Tune and Coupling Feedback Implementation and Results from RHIC

1. Tune and Coupling Feedback Implementation and Results from RHIC Peter Cameron

2. Outline • A Brief History • Coupling and Coupling Feedback • RHIC Run 7 • anomalous BTF • tune scalloping • Plans • ‘hybrid’ tune tracker • chromaticity feedback • continuous head-tail studies

3. Brief History Run00LF Schottky Pickup, first tune tracker based on SRI lockin amplifier Run01tune tracker based on RHIC BPM module • experience gained with tune tracking, preparations for tune feedback begin Run03tune tracker based on Pentek VME DSP system • first successful tune feedback ramp was first try with gold (thru transition!) • performance good, but not consistent – failed ramps with no understanding of why Run04mechanism of failure of tune feedback in the presence of coupling is finally understood • efforts to ramp with tune feedback end • within one week the ‘eigenmode projection’ method of coupling measurement is running Run05no direct progress with tune/coupling feedback, however • early in the run Rhodri and Marek visit, first installation of 3D AFE in RHIC • understanding of mains harmonics is developed • last day of the run, BBQ tracks tune up the ramp Run06VME BBQ tune tracker - protons • AP/Controls finally ready for coupling feedback • first attempt delivers decoupled beam to full energy with ~.001 tune control • continued good success through the run, but mains harmonics prevent transition to full operational status • last week of run - feedbacks used for first successful acceleration of polarized beam to 250GeV • the need for chromaticity control becomes ever more obvious Run07VME BBQ tune tracker – gold ions • chrom feedback planned for startup, but Controls Interface not completed • discovery of ‘anomalous BTF’ at injection • discovery of ‘tune scalloping’ on the ramp • mains harmonics plus tune scalloping gives a new problem

4. Only horizontal tune shows up in horizontal FFT Vertical mode shows up & frequencies shift D Measurement of Coupling using a PLL Tune Tracker courtesy Rhodri Jones Start with decoupled machine Gradually increase coupling Fully coupled machine: D = |C-| FFT of Horizontal Acquisition Plane Ver Set Tunes V H Amplitude Hor Frequency

5. Measurement of Coupling using a PLL Tune Tracker Tracking the vertical mode in the horizontal plane & vice-versa allows the coupling parameters to be calculated

6. Measurement of Coupling using a PLL Tune Tracker courtesy Rhodri Jones Tunes entirely defined by coupling – tune feedback would break here Fully coupled

7. Tune Loop Simulink Diagram • PLL loop BW - a few Hz to 10Hz • Magnet loop BW ~1Hz, limited by magnet/power supply BW • Coupling loop has similar structure • Coupling loop BW ~0.1Hz • Loops are both ‘Single Input, Single Output’

8. Outline • A Brief History • Coupling and Coupling Feedback • RHIC Run 7 • anomalous BTF • tune scalloping • Plans • ‘hybrid’ tune tracker • chromaticity feedback • continuous head-tail studies

9. RHIC Run 7 0. MAINS HARMONICS REMAIN THE PRIMARY OBSTACLE 1. Anomalous beam transfer function – cause is not understood (next slide) 2. Tune, coupling, and chromaticity drifts due to persistent currents at injection (also next slide) 3. ‘Tune scalloping’ 4. Mains harmonics plus tune scalloping gives a very tight ‘box’ 5. Effect of chromaticity and tune scalloping on tune feedback 6. Large noise introduced when magnet loops were closed – again, from anomalous BTF? Kills chrom feedback at injection 7. A variety of other problems • IPM, Artus, injection damper, button monitor all turning themselves on at inappropriate times • 5 FEC VME crate would hang frequently • ‘cache corruption’ problem would break feedback ramps

10. conventional delta-wye 6 phase 12 pulse Df=30 deg

11. firing order: A1, A2 B1, B2 A3, A4 B3, B4 A5, A6 B5, B6 Phase between A and B bridges depends on mains voltage (booster on/off, ???...) With phase shift between A and B bridges, this reproduces the pattern used in the simulation. LOAD RHIC 6 phase 12 pulse 1 3 5 A 4 6 2 Df=30 deg 1 3 5 B 4 6 2

12. The simulation • 12 free parameters for main dipole PS • turn-on time of each of the 12 SCRs • also controls turn-off time of preceeding SCR • 24 free parameters for simulation • turn-on time of each of the 12 SCRs • pulse length of each of the 12 phases • SCR rise time ~10usec • Simulation time resolution ~14usec • this is what was convenient • later reduced to ~7usec, no qualitative difference

13. familiar pattern

14. 3D AFE M Turn BPM Simulation

15. Seen here mains harmonics IPM firing Artus firing power supply oscillating?

16. Mains Harmonics in Time Domain Vertical Artus kicker firing 1.67msec 720Hz Amplitude is tens of microns BBQ sensitivity is ~10nm

