Review of the e p feedback experiments
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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.

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Review of the e-p feedback experiments

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Review of the e p feedback experiments

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

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


Low level rf system

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


Setting the timing

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


Complicating factors

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

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

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


Long bunch

Long Bunch

IU e-Cloud Feedback Workshop March 13, 2007

LA-UR-07-1613


Bpm v signal

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


Bpm v signal1

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


Bpm v signal2

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


Betatron sidebands

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


Experiments

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

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

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

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


Beam in the gap

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

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

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

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

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


Signal fidelity phase errors

Signal Fidelity – Phase Errors

  • Phase errors in power amplifiers and cables

IU e-Cloud Feedback Workshop March 13, 2007

LA-UR-07-1613


Comb filters

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


Comb filters1

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


Results of comb filters

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

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

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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