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Review of the e-p feedback experiments

Review of the e-p feedback experiments. Rod McCrady Los Alamos National Lab. Overview. Pickup, process v , feedback 4 turns later Q = 2.1875, 4 ×Q = 8.75 Cables and LLRF require >3 turns. Signal Processing. RF amp. Kicker. Beam. Pickup. . Monitor. . Filter. Variable Attenuator.

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Review of the e-p feedback experiments

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  1. Review of the e-p feedback experiments Rod McCrady Los Alamos National Lab IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  2. Overview • Pickup, process v, feedback 4 turns later • Q = 2.1875, 4×Q = 8.75 • Cables and LLRF require >3 turns Signal Processing RF amp Kicker Beam Pickup IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  3. Monitor  Filter Variable Attenuator  RF switch Input Level Control  Variable Attenuator  Fiber Optic Delay Variable Delay Gain Control Low-Level RF System • We have plenty of signal strength • Fiber optic link compresses at -14dBm IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  4. LLRF  Oscilloscope Beam Pickup Kicker LLRF  LLRF  Oscilloscope Oscilloscope Beam Beam Pickup Kicker Pickup Kicker Setting the timing Use kicker as “BPM” Mark time of arrival of 1µpulse on 5th traversal Observe time of arrival of pulse from PAs (This will be from the 1st traversal) Adjust delay so that damper pulse from 1st traversal arrives when beam arrives on 5th traversal IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  5. Complicating factors • Short store time • Complicates measurements and system diagnosis • Long bunch • A few complexities introduced by this • v signal from BPM • (dy/dt)×I(t) • Broad band • Rapid growth IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  6. Factors Limiting Performance • System gain • System bandwidth • Power amplifiers • Kicker • Signal fidelity • Especially phase • Optimization of betatron phase advance • Beam in the gap • Longitudinal “noise” • Onset of horizontal instability IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  7. Long bunch & Short store time • Short store: difficult to use spectrum analyzer, etc. • Very little frequency information on-line • Frequencies change: IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  8. Long Bunch IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  9. BPM v signal • Need beam position quickly (<1s) with wide bandwidth (10 to 300MHz) • v(t) = Vtop(t) – Vbottom(t) • v  intensity • Looks like derivative of position in bandwidth of this system • 90 phase shift at all frequencies • Cannot compensate with a delay IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  10. BPM v signal Signal at upstream end of stripline electrode: Difference of top and bottom electrodes (v): For an oscillating beam: Note 90 phase shift at all frequencies. Looks like derivative of position.   and sin  cos IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  11. Vin Vout R C BPM v signal • One could integrate the v signal • We tried a passive integrator • 1/ response was unpalatable • Reduced signal level • In retrospect, maybe not a big deal • Other ideas • Another differentiator: • Comb filter also gives 90 phase shift • We haven’t seen any benefit from comb filters • Different pickup type • Buttons • Slotted coupler C Vin Vout R IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  12. Betatron Sidebands • Why are they present in the v signal: • Beam pulse traverses BPM at fR=2.8MHz (revolution frequency) • Revolution harmonics n × fR • Position changes turn-to-turn due to betatron motion • f = Q × fR = (k+q) × fR • A BPM only knows about q, the fractional tune • fR is modulated by q × fR • Betatron sidebands: (nq)×fR (upper and lower sidebands) • Lower sidebands are associated with instabilities IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  13. Experiments • Explore limitations of the system • Elucidate complicating factors • Improve performance of the system ! • Drive / damp • Noise-driven beam • Tests of system fidelity • Investigate effects of saturation in the LLRF system • Tests of comb filters • Effects of longitudinal noise • Compare Qthr with/without damping • Grow / damp IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  14. Drive - Damp • Signals are complicated by synchrotron motion of beam • Hoped to compare passive vs. active damping rates • Next time use coasting beam IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  15. Noise-Driven Beam • Does it “damp” as well as feedback does? • One of my darkest fears • Does it initiate instability? • Does it interfere with coherence? • Makes the beam more unstable. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  16. Monitor WM41 top 1  300MHz LPF  RF switch Variable Attenuator 2 Input signal level control  WM41 bot PM44 bot 8.5dB gain 1  Variable Attenuator F.O. Tx F.O. Delay F.O. Rx 17dB 2 Gain Control. PM44 top -8dB -8dB Effects of saturation • Re-configured system • Monitor input 150mVp-p no compression • Attenuator for input level • Attenuator for gain • Operating in compression is better • What’s the benefit? • Damping early? • Compression is OK? IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  17. Beam in the gap • Compare conditions at low Vbuncher to intentional BIG • Explore both axes of threshold curve IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  18. Longitudinal noise • Problem: v signal has intensity information • PSR fR = 72.00×flinac  micropulse stacking • 2006: changed to fR = 72.07×flinac • Longitudinal noise was reduced • 402.5MHz is ~USB of mode 144 when using 72.07 • But no improvement in damper performance 72.00 72.07 IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  19. Less longitudinal noise, but… • 402.5MHz is ~USB of mode 144 when using 72.07 =2×linac frequency • Vertical oscillations at 402.5MHz IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  20. Vary the vertical tune • How perfect does the betatron phase advance need to be? • Can give some indication of what frequencies matter • Found that several 1/100ths units on vertical tune made little difference. • 3.18 to 3.20 IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  21. Vary the Timing • Increase & decrease LLRF system delay till damping is clearly worse • How perfect does the betatron phase advance need to be? • Can give some indication of what frequencies matter • ~90  ~2ns  100 to 150MHz IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  22. Signal Fidelity – Phase Errors • Phase errors in power amplifiers and cables IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  23. coax   IN OUT FO xmitter FO rcver Optic fiber Comb Filters • To filter out revolution harmonics • Wasted power • Closed orbit offset • Subtract signal from time-delayed signal (t=Rev) • Similar to stripline BPM • 90 phase shift at all frequencies • ? Might help mitigate dy/dt from v signal ? • 180 phase shift from one passband to the next IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  24. Comb Filters • 180 phase shift from one passband to the next • Damping in one passband means driving in the next • Two ways to deal with it: • Twice as many passbands Only LSBs matter anyway 2) Two comb filters in series Lose 90 phase shift • Time domain picture • Which “turns” to feed back • One positive, one negative IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  25. Results of Comb Filters • Revolution harmonics reduced • Signals to kicker: • Ultimately, no better damping achieved IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  26. Instability in the Horizontal Plane • If we control the vertical motion, will the intability show up in the horizontal? • Some predictions of instability tune • In PSR: Qh / Qv = 3.2 / 2.2 IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

  27. Experiments: To Do • Understand mechanisms for frequency spread • Coasting beam • Why does system perform better in compression • Damp early, then turn off damper • Turn on damper late, without early damping • Can we get a better input signal? (other than v) • What frequencies really matter? IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613

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