17. main dipole power supply turns off Chrom needs fixing here main dipole power supply turns on

19. Anomalous beam response at injection • tunes separated and well decoupled • not power line, synchrotron freqs • similar in all 4 planes • serious obstacle to acquiring lock • serious contributor to ‘noise’ • not seen with 245MHz PLL • disappeared with start of ramp • not understood – speculation on power supply regulation/phase shift at low current Blue Vert >30dB above noise floor! ‘Unexcited’ spectrum at betatron line • This difficulty with acquiring lock was aggravated bypersistent current decay, causing large and fast drifts at injection • tune ~.02 (tunes cross!!!) • coupling - full coupling • chromaticity ~3 units • These drifts are compensated for Run 8?

20. The consequence of the previous slide - procedure to lock the tune trackers in the presence of multiple peaks and persistent current decay was ~40 steps! • take BTFs to determine where the optimum line is for locking • reduce proportional gains (can’t acquire initial lock with optimal gains) • set the search windows so that H cannot lock on V, and V cannot lock on H • turn on the H kicker, acquire a lock. This inevitably is not a lock on the line you want, but rather a lock on the first line the BBQ comes to. • nudge the search window to push the BBQ onto the correct line. This often requires repeated nudges. • tweak the phase to optimize response amplitude - because of many peaks BTF app does not reliable give optimum phase information, has to be done manually • turn up proportional gain • adjust desired amplitude for kicker feedback • open the search windows • turn on kicker feedbacks • repeat steps 1-10 for vertical. If we are running both beams repeat this whole thing for the other ring • tell shift leader BBQ is ready

21. ‘Tune Scalloping’ with TF loops open • BBQ very precisely drives a slice up out of the tune distribution • tune shifts as amplitude increases (sextupoles dominate? Large octupole for gt?) the slice moves off resonance, depopulates • BBQ falls off that slice, locks back in the center of the distribution • the process repeats • Contributing factors • large kicker excitation • large loop gains • small chromaticity Ramp 8245 Kicker turned down here, then started locking on mains harmonics scalloping dots are Artus kicked tune measurements • Makes for a tight ‘box’ • need large kicker excitation because of mains harmonics • need large loop gains for reliable tracking of fast tune changes • chromaticity feedback not yet implemented – chrom control is not particularly good • A delicate balance to tune the tracking Limit of tune shift defined by dynamic aperture? Indication of small chromaticity

22. Three successive wiggles ramps during beam commissioning second ramp – turn down kickers, BBQ captured by mains Ramp 8254 first ramp – we see scalloping Ramp 8253

23. Third ramp, and effect of scalloping on chrom measurement This is NOT problem with chrom algorithm phase third ramp – turn kickers back up, again see scalloping tunes Ramp 8255

24. During Run 7 BBQ worked very well, despite many issues 0. MAINS HARMONICS REMAIN THE PRIMARY OBSTACLE 1. Anomalous beam transfer function – cause is not understood (next slide) 2. Tune, coupling, and chromaticity drifts due to persistent currents at injection (also next slide) 3. ‘Tune scalloping’ 4. Mains harmonics plus tune scalloping gives a very tight ‘box’ 5. Effect of chrom and scalloping on tune/chrom measurement/feedback 6. Large noise introduced when magnet loops were closed – again, from anomalous BTF? Kills chrom feedback at injection 7. A variety of other problems • IPM, Artus, injection damper, button monitor all turning themselves on at inappropriate times • 5 FEC VME crate would hang frequently • ‘cache corruption’ problem would break feedback ramps

25. Saturday – ramp 8211 (the first wiggles ramp) Chromaticities and tunes The first wiggles ramp came 10 ramps after the first successful TF ramp. Chrom was not corrected until the following Wednesday evening Chrom measurement affected by tune scalloping here dots are Artus kicked tune measurements

26. Effect of chrom on Tune and Coupling Feedback – ramp 8217 Loops open ‘ungracefully’, BBQ continues to track Effect of chrom on TF

27. Scalloping with TF loops closed Loop gains and kicker excitation turned down here, back up here, down again here Vert amplitude and phase Chrom small here Ramp 8667 IBS lattice horiz amplitude and phase H tune vert tune scalloping with TF on V tune

28. Chromaticity via radial modulation with BBQ in RHIC Run 7 two successive ramps for correcting and measuring result second ramp Chromaticity corrected here and here first ramp With ongoing tuning we eventually had reasonably good chrom measurements on the ramp

29. Simultaneous tune and coupling feedback in both rings , ~97% transmission Ramp 8498 Yellow beam current H tune V tune With ongoing tuning we eventually had good feedback ramps Loops closed thru transition Blue beam current H tune V tune Slight tune swing down to keep between mains during ramping

