1. July 9-11 2014 LEReC Review 9 - 11July 2014 Low Energy RHIC electron Cooling Jorg Kewisch, Dmitri Kayran Electron Beam Transport and System specifications

2. July 9-11 2014 LEReC beam structure in cooling section Example for  = 4.1 (E ke =1.6 MeV) 110 nsec, f=9.1 MHz 1.42 nsec 30 electron bunches Electrons: f_SRF=703.518MHz Rms length=3 cm, I_peak=0.4 A Q_e=100 pC 9 MHz RHIC RF

3. July 9-11 2014 Electron Beam Energies Gun Energy [MeV] GammaCharge per bunch [pC] Beam Power [kW] 1.64.1310045 1.84.5210051 2.04.9110056 3

4. July 9-11 2014 Low-Energy RHIC electron Cooler (LEReC), 2018 Cooling sections Beam transport lines Beam dump 704 MHz cavity 704 MHz SRF gun e-e- e-e- e-e- e-e- e-e- IP2 DX 2.1 GHz cavity

5. July 9-11 2014 2 O’clock Interaction region 5

6. July 9-11 2014 Merging Electrons and Ions (Dog Leg) 6

7. July 9-11 2014 Cooling section Use two solenoids at the start of the section to match the beam size to 4.3 mm Continuous weak focusing to keep the divergence of the beam small and the radius constant Use two solenoids to reduce the beam size at the end to avoid over-focusing by the weak focusing of the U-turn dipoles

8. July 9-11 2014 Lattice : cooling section blue, turn around, cooling section yellow

9. July 9-11 2014 Beam Transport in the low energy cooler No acceleration to high energies, space charge is an issue for the whole accelerator Long transport –Strong focusing elements produce continuous emittance compensation –Space charge pushes head and tail of the bunch in opposite direction. Energy spread increases continually 9

10. July 9-11 2014 What to do about it? Longitudinal bunch shaping: The energy spread growth depends on the longitudinal density distribution: Beer can is best, Gaussian is mediocre, Triangular is bad Longitudinal profile after the 3 rd harmonic cavity (energy spread is small): 10 Starting with beer can Starting with trapezoid

11. July 9-11 2014 What else to do about it? Make the bunch long as quickly as possible –Adjust the phase of the gun to create energy chirp –Bunch length increases due to velocity spread –Remove linear chirp in the 5 cell cavity –Remove non-linear chirp in the 2100 MHz cavity  Does not work if bunch is too long  Too much energy spread hurts transverse emittance 11

12. July 9-11 2014 Bunch length and minimum energy spread after the 3 rd harmonic Cavity 12

13. July 9-11 2014 Emittance degradation through Chromaticity in Solenoids between Gun and 5 cell Cavity 13

14. July 9-11 2014 What else to do about it? “Hot” particles don’t cool, but don’t do harm either –Put 10% more charge into the bunch –Ignore the 10% hottest particles 14

15. July 9-11 2014 Simulations All simulations were made using the code PARMELA PARMELA assumes for the space charge calculations that the beam is round. If the aspect ratio is large the space charge in the smaller direction is underestimated For this simulations we used solenoid focusing only. If quadrupoles are used a 3-D program like ImpactT must be used. A multi-threaded optimizing program (written at BNL) was used which launches PARMELA for the function evaluation. The WINE program was used to allow running PARMELA, which is closed source and MICROSOFT only, in an UNIX environment A typical optimization run with 5 parameters takes 36 hours. 15

16. July 9-11 2014 Orientation All calculations are done at 1.6 MeV. Higher energies are easier. 16

17. July 9-11 2014 Envelope 17

18. July 9-11 2014 Maximum Envelopes and Beam Pipe Dimensions 18

19. July 9-11 2014 Transverse Temperature after 10% cut 19

20. July 9-11 2014 Longitudinal Phase Space at the Cathode 20

21. July 9-11 2014 Longitudinal Phase Space at the Gun Exit 21

22. July 9-11 2014 Longitudinal Phase Space at the 5 cell Entrance 22

23. July 9-11 2014 Longitudinal Phase Space at the 5 cell Exit 23

24. July 9-11 2014 Longitudinal Phase Space at the 5 cell Exit 24

25. July 9-11 2014 Longitudinal Phase Space at the 3 rd Harmonic Exit 25

26. July 9-11 2014 Longitudinal Phase Space at minimum energy spread 26

27. July 9-11 2014 Energy Spread Disclaimer: The parameters for this calculation are different from ones above 27

28. July 9-11 2014 Energy Spread in the cooling sections 28

29. July 9-11 2014 Summary Getting required electron beam parameters (transverse angular spread and momentum spread) is most challenging for lowest energy of LEReC operation. A scheme with RF manipulation was developed which produced satisfactory beam quality. Beam lattice and dynamics can be further optimized. Complete gun-to-dump calculations are planned to –Find optimal focusing in the transport section –Match flat envelope in cooling section –Optimize dispersion matching 29

30. July 9-11 2014 Backup slides 30

31. July 9-11 2014 Cooling sections U-turn dump 2.1 GHz warm cavity 704 MHz SRF cavity 704 MHz SRF gun Focusing Solenoids Dipoles quadrupoles v Layout – not to scale, not showing all magnets in their locations Transport quadrupoles