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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. - PowerPoint PPT Presentation
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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
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 ampSignal
Processing
Beam
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 OpticDelay
Variable Attenuator
Gain Control
Variable Attenuator
Input Level Control
VariableDelay
IU e-Cloud Feedback Workshop March 13, 2007
LA-UR-07-1613
Setting the timingUse kicker as “BPM”
Mark time of arrival of 1µpulse on 5th traversal
LLRF
Oscilloscope
Pickup Kicker
Beam
LLRF
Oscilloscope
Pickup KickerBeam
LLRF
Oscilloscope
Pickup KickerBeam
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
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
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
Long bunch & Short store time• Short store: difficult to use spectrum analyzer, etc.
– Very little frequency information on-line
• Frequencies change: Injectedf201.25 MHz
Bunchedf201.25 MHz
IU e-Cloud Feedback Workshop March 13, 2007
LA-UR-07-1613
Long BunchCoherent transverse oscillationCoherent transverse oscillation...After synchrotron motion
IU e-Cloud Feedback Workshop March 13, 2007
LA-UR-07-1613
BPM v signal• Need beam position quickly (<1s) 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
BPM v signalSignal 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
tytybeam sin)( 0
sbbbU c
LtyFtItyFtICtV
11
where)()()()()(
)()()()(2)()()( bottom,top, tytItytIaCtVtVtV bbUUU
)sin()sin(2)( 00 ttyaCItVU
200 400 600 800
Pickup Response
for f300MHz
tytydt
dbeam cos)( 0
)
2(cos
2sin4)( 00
tyaCItVU
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
Vin Vout
R C
tt sinsin 2Vin Vout
R
C
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 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: (nq)×fR (upper and lower sidebands)
• Lower sidebands are associated with instabilitiesBeam Position
q2
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 Qthr with/without damping
• Grow / damp
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
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.
IU e-Cloud Feedback Workshop March 13, 2007
LA-UR-07-1613
Effects of saturation• Re-configured system• Monitor input
150mVp-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. Tx F.O. RxF.O. Delay 17dB
Variable Attenuator
Gain Control.
8.5dB gain
WM41 top
WM41 bot
-8dB
-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-07-1613
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
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
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
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
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
t
4ns
Damping
IU e-Cloud Feedback Workshop March 13, 2007
LA-UR-07-1613
Signal Fidelity – Phase Errors• Phase errors in power amplifiers and cables
2 Cables
-20
-15
-10
-5
0
0 50 100 150 200 250 300f (MHz)
Phas
e (d
eg)
Power Amplifier (0dBm)
-10
0
10
20
30
40
50
0 50 100 150 200 250 300f (MHz)
Phas
e (d
eg)
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
FOrcver
IN OUTFO
xmitter
40.5 41 41.5 42 42.5 43 43.5
60
40
20
0
LSB
180°
)cos(sin(...)sin tt
0 1 2 3 4 5 6 7Mode #
605040302010
0
esnopseR
Bd
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
40.5 41 41.5 42 42.5 43 43.5
60
40
20
0
LSB
360°
0
1
2
3
4
5
6
2q
• Time domain picture– Which “turns” to feed back– One positive, one negative
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
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: Qh / Qv = 3.2 / 2.2
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?