This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Adney, J.

Anderson, D.R.

Ball, M.S.

Caussyn, D.

Ellison, M.

Ellison, T.J.P.

Ferry, J.

Friesel, D.L.

Hamilton, B.

Lee, S.Y.

Nagaitsev, S.

Reistad, D.

Schwandt, P.

Sedlacek, M.

Sundquist, M.

English

IUScholarWorks

DESIGN OF A 6 MeV ELECTRON COOLING SYSTEM FOR THE 
SSC MEDIUM ENERGY BOOSTER 
Dan R. Anderson, Mark S. Ball, David Caussyn, Michael J. Ellison, 
Tim J.P. Ellison, Dennis L. Friesel, Brett Hamilton, S.Y. Lee, 
Sergei Nagaitsev, and Peter Schwandt 
Indiana University Cyclotron Facility, Bloomington, Indiana 47408 
Jim Adney, Jim Ferry, and Mark Sundquist 
T h e  National Electrostatics Corporation, Midd leton, Wisconsin  55562- 091 0 
Dag Reistad 
T h e  Svedberg Laboratory, Uppsala, Sweden 
Miro Sedlacek 
T h e  Royal Inst i tute of Technology, Stockholm, Sweden 
A 6 MeV-2 A electron cooling system is being designed for the SSC Medium Energy 
Booster (MEB). This system has the potential of decreasing the beam emittance by a factor 
of 2 in less than 20 s, consequently increasing the initial SSC luminosity by the same factor. 
Alternately, the required number of particles per bunch (and therefore synchrotron radi- 
ation and required cryogenic cooling power) can be reduced while keeping the luminosity 
const ant; or this system can compensate for unexpected emitt ance-increasing effects while 
relaxing the closed orbit error and dynamic aperture requirements in the following ma- 
chines. This report summarizes the system design, and the status of the proof-of-principle 
electron beam recirculation tests to be carried out at the National Electrostatics Corp. 
The lower limit for the beam emittance in the SSC is determined by the space charge 
tune shift, A&,, ~s 0.33, in the Low Energy Booster (LEB) at injection (1.46 GeV/c). 
After acceleration to 12 GeV/c, however, A&,, is reduced to 0.02 in the LEB before 
extraction and is 0.065 at injection in the MEB. We estimate that the beam emittance can 
be reduced by greater than a factor of 3 in the MEB at injection using an electron cooling 
system.' The specifications for this system are summarized in Table I. 
The cooling rate (emittance e-folding time), using values from Table I, is estimated2 
to be less than 30 s, increasing the SSC fill (fill + ramp) time by less than 50% (30%). 
Proton beam emittances, or electron beam temperatures a factor of three higher than 
estimated will still allow cooling times less than 60 s. Enhancements in the cooling rate 
due to magnetized cooling effects are not expected. Intrabeam scattering3 does affect the 
equilibrium longitudinal emittance, but has no effect on the transverse beam emittance. 
The projected proton beam emittance as a function of time is shown in Fig. 1. 
TABLE I 
Electron Beam Current 
Cathode Radius 
Electron Beam Radius 
Cooling Region Length 
Coulomb Logarithm 
Cool Reg. Beta Functions 100 m 
Cathode Temperature TeI 0.12 eV/k 
roton rms normalized emittance EN 0.7 ?r pm 
0 2 0 4 0 6 0 8 0 100 120 
T i m e ,  s 
Figure 1. The solid (dashed) line shows the projected proton beam emittance in the MEB 
as a function of time for a ring pressure of 1 x lo-' (1 x Ton N2-equivalent. 
Layout 
Figure 2 is an overall view of the proposed electron cooling system in the MEB.* This 
configuration was chosen because it provides the shortest possible path (and thus highest 
beam quality) from the cathode to the cooling section. The electron beam is generated by 
a dispenser cathode located in the terminal of a 6 MV Pelletron accelerator. Two solenoids 
following the first 90' bend produce the required beam size (an increase from r = 3.2 mm 
at the cathode to r = 4.5 mm in the cooling region) and convergence (20 ,wad) at the 
beginning of the cooling straight section. Following this straight section, the beam is then 
transported back to the 6 MV terminal and collected. 
MEB Electron Cooling 
System Layout  
(elevation view) 
I 
\+------ Cooling Straight 
Figure 2. Electron cooling system layout in the MEB. 
128 
The electron and proton beams must be aligned with a tolerance of less than 40 prad 
to preclude "effective" temperatures in excess of the cathode temperature; the electron 
beam divergence must also be less than this value. Figure 3 shows a layout of the electron 
beam optics, alignment, vacuum and diagnostic systems in the electron cooling region. 
The electron confinement system5 consists of weak solenoids (focal length of 95 m) located 
