EBIS review, BNL 27-28 January 005
Review
of the BNL
EBIS Pre-Injector Project
BNL, January 27-28, 2005
Rick Baartman, TRIUMF, Vancouver, Canada
Reinard Becker, University of Frankfurt/M, Germany (chair)
Masahiro Okamura, RIKEN, Saitama, Japan
Peter Ostrumov, ANL, USA
EBIS review, BNL 27-28 January 005
Summary
The present RHIC pre-injector, consisting of negative ion sources, two Tandems and a 860
m long transfer line to the booster synchrotron is becoming fragile, due to the age of the
tandems, which will run out of support by the manufacturer. A modern replacement should
avoid the long transfer line, provide similar or more ions and – most – important, need less
personal and investment for maintenance. It also has to serve the needs of the Nasa Space
Radiation Laboratory (NSRL), which runs experiments with light ions, extracted from the
booster.
The project of a new pre-injector, based on an electron beam ion source (EBIS), which has
been developed by the staff of BNL and presented to the review board during January 27-28,
2005, meets these requirements in an excellent way. The same ions, like Au32+ can be injected
with even higher intensity, and starting from highly charged positive ions makes it possible to
inject ions of noble gases and those of the first and second row of the periodic table, which are
not available as negative ones. All the ions, which have been available for the NSRL are also
available with the EBIS in a time sharing fashion, without conflict of serving RHIC. The
higher booster injection energy of 2 MeV/amu by the LINAC avoids beam losses and is a
significant improvement for RHIC performance as well as for the NSRL.
The pioneering and outstanding development of a high current EBIS at BNL – E. Beebe
and A. I. Pikin have been awarded with the brightness Prize of the International Ion Source
Conference 2003, Dubna - has resulted in an ion source for synchrotron injection which has
no alternative, not by ECR ion sources, nor by laser ion sources. ECR ion sources at present
are not able to provide the intensity of several mA nor the charge states, additionally by the
memory effect fast switching between ion species is impossible. Laser ion sources are
suffering from missing shot-to-shot reproducibility and cannot meet the fast pulsing
requirements.
The combination of the BNL high current EBIS with a short LINAC, consisting of a RFQ
and a short IH type rf accelerator, will provide for RHIC and the NSRL a versatile and
flexible injector, which combines state-of-the-art components, resulting in low maintenance
cost, and offering the possibility for later intensity upgrades.
The presentation of the project has impressed the review panel by its carefully worked out
details, especially for the EBIS. The LEBT will need some more design considerations, while
the RFQ and IH LINAC may be copied from similar existing ones at GSI and at CERN with
little changes. From the technical point of view the realization of this projects is very
promising and shows very little risk.
EBIS review, BNL 27-28 January 005
RECOMMENDATIONS
The review panel has discussed the presentations in detail and wants to give the following
recommendations, in order to promote the project.
EBIS:
The design of the LEBT, including the matching of extracted ion pulses to the RFQ,
requires measured beam parameters, such as the transverse emittance and the longitudinal
beam spread. These measurements are needed for different ion species and for different ion
currents, including highest possible peak currents.
Fast ion extraction by dropping the extraction barrier voltage and supplying the drift tubes
with interpolating voltages for an axial electrostatic gradient may introduce a too high
longitudinal energy spread, which will also couple to the transverse energy spread. A more
gentle extraction scheme by additionally lowering the drift tube potentials during the
extraction pulse my result in a minimum longitudinal energy spread.
LEBT:
More studies, including 3-D simulations are needed in the presently proposed layout of the
LEBT. Particularly, our recommendations are:

Create 3D computer model of the LEBT which starts from the extraction electrodes
and ends at the RFQ entrance. The model should include space charge of multicomponent ion beam. Also, image fields on various LEBT equipment can be
important.

Analyze beam dynamics and compare with beam measurements. Apply iterative
procedure to achieve complete agreement between simulations and measurements.

Once verified, the model can be applied for the design studies of the LEBT with
projected EBIS parameters.

Use the model to minimize emittance growth in the LEBT and define optimum
injection energy into the RFQ.

Provide emittance values, energy spread and intensities of all beam components at the
entrance of the RFQ for its appropriate design.
The transport of a 10 mA, 17 keV/u beam is not a simple matter, so design of the LEBT
should be developed experimentally on the existing test stand. In particular, the proposed
EBIS review, BNL 27-28 January 005
scheme of EBIS extraction in a straight line to the RFQ requires at least 80 cm of drift to
bypass the switch where the external beam is injected. This may be too long a drift for the
space charge dominated beam. A possible alternative is to bend the extracted beam. This
achieves 2 benefits: it allows selection of the desired charge state, and, if the dipole is doublyfocusing, controls the space charge effect. This deserves more studies.
The separation of unwanted charge states will require at least one bending magnet if the
energy spread of the beam is small and below ~1%. In the case of a larger energy spread an
achromatic bending system may be required. Elimination of unwanted ion beam components
will result in: a) reduced space charge in the LINAC; b) reduced rf power to compensate
beam loading both in the RFQ and IH-structure; c) most relevant data will be obtained from
beam diagnostics equipment along the LINAC and HEBT. This can significantly save beam
tuning time and beam matching to the booster.
LINAC:
The combination of RFQ and IH-structure has been proven as a reliable technology for
acceleration of heavy ions. Therefore the BNL choice of the new injector based on room
temperature RFQ and IH structure sounds entirely feasible. The difference of the BNL design
compared to similar existing systems is a higher pulsed beam current. A low level of beam
losses both in the RFQ and IH-structure will be desirable. It is well known that the
longitudinal acceptance of the IH structure designed for KONUS-type beam dynamics is
lower than for standard ion accelerators. Therefore, careful design of the whole chain of
injector sections such as LEBT, RFQ, MEBT and IH-structure is necessary to preserve both
beam emittances and intensity along the injector. For the final design verification it is
desirable to use simulation codes that include the simultaneous transport and acceleration of
multi-component ion beams. The projected beam losses of 10% in the RFQ and 15% in the
IH structure seem too high. By optimization of the beam dynamics these losses can be
significantly reduced without cost impact. It is worth to check the present beam performance
in the CERN lead injector LINAC-3 which is similar to the BNL proposed design. In an early
stage of this project some appreciable transverse emittance growth has been reported.
A permanent magnetic quadrupole at the end of the RFQ proposed in the current design
looks inappropriate with respect to fast switching of ions with different q/m. Several of the
last cells of the RFQ can be designed in order to optimize the beam matching with the MEBT.
The end regions of the RFQ should be designed to take into account fringe fields.
The existing IH-structures consist of electromagnetic triplets inside the resonator. It is
reasonable to perform analysis of integrated electric focusing and compare it with magnetic
focusing. There are several ways to apply rf focusing. One of them is replacing of
electromagnetic triplet by rf focusing section which can provide the same properties as a
focusing triplet. BNL can make a later decision depending on the progress in this field.