Beam Tests of DFS & WFSat FACET

Andrea Latina, J. Pfingstner, D. Schulte, D. Pellegrini (CERN), E. Adli (Univ. of Oslo)

With the help of F.J. Decker, and N. Lipkowitz (SLAC)

AWLC 2014 – Fermilab – May 14, 2014

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Overview

• Motivations and objectives

• Summary of the results

• Analysis of the results

• Conclusions

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Beam-based alignment testsWe proposed automated beam-steering methods to improve the linac performance by correcting orbit, dispersion, and wakefields simultaneously.

Our technique is:• Model independent• Global• Automatic• Robust and rapid

We base our algorithms operate in two phases: automatic system identification, and BBA

It is a considerable step forward with respect to traditional alignment techniques.

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Recap on Dispersion-Free Steering and Wakefield-Free Steering

• DFS: measure and correct the system response to a change in energy

(we off-phased one klystron either in sectors S02 or in S04, depending on the case)

• WFS: measure and correct the system response to a change in the bunch charge

(this time we used 70% of the nominal charge, 2e10 e- and 1.3e10 e-)

Recap of the equations

Simulation: WFS weight scanSimulation: DFS weight scan

woptimal = ~40

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The SLAC linac

(*) Emittace measurements:• S02: 7 wires (only 5 used)• S04: quad-scan (1 wire)• S11: 4 wires (only 3 used)• S18: quad-scan (1 wire)

• Divided in 100m long sectors• Energy = from 1.19 GeV to 20.3 GeV• Bunch length = from 1.0-1.5 mm in S02 to 20 μm in S20• Nominal charge = 2e10 e- (test charge = 1.3e10 e-)• Nominal emittances: X = 2.5 x 10-5 m ; Y = 0.2 x 10-5 m

Orbit feedbacks (slow):• S03-04, S06, S11, S15: orbit correction• S09, S17-18: energy correction

* * * *

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Overview of the tests performed

2013:1) Dispersion-Free Steering

in sectors LI04 – LI08

2014:2) Wakefield-Free Steering

in sectors LI02 – LI04

3) Wakefield-Free Steering and Dispersion-Free Steering simultaneouslyin sectors LI02 – LI04

4) WFS and DFS over longer sections of the LINACin sectors LI05 – LI11

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March 2013: Tests of DFSSectors LI04 thru LI08 (500 meters of Linac)• 52 correctors and 52 bpms (one every two)• Dispersion created off-phasing one klystron in sector LI03 by 90o

Dispersion got reduced by a factor 3-4 in X and Y

8Before correction After 3 iterations

Incoming oscillation/dispersion is taken out and flattened; emittance in LI11 and emittance growth significantly reduced.

After 1 iteration

S19 phos, PR185 :

March 2013: DFS and Emittance Reduction

Emittance at LI11 (iteraton 1)X: 43.2 x 10-5 mY: 27.82 x 10-5 m

Emittance at LI11 (iteration 4)X: 3.71 x 10-5 m Y: 0.87 x 10-5 m

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March 2014: Tests of WFS in LI02-04

• We measured the wakefield effects by using a test beam with 80% of the nominal value

• We used all correctors and all bpms.

• Notice: The wakefield is measured as orbit distortion due to the difference in bunch charge

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Tests of WFS in LI02-04, March 2014Vertical Wakefield orbit = Y_test_charge – Y_nominal

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Tests of WFS in Sectors LI02-04Horizontal Wakefield orbit = X_test_charge – X_nominal

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Tests of WFS in Sectors LI02-04WFS convergence plot.

Apply WFS with optimal weight=40.

Emittance at start of our shift was:X = 2.79 / 1.07 x 10-5 mY = 0.54 / 1.12 x 10-5 m

Emittance after correctionX = 3.38 / 1.01Y = 0.12 / 1.16 ; 0.17 / 1.20

Nominal emittances should beX = 2.5 x 10-5 mY = 0.2 x 10-5 m

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WFS weight scanWeight scan vs. emittance. We tried w = 4, 40, 160, 400.

From simulation, one expects something like the black line in the plot:

Vertical emittance measured in sector 04 (quad scan)-w = 0 initial vertical emittance: 0.56 / 1.10-w = 4, vertical emittance = 0.36 / 1.63-w = 40, vertical emittance = 0.12 / 1.16 (re-measured: 0.17 / 1.20)-w = 160, emittance not measurable -w = 400, emittance not measurable

Conclusion:• Emittance scan gives expected results• No time for measuring more points

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Sometimes in Sectors LI02-04First test of combined test of DFS+WFS. Notice machine “hiccups”.