every 2 m. Each solenoid provides just enough focussing to compensate the electron beam 
expansion due to its space charge. An error in the beam size or divergence causes the beam 
envelope to modulate about the equilibrium size with a wavelength of 65 m (the plasma 
wavelength). Such a modulation can be detected using single-pass flying wires. The 
electron and proton beams are aligned using nonintercept ing beam posit ion monitors with 
a resolution of 10 pm. The p-met a1 shielding attenuates the magnetic fields from the earth 
and other stray sources. The degree of space charge neutralization must be kept below 
0.06% to prevent "pinching." This is accomplished by a design pressure of 1 x lo-' Torr, 
provided by nonevaporable getter pumps, and ion clearing electrodes located every 2 m. 
The gradient electrodes are used to accelerate ions to the clearing electrode system. We 
plan on assessing the effect of this beamline on the MEB impedance budget in the future 
using the Hewlet t-Packard program High Frequency Structure Simulator. 
MEB Modifications 
An alternate scheme for the MEB long straight section proton beam optics design has 
been ~ r e a t e d . ~  This scheme leaves the basic ring FODO structure unaltered and preserves 
the dispersion suppression while reducing the number of quadrupoles in the insertion by 2, 
increasing the magnet free length from 20 to 45 m, and increasing the beta functions from 
/ 1 j . L  m e t a l  shield BPM Assy & S tee r ing  Coi I s  G r a d  i e n t  E l ectrodes 
Figure 9. Electron confinement, alignment, vacuum, and clearing system. 
about 25 m to 100 m. This modification significantly increases the cooling rate, which 
scales approximately as the product of the cooling region length and the square root of the 
bet a functions. These modifications, however, move the ring 0.87 m closer to the center of 
the tunnel in the insertion region, and decrease the ring circumference by 6 cm. 
A ring pressure of 1 x Torr Nz-equivalent is needed to prevent multiple scattering 
from competing with the electron cooling process.397 At this pressure, the equilibrium 
emittance would be < 0.09 npm for a 25-s cooling time. The present MEB vacuum 
pressure specification is 5 x Torr. The reduced pressure requirement could be met 
by a combination of lower outgassing rate (using improved surface preparation techniques) 
and inexpensive nonevaporable getter pumping. In-situ baking should not be necessary to 
achieve this value. 
Although the space charge tune shift at injection allows the beam emittance to be 
reduced by at least a factor of 3, the tune shift would exceed 0.3 at transition, necessitating 
a transition gamma jumping system.8 Although such a system is not presently included in 
the SSC design, space has been reserved in the ring for such a system. 
Since space charge plays such an important role in the electron beam optics,5 it was 
initially thought that the proton beam would need to be nearly completely debunched. 
Further work, however, has shown that although the proton beam space charge leads to 
"non-ideal" electron optics, these distortions do not necessarily significantly affect the 
cooling process. Solutions for the electron beam optics design have been found for the 
cases of no debunching, partial debunching, and complete debunching. Consequently, the 
system can be easily adapted to any conceivable MEB operating mode without altering 
the electron beam confinement system hardware. 
Recirculation Tests 
. 
To date, there have been two electron recirculation systems built that are similar to 
the one we propose to use at the SSC MEB. The UCSB FEL driver1' has recirculated 
currents up to 3 A with collection efficiencies as high as 99.7% while operating in a pulsed 
mode; the NEC/FNAL/Univ. of Wisc.ll system, which operated with DC current, demon- 
strated collection efficiencies as high as 99.99%, though current was limited to 0.12 A. A 
recirculation test system will be built at the National Electrostatics Corp. using an exist- 
ing 2 MV Pelletron. The increase in energy from 2 to 6 MeV should not pose a problem 
since it involves no fundamental changes in technology. 
The beamline12 that joins the pair of Pelletron acceleration tubes to be used in the 
test system is shown in Fig. 4. The electron optics for this beamline have been modelled 
using a version of TRANSPORT which includes the effects of space charge. The transfer 
line produces a beam waist at the middle of the 180' bend, and is consequently symmetric 
