Review of the e-p feedback experiments

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


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


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


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


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


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


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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


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Long BunchCoherent transverse oscillationCoherent transverse oscillation...After synchrotron motion


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


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


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


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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


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


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


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.


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?


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Beam in the gap• Compare conditions at low Vbuncher to intentional BIG

• Explore both axes of threshold curve


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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


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


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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


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


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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)


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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


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


LA-UR-07-1613

Results of Comb Filters• Revolution harmonics reduced

– Signals to kicker:

• Ultimately, no better damping achieved


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


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?

Review of the e-p feedback experiments

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