1400 amps/mm2. Larbalestier set out improve
this parameter in manufactured wire. Through the collaboration of materials scientists,
magnet builders and industry in the Low Temperature Superconducting Wire (LTSW)
workshop, the critical current density of the wire available to the SSC reached ~3600
amps/mm2, well above the 2800 amps/mm2 specified by the SSC Central Design Group.
Using the same strategy, Larbalestier and his colleagues, tackled the problem of critical
current density in Nb3Sn with similar success, making long Nb3Sn magnets the baseline
candidate for the LHC luminosity upgrade.
Following the 1982 Laser Accelerator Workshop, the first demonstration of particledriven plasma wave acceleration was done by a university of Wisconsin graduate student,
James Rosenzweig, working with a senior accelerator physicist, Jim Simpson, on an old
21 MeV electron linac at ANL. At UCLA Professor Chan Joshi began assembling the
experiment that first demonstrated laser-plasma acceleration with a two-frequency laser
in which the difference of the two light frequencies equaled the plasma frequency of the
acceleration medium. This early work work laid the foundation for the research at the
SLAC FACET facility and for the BELLA facility at LBNL under the direction of Wim
9
In this context symplectic means area-preserving in phase space.
8
Leemans, one of Professor Joshi’s former students. The role of the national laboratories
in this research might never have begun except for the success of the university programs
in demonstrating the feasibility of the concepts.
NSF also funds accelerator research in support of accelerator facilities at Cornell,
Michigan State and Indiana University. MSU pioneered compact, superconducting
cyclotrons for nuclear physics. The Cornell’s contributions to beam dynamics and its
SCRF work certainly fit the definition of AARD. In fact, the Cornell SCRF work was the
backbone of the U.S. effort until the heavy-ion accelerator at ANL and CEBAF
construction projects greatly expanded the effort. The Cornell program continues to be a
world leader in exploring the optimization of SCRF for broad application in accelerators
of all purposes.
University research in accelerator physics makes an essential contribution to training
PhD’s in accelerator science. At the time of the Tigner sub-panel, the majority of
accelerator physicists came into the field from other disciplines, mostly high-energy
particle physics and a few from plasma physics. Since the start of the OHEP-funded
university program in accelerator science began in 1982, most of the new, Ph.D.
accelerator physicists have been university-trained, although many do thesis research at
national laboratories, but not on short-term problems. Now four of the six largest
producers of U.S. accelerator-Ph.D.s appear to be threatened by current funding practices
and strategies in the OHEP program. Moreover, no other DOE Office of Science program
funds advanced accelerator R&D. The offices of NP, BES, and FES limit their R&D
funding to near-term research that has direct bearing on project improvement and
construction. That restriction is based on the Office of Science policy that OHEP is the
steward of accelerator science and technology.
Summary
The 1980 HEPAP sub-panel, chaired by Tigner, provided a broad and encompassing
template for the management of advanced accelerator research that was largely followed
until approximately 2010. It gave full recognition to the importance of AARD in
advancing the physics of particle beams and in all of the fundamental enabling technical
areas for accelerator design and construction: beam generation, magnets, accelerating
structures and systems, accelerator theory, and instrumentation. That work has proved to
be highly beneficial both in support of proposed or actual construction projects and as a
foundation for further advances in accelerator science and technology.
The subpanel also facilitated the initiation of a strong, university-based advanced
accelerator R&D effort that has contributed richly to advancing the field of accelerator
science, to establishing new faculty lines in accelerator science, and to the training of
PhD’s. During the past few years that successful program direction, particularly the
university component, appears to have been lost. Restoring that component will provide
a strong return on investment for the future of US science in its many aspects.
9