30. Outline • A Brief History • Coupling and Coupling Feedback • RHIC Run 7 • anomalous BTF • tune scalloping • Plans • ‘hybrid’ tune tracker • chromaticity feedback • continuous head-tail studies

31. ‘Hybrid’ tune tracker • Dynamic range at transition was the showstopper for the 245MHz PLL • 3D AFE and BBQ appeared to be the solution to this problem • Large dynamic range of the diodes – first components in the signal paths • ~160dB suppression of the revolution line plus ~10nm sensitivity • However, BBQ operates in the coherent spectrum • We discover mains harmonics, which are ~70dB above BBQ noise floor, ~50dB more severe in RHIC than Tevatron or SPS • We discover the ‘anomalous BTF’ at injection with ions • No obvious solution on the horizon for either of these problems in the coherent spectrum, they both are solid obstacles to making tune tracking feedback operational • lock acquisition at injection cannot be automated • needed kick (due to mains harmonics) blows up emittance • So how about a ‘hybrid’ system? Direct diode detection of 245MHz pickup • Some data from last run (following slides), many questions • Noise floor? At high frequency? • Coherence at high frequency? • Performance of 3D AFE peak detector with resonant pickup? • Phase compensation? Mix in tunnel?

32. Ramp 8687 (early May) Yellow LF Schottky 245MHz signal as seen thru 3D AFE Ramp 8687 yellow LF Schottky Yellow 1m long stripline signal as seen thru 3D AFE • What is this??? • ‘clock’ jitter? Blue is master • higher order transverse modes? • we consistently see stronger beam response below gt • else? Ramp 8687 yellow BBQ

33. Ramp 8687 around transition Yellow LF Schottky 245MHz signal as seen thru 3D AFE Yellow 1m long stripline signal as seen thru 3D AFE

34. Ramp 8687 time domain around transition IPM firing Yellow LF Schottky 245MHz signal as seen thru 3D AFE Yellow 1m stripline signal as seen thru 3D AFE -13 sec 0 +12

35. Possible ‘Hybrid’ Tune Tracker Block Diagram motion control pickup sum mode 122.5 MHz bandpass diode doubler 61.25 MHz bandpass diode doubler 30.625 MHz bandpass AGC beam 245MHz pickup 245MHz kicker diode doubler 245 MHz cavity filter 245 MHz cavity filter tunnel 1002a existing BBQ processing existing 3D AFE • Advantages of this approach to ‘hybrid’: • minimal phase compensation needed • excite the same spectrum you are measuring • ‘bleedthru’ less problematic • minimal modification to existing BBQ

36. Outline • Transition Chromaticity Monitor • ‘Hybrid’ tune tracker? • Chromaticity measurement and feedback • Other Stuff • Beam transfer functions • Streaming audio • Coupling echoes?

37. Run 8 Plan for Chromaticity Feedback (assuming present BBQ tune tracker) • Scenario 1 – no anomalous BTF at injection with ions • Early on (before any ramps in blue?) • turn on tune and coupling feedback, turn on radial wiggles, assess quality of chromaticity measurement (all hardware and software is in place) • If good, ramp in this configuration (it will improve chrom measurement on the ramp, therefore will speed ramp development) • When controls infrastructure is sorted out, do APEX ramps with chrom feedback • During proton ramp development – use chrom feedback (250GeV!!!) • Scenario 2 – anomalous BTF at injection with ions, no anomalous BTF at injection with protons • During APEX at store with ions • turn on tune and coupling feedback, turn on radial wiggles, assess quality of chromaticity measurement • If good, close chromaticity feedback loop and explore • During proton ramp development – use chrom feedback • Scenario 3 – anomalous BTF at injection with ions AND protons • Hybrid tune tracker

38. Dp we excite ‘on-momentum’ Dt longitudinal beam profile

39. (with positive chromaticity) • In betatron phase • particles from the head arrive at the tail early • particles from the tail arrive at the head late Dp Dt (scaled) difference signal is time shifted dt sum signal

40. (scaled) difference signal is time shifted sum signal the (scaled) difference of the sum and difference signals looks like this

41. FFT showing relative amplitudes of the 3 signals for a 0.1 radian phase shift and 1ns rms bunch length sum signal the (scaled) difference of the sum and difference signals rectified difference of the sum and difference signals

42. zoom on the previous FFT sum signal the (scaled) difference of the sum and difference signals rectified difference of the sum and difference signals

43. a possible processing chain BBQ AFE hi- pass D S D S off- set BPM

44. Other Stuff • Beam transfer functions – improvements? • White noise? Chirped? • Beam-beam transfer functions? • Coupling BBTF for LHC orbit feedback - Steinhagen • Streaming audio of BBQ – Internet radio • Coupling echoes?

45. Backup material