1. SPS proton beam for AWAKE E. Shaposhnikova 13 th AWAKE PEB Meeting 29.09.2015 With contributions from T. Argyropoulos, T. Bohl, H. Bartosik, S. Cettour Cave, V. Kain, A. Lasheen, H. Timko 1

2. Scope of the talk (1/2) Questions from Patric Muggli: Understanding how the experiment could possibly happen in terms of proton beam time and parameters. What to expect in terms of sharing the beam (i.e., not using the beam when not needed) The possibility of giving the beam to another experiment and being able to get it back later (not 24h/day run), Reproducibility of the bunch from event to event The possibility of a non-Gaussian bunch current profile And anything you think could be important for AWAKE regarding the p+ beam… d 2

3. Scope of the talk (2/2) Questions from Allen Caldwell: How the p-bunches are allocated (how to make a request, what is the decision process on the different requests, how much flexibility, …)? The flexibility that we have on the properties of the bunches (range of charge and length of bunch) How long in advance bunch parameters need to be requested? Range of variations of parameters we can expect shot-to-shot, etc. Some of these numbers written down in the Design Report, but… 3

4. Outline SPS beams and cycles Beam requests, flexibility AWAKE cycle Bunch parameters (intensity, bunch length, profile, transverse emittance): – Available parameter range – Short-to-short variation Future plans (studies, MDs, improvements) 4

5. SPS beams and cycles LHC beams (450 GeV) – individual bunch - 7.2 s) – 25 ns, 50 ns (22.8 s) The LHC filling (from 20 min to a few hours) has a priority and LHC cycle is not always compatible with other cycles/beams  Need to be seen how LHC is sensitive to change of supercycle (SC), can be tested this year Fixed target beam: SFTPRO (400 GeV) - long flat top  Limitation to the rms current in power supplies Ion beams (FT and LHC) HiRadmat (material tests) MD (machine development) beams 5

6. SPS supercycle The supercycle length is defined in number of PS basic periods (BP) = 1.2 s http://op-webtools.web.cern.ch/opwebtools/vistar/vistars.php?usr=SPS1 The supercycle length and composition can be changed by operators from one SC to the next 6

7. SPS supercycle If beam is not requested the corresponding cycle can be either removed from supercycle or replaced by “economy” cycle 7

8. Beam requests Beam to the AWAKE experiment will be given during allocated time period (SPSC) All requests should go via CCC (SPS island) Beam can be taken only when needed If beam is not extracted, it is dumped on beam dump – should be avoided by removing the beam request For short periods without beam the composition of SC can be kept the same For longer periods it can be replaced by “economy” cycles It is also easy to change the SC and remove the AWAKE cycle from the SC (can sometimes perturb other users) => Present SPS operation is very flexible 8

9. AWAKE cycle 9

10. AWAKE cycle in the SPS supercycle AWAKE cycle can be similar to the LHC pilot (26 => 450 GeV/c, 7.2 s long, 4.2 s acceleration time) – short flat bottom (one injection) – 7.2 s long cycle (an integer of PS BP 1.2 s) – Extraction energy of 400 GeV/c => ~10% shorter acceleration (and ramping down) time – Minimum 845 ms flat top for synchronization (T. Bohl) Typical SPS Supercycle length is ~40 s, but a few AWAKE cycles in one supercycle are also possible 10

11. AWAKE magnetic cycle cycle length: 7200 ms flat bottom: 60 ms ramp: 3715 ms flat top: 900 ms Cycle generated by S. Cettour Cave (OP) 11

12. AWAKE momentum program Cycle: AWAKE_1Inj_FB60_FT_900_Q20_2015_V1 12

13. AWAKE cycle in SPS supercycle We can’t have SFTPRO +AWAKE (18 s): rms current is too high Minimum SPS SC length is 14.4 s, but its composition is determined by maximum possible rms current in main power supplies 13

14. Possible supercycle composition SFTPRO + AWAKE+AWAKE SFTPRO + AWAKE+MD1 Example of short SC: 25.2 s with one or two AWAKE cycles 14

15. AWAKE beam parameters 15

16. AWAKE voltage programs Need to use two RF systems for beam stability – 200 MHz RF system with maximum V 200 = 7.5 MV (maximum voltage available before the LS2) – 800 MHz RF system with V 800 =V 200 /10 Voltage programs should be optimized to provide maximum beam stability during ramp Sharp voltage increase on flat top for bunch rotation (shortening) 16

17. MD studies: voltage programs Voltage (V) Time (s) LHC pilot cycle (LHC FAST): acceleration time 4.2 s to 450 GeV AWAKE cycle: acceleration time 3.7 s to 400 GeV constant bucket area 0.5 eVs 17 high voltage

18. Bunch rotation on the flat top (1/3) 18

19. Bunch rotation (2/3) (SPS tests in 2012) 200 MHz voltage program (double RF) Bunch length through ramp instability Bunch rotation 19 H. Timko et al.