about that point. Two solenoids provide enough flexibility to give both the required focal 
point position as well as a choice of beam size at the symmetry point. A quadrupole will be 
inserted between the two 90" dipoles to make the entire bend achromatic. Beam diagnostic 
systems will include non-intercepting beam position monitors ,13 single-pass flying wire 
scanners, and rotating wire beam profile monitors. The first two systems can be used to 
monitor the beam with beam currents up to the full design limit of 2 A. A beamline pressure 
of less than 1 x Torr will be maintained using a combination of non-evaporable getters 
and ion pumping. 
[XI Solenoid n 
X/Y BPM's 
X/Y S t ee r e r s  
Ion ~ a u g e / ~ o u g h  P 
Gate Valve 
Flying Wire Assy 
Roughing Port 
& Ion Pump Wire S c a n n e r  
& 
' Clearing Electrodes  
Flying Wire Assy 
'B X/Y BPM's 
- -  X/Y S t ee r e r s  
Dipole 
u 
1 
inches 
12 
Quad 
w/ Clearing Electr .  & BPM's 
Electron Beam Transfer Line 
o r t  
Figure 4. Recirculation test system transfer beamline. 
Summary and Schedule 
A design report for this cooling system is currently being prepared, and will be finished 
by the end of the summer 1992. Detailed design and procurement are underway for the 
test electron recirculation system. The system is scheduled to be assembled during the first 
part of 1993, and tests will then be carried out during the following year. A design review 
by outside experts was held in March 1992. The committee report strongly supported the 
project. 
We wish to thank Don Edward, Rod Gerig and Mike Syphers (SSC); Fred Mills 
(FNAL); Luis Elias (U. of FL, CREOL); and Ed Berners (U. of Notre Dame) for use- 
ful comments and suggestions. We also especially grateful to Delbert Larson (SSC) for 
his many useful contributions. This work is supported by the Texas National Research 
Laboratory Commission under grant number RGFY9158. 
1. Tim Ellison and S.Y. Lee, "Study of the feasibility of decreasing the emittance of 
beam in the SSC by electron cooling in the SSC Medium Energy Booster" (proposal 
to the Texas National Research Laboratory ~ommissioh, 1990). 
2. Tim Ellison and Christopher Mert on, "Numerically- calculated electron cooling rates 
for a disk-shaped Maxwellian electron velocity distribution" (IU/AP/MEBEC/005a, 
October 1991, unpublished). 
3. D. Reistad, "Intrabeam and multiple scattering in the MEB" (IU/AP/MEBEC/004a, 
September 1991, unpublished) 
4. D.R. Anderson and T.J.P. Ellison, "Electron cooling system layout in the MEB,'' 
(IU/AP/MEBEC/Oll, March 1992, unpublished). \ 
5. D. Reistad, "Electron beam optics for proposed 6 MeV electron cooler at MEB," 
(IU/AP/MEBEC/003, March 1991, unpublished). 
T. J.P. Ellison and S. Nagaitsev, "Electron beam optics and electron/proton beam 
alignment in the medium energy booster electron cooling region," (IU/AP/ 
MEBEC/OO9, February 1992, unpublished). 
6. S.Y. Lee, "A high betatron amplitude insertion for the SSC MEB lattice" (IU/AP/ 
MEBEC/007, unpublished); S.Y. Lee, "A high P value lattice for the SSC Medium 
Energy Booster straight section for electron cooling and slow extraction," these pro- 
ceedings. 
7. S.Y. Lee and Tim Ellison, "Vacuum pressure requirement of MEB in the electron 
cooling scenario," (IU/AP/MEBEC/OOl, 25 February 1991, unpublished). 
8. S .Y. Lee, "Instabilities in electron-cooled MEB beam," (IU/AP/MEBEC/OlO, March 
1992, unpublished). 
9. Cleon Manz and Rod Gerig, "The Medium Energy Booster at the SSC Laboratory," 
IEEE PAC Trans., V. 1, p. 123, May 6-9, 1991, San Francisco. 
10. Luis R. Elias and Gerald Ramian, "Recirculating electrostatic accelerators," IEEE 
Trans. Nucl. Sci., NS-32, No. 5 (Oct 1985) 1732. 
11. D.J. Larson, J.R. Adney, D.R. Anderson, F.E. Mills and M.L. Sundquist, "Operation 
of a prototype intermediate-energy electron cooler," Nucl. Instrum. and Meth. A311, 
p. 30-33 (1992). 
12. D.R. Anderson, T.J.P. Ellison, "Electron transfer beamline for the SSC Medium En- 
ergy Booster electron recirculation test system" (IU/AP/MEBEC/008, January 1992, 
unpublished). 
13. M.S. Ball, T.J.Po Ellison, B.J. Hamilton, "Position measurement system for the SSC 
Medium Energy Booster Electron Recirculation test system" (IU/AP/MEBEC/OOG, 
October 1991, unpublished). 


Design of a 6 MeV Electron Cooling System for the SSC Medium Energy Booster

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Design of a 6 MeV Electron Cooling System for the SSC Medium Energy Booster

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