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

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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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 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 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 Long Bunch

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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 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 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?

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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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 But no improvement in damper performance

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

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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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 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

IU e-Cloud Feedback Workshop March 13, 2007 LA-UR 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?