1. Cooler Ring Design Status - July 2017
S.V. Benson, July 20, 2017

2. fast extraction kicker
Baseline is the CCR
Same-cell energy recovery in SRF cavities
Uses Yulu’s kicker design to inject and extract from CCR
Assumes high charge, low rep-rate injector (with subharmonic acceleration and bunching)
ion beam
magnetization flip
top ring: CCR
bottom ring: ERL
injector
beam dump
linac
fast extraction kicker
fast injection kicker
De-chirper
Re-chirper
circulating bunches
extracted
injected
septum
vertical bend
B < 0
B > 0

3. Strong Cooler Specifications (Electrons)
Energy 20–55 MeV 1
Charge 2.0 (3.2) nC
CCR pulse frequency MHz
Gun frequency 43.3 MHz
Bunch length (tophat) 2 cm (23°)
Thermal emittance <19 mm-mrad 2
Cathode spot radius 3.14 mm
Cathode field 0.05 T 3
Gun voltage 350 kV
Normalized hor. drift emittance 36 mm-mrad
rms Energy spread (uncorr.)* 3x10-4
Energy spread (p-p corr.)* <6x10-4
Solenoid field 1 T
Electron beta in cooler 37.6 cm
Solenoid length 2x30 m 4
Bunch shape beer can

4. Cooler Specifications (protons)
Case 1 – 63.3 GeV center of mass energy
Energy 100 GeV
Particles/bunch 2.0x1010
Repetition rate MHz
Bunch length (rms) 2.5 cm
Normalized emittance (x/y) 1.2/0.6 mm-mrad
Betatron function in cooler 100 m (at point between solenoids)
Case 2 – 44.7 GeV center of mass energy
Particles/bunch 6.6x109
Repetition rage MHz
Bunch length (rms) 1.0 cm
Normalized emttance (x/y) 1.0/0.5 mm-mrad
Betatron function in cooler 100 m (at point between solenoids)
Ion ring lattice may be coupled or dispersed in solenoid.

5. Critical Design Tasks
Verify cooling model with Derbenev thesis formulas
Is it really good to have an electron beam smaller than the proton beam? Should we scan the position and time?
Can we use x/y coupling and dispersion in the proton ring to balance cooling?
Can we recirculate the beam (CCR design)?
How do we make a 3.2 nC bunch and accelerate to 55 MeV?
Solenoid non-uniformities (tools?)
Need Merger Design.

6. New Issues
Would like to preserve the ability to run at MHz in the CCR and 86.6 MHz from the gun.
Will use 433 MHz for lower energy injector cavities (now optimizing on bunch uniformity and magnetization).
Looking at RF gun to increase beam quality.
Do we want to have harmonic RF in the injector?
Potential RF controls issue
Do we want to lengthen the bunch in the cooler?
This is not looking essential yet.
Can we get rid of the ERL arcs?
Space charge must be compensated (both longitudinal and transverse)
Laslett tune shift is 0.5 at 20 MeV
DC cooling at 8 GeV looks good for protons. Have to check on ions now.

7. Layout

8. Beam Envelopes and Sigmas

9. Chromatic Properties

10. Performance Linear optics fine Aberrations not too bad Chromatics good
Need to check residual geometric terms
Spatial terms ~unity => contribution ~tens-hundreds of microns
Path length terms (T533=T544) ~ten => mm-ish path length changes… potentially problematic?
CSR is a problem
Low symmetry, periodicity => optics balance breaks down
phasing only weakly constrained
beam envelopes not identical
Preliminary simulation: 36 mm-mrad => 47 mm-mrad
Need to make some modeling tweaks to look at space charge
(separate sextupole from quads)

11. Alternate Layout - Delete the ERL RingIS IK EK ES
Kick
Kick
Use two kickers and two septa in the Cooler Ring
Path through RF cavities is λ/2 (15.75 cm) longer than direct path
The extracted bunch arrives at the RF cavities λ/4 late
Injected bunch is λ/2 before extracted bunch, minimizing wakefield effects
Injected bunch starts λ/4 ahead, arrives at injection septum at correct time
Example: λ/4 delay can be created with 2.3° bend and a distance of 2 meters
Gun and dump lines would then be ~23° from ERL cavity axis
PSTK
PREK
Kick
Kick
RF Cavities
HRD
HDD
Dump
Injector
Gun

12. Comparison with Present Layout
Advantages
No ERL Arcs!
Chirp/re-chirp can be provided by ERL Linac rather than separate cavity. Eliminates two cryo-modules.
Disadvantages
Matching more complicated as space is compressed
Cannot use ERL ring as distributed collimator for halo
Use transfer line instead – not as good
Must produce proper bunch length in injector
No longitudinal flexibility
Requires harmonic RF or much lower linac frequency
Note: this layout would also work with twin-axis cavity
Only change is that the path length difference would be λ not λ/2

13. Conclusions
Continuing to look at “real world” problems (timing, aberrations, clearances, space charge, etc.)
8 GeV cooling looks reasonable with half solenoid and 2 A. Have to look at ion cooling.
Still trying to get good injector solution but a lot of progress has been made. Now incorporating acceleration before bunching and looking at an RF gun.
CCR arc requires a fair amount of periodicity. A simple design results in beam destruction via CSR.
Considering new architectures to see if they make life easier.

14. Backups

15. Notes on Specifications
1 This assumes that the maximum proton energy is 100 GeV. If the maximum energy changes to 200 GeV, this must increase to 110 MeV. All the specifications below assume a beam energy of 55 MeV. 2 The measured thermal energy is 0.16 eV for CsKSb. This means that the thermal emittance is 0.56 mm-mrad/mm (rms) (see Bazarov et al. arXiv: v1 “Thermal emittance measurements of cesium potassium antimonide photocathode”). Due to space charge we presume that there will be some effective growth in the emittance up to >1 mm-mrad. For a 1.1 mm radius spot size (rms spot size is 0.78 mm) the calculated thermal emittance should be 0.44 mm-mrad. We have to keep the emittance growth less than factor of We think the cathode field is limited by technical reasons to 0.2 T. We have chosen 0.1T to keep the beam large and reduce space charge forces. 4 The ion ring lattice is being designed with ~70 meters of space available for the cooling channel. The baseline assumes that we use all of this space with an angular momentum reverser in between. The second space has a solenoid of opposite polarity. 5 It is assumed that the proton beam emittance and energy spread are in equilibrium such that the IBS growth is matched by the cooling rate. The calculated horizontal spot size in the cooler is ~1 mm for these parameters. We will assume that the proton beam is uncoupled so that the vertical emittance is eventually much smaller than the horizontal emittance.