20. Bunch rotation (3/3) T. Bohl Optimal extraction time T opt => Optimal extraction time (shortest bunch length) depends on bunch intensity => Unstable bunches perform coherent quadrupole oscillations during ramp and on flat top T opt depends on bunch intensity Q20, (2.7-3.7)x10 11 Q26 optics unstable N=(2.7-3.6)x10 11 20 Q20 optics N=(2.2-2.6)x10 11

21. Parameter range Intensity: (0.5 - 3.5)x10 11 – Bunch intensity is normally adjusted in the PSB by changing the number of injected turns from Linac – Change in bunch intensity will be requested via the CCC and for low emittances may take some time for adjustment by operators Bunch length is defined by longitudinal emittance and voltage (maximum available voltage at 200 MHz is 8.0 MV now and 14 MV after 2021) – Longitudinal emittance is also normally adjusted in the PSB (using controlled longitudinal emittance blow-up followed by voltage dip) – Can be increased in PS and SPS by controlled BUP, mismatched RF caption – Minimum possible bunch length is increasing with intensity 21

22. Measurements and simulations RF: V 200 = 2 MV & V 800 = 0.2 MV  Particle simulations (code BLonD) using the SPS impedance model are in good agreement with single bunch measurements 2012 measurements T. Argyropoulos et al 22

23. Shot-to-shot variation of bunch parameters Variation of injected bunch parameters (SPS injectors: Linac2 PSB PS SPS): Typically ±10%, up to ± 15% (depends on settings, can vary from day to day) – need more statistics for AWAKE parameters Sources of variation in the SPS: – Systematic increase of bunch length with intensity (stable bunches) – Variation due to longitudinal instability – Bunch rotation with fixed extraction time 23

24. Proton bunches at extraction (450 GeV): bunch shape (1/2) Before rotation: 4σ = (2.38 +/- 0.04) ns After rotation 4σ = (1.25 +/- 0.024) ns Gaussian fit  Bunch profile can be well approximated by Gaussian (but with smaller tails) for intensities in the range (2.7-3.7)10 11 T. Bohl 24

25. Bunches at extraction: bunch shape (2/2)  Stable bunch is symmetric  Left half of the bunch should be used for measurements (right part is perturbed by the PU transfer functions)  Bunch shape (before rotation) fit ~ (1-t 2 /t b 2 ) 1.5 T. Argyropoulos et al 25

26. Bunch length before rotation Simulations of bunch stability through ramp with full SPS impedance model A. Lasheen => In simulations all bunches with intensity above 2x10 11 are unstable during this short cycle (to 450 GeV) => Max possible emittances are defined but the length of the cycle (and maximum voltage available) stable unstable rms FWHM stable unstable 26

27. Bunch length after rotation (2 => 7.8 MV) Simulations of bunch rotation with full SPS impedance model A. Lasheen FWHMrms stable unstable 27

28. Unstable bunches: N=3x10 11 (different bunch lengths) Simulations of bunch rotation with full SPS impedance model A. Lasheen FWHM rms We should be slightly more stable at 400 GeV (~ 10% in intensity) 28

29. Simulations: minimum bunch length on the flat top High voltage program (flat top 7.2 MV) 29 stable unstable A. Lasheen

30. Transverse emittance => Transverse emittance is increasing with intensity, mainly due to the space charge effects in all accelerators of the chain => Will be improved after LS2 with higher injection energy in PSB and PS (main motivation for these LIU upgrades) H. Bartosik 30

31. Summary of beam parameters ( tests in 2012) optics Q26 (higher I)Q20 (lower I)Q20 (higher I) Intensity (p/bunch) (2.66—3.61)  10 11 Average: 3.1  10 11 (2.19—2.56)  10 11 Average: 2.4  10 11 (2.68—3.73)  10 11 Average: 3.3  10 11 Bunch length after rotation (1.3—1.7) ns Average: 1.52 ns (1.1—1.3) ns Average: 1.19 ns (1.25—1.6) ns Average: 1.34 ns Adiabatic shortening1.65 ns1.6 ns Transverse emittance(1.6—2.4) µm at (2.6—2.7)  10 11 (1.3—1.7) µm(1.8—2.8) µm H. Timko et al. 31 Fit to existing data: bunch length = 0.78 + 0.17xN/10 11 [ns]

32. Conclusion The SPS operation is flexible and allows fast changes of the SPS supercycle 6D-bunch size increases with intensity SPS bunches are unstable for intensities above ~2x10 11 If significant shot-to-shot variation in bunch length and shape is acceptable then we can run in unstable regime, otherwise controlled longitudinal emittance blow-up during ramp can be applied, but bunches will be longer Beam Quality Monitor (BQM) can be used to extract only bunches with required parameters (need some work on BQM, used now mainly for the LHC beam) 32

33. Future plans Measurements for the new 400 GeV cycle – Optimum voltage program for beam stability – Bunch emittance as a function of intensity – Bunch rotation @400 GeV with jump to unstable phase Simulations for the 400 GeV cycle – Bunch emittance as a function of intensity – Further optimization to make bunches shorter (use of 800 MHz RF system,…) The 800 MHz RF (2 nd cavity) commissioning and LLRF upgrade can help to improve bunch stability (2016) SPS upgrades planned during LS2 (2019-2020): – Impedance reduction (~shielding of ~200 vacuum flanges) should significantly improve longitudinal beam stability – The 200 MHz RF upgrade (doubling RF voltage) should help to shorten bunches – Transverse emittance will also be reduced due to Linac4 and upgrade of PSB 33