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

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

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

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

5. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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

6. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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

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

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

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

10. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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

11. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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 V in V out R C V in V out R C

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

13. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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 Q thr with/without damping Grow / damp

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

15. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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.

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

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

18. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 Longitudinal noise Problem:  v signal has intensity information PSR f R = 72.00×f linac  micropulse stacking 2006: changed to f R = 72.07×f linac Longitudinal noise was reduced –402.5MHz is ~USB of mode 144 when using 72.07 But no improvement in damper performance 72.0072.07

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

20. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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

21. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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

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

23. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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  coax  Optic fiber FO rcver INOUT FO xmitter

24. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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: 1)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

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

26. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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: Q h / Q v = 3.2 / 2.2

27. IU e-Cloud Feedback Workshop March 13, 2007 LA-UR-07-1613 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?