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Sometimes in Sectors LI02-04Test of DFS: LI02-LI04. Divergence.gain = 0.5svd = 0.7wdfs = 40

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Tests of simultaneous DFS + WFS in LI05-LI11Problems:• Very unstable machine

• Damping ring extraction kicker• NRTL energy jitter• Earthquake ?

• Initial config problems with scavenger line (3h to recover)

Emittance at shift start:- X = 4.186 / 1.1- Y = 0.445 / 1.06

Emittance 6h later, before applying BBA- X = 11.21 / 1.19- Y = 0.91 / 1.12

Emittance after correction:- X = 9.50/1.04- Y = 1.06/2.40 (improvement in X)

Not conclusive

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Tried a few interesting things:1) simultaneous X and Y correction2) with all coupled information3) re-measurement of the golden orbit after 5 or 6 iterations, to update the

reference for the orbit correction, y0

Emittance Y:--> from 1.58 x 10-5 m vertical emittance before correction4) down to 0.50 after few iterations of fully coupled correction5) to further 0.40 after resetting the target orbit during the correction

(equivalent to correcting without orbit constraint)

Further tests in Sectors LI05-11Extra beam-time

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Analysis

• Try to understand divergence in simulation (see next slides)

• Stability analysis proved that the choice of gain, g was correct, and that the system is stable even in presence of potential corrector errors:– bn : bpm readings at iteration n

– δn: relative correction– R: ideal response matrix– R tilde: erroneous matrix representing eventual corrector erroes

if the absolute value of all eigenvalues of (I-gRR) < 1, the system is stable

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Singular Values, DFS+WFS, w=40

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Correcting a simulated LINACwith the measured response matrices

…including:

• Injection jitter• Misalignments• BPM resolution error (3 microm)• Transverse and Longitudinal Wakefields

Picking N progressive singular values at time

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Correction using N=2 singular values

norm_OrbitX = 8.13995norm_OrbitY = 25.8351norm_DispX = 1.29383norm_DispY = 2.99051norm_WakeX = 0.905165norm_WakeY = 1.17392

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N=3 singular valuesnorm_OrbitX = 10.3687norm_OrbitY = 22.6852norm_DispX = 1.53973norm_DispY = 3.02105norm_WakeX = 0.729164norm_WakeY = 0.998268

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N=4 singular valuesnorm_OrbitX = 7.61432norm_OrbitY = 19.1609norm_DispX = 1.03749norm_DispY = 1.40887norm_WakeX = 0.50546norm_WakeY = 0.734156

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N=5 singular valuesnorm_OrbitX = 6.72384norm_OrbitY = 22.656norm_DispX = 0.811785norm_DispY = 1.34545norm_WakeX = 0.380037norm_WakeY = 0.869225

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N=6 singular valuesnorm_OrbitX = 7.31326norm_OrbitY = 23.0469norm_DispX = 1.04246norm_DispY = 1.38634norm_WakeX = 0.435169norm_WakeY = 0.917698

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N=7 singular values

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Singular Values, DFS+WFS, w=40

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FACET-specific problems

• The response matrix measurement is very slow– Takes ~2 hours for 48 correctors / 1 matrix

• Large jitter in the horizontal axis makes the X axis harder– Damping ring extraction kicker– RF system of NRTL bunch compressor

• Machine “hiccups“, LEM– LEM (linac energy management) http

://www.slac.stanford.edu/grp/ad/op/LEM/index.shtml– Impact to be studied

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Speeding up the response matrix measurement

1) While measrung the response of dispersion in S02-S042) Optimize speed in measurements3) Test a feed-forward system to stabilize the orbit during correction

Worked with Nate Lipkowitz to speed up the system identification procedure.

Overall 30% speed up measured

Time required to set corrector and read bpms

SPEED UP ACCOMPLISHED.

Still quite slow.

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New tools developed“CERNBBA” Tools:

(top) System Identification(bottom) Beam-Based Alignment

Tests foreseen at Fermi (Elettra) and ATF2 (KEK), …

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Conclusions and future plans• Applying DFS and WFS, the vertical emittance got reduced almost

systematically• Horizontal axis more difficult

• Sometimes observed instability/divergence:• Might be related to noise in the measurement of the response

matrices (counteracted with SVD cuts)• Tests of convergence showed that the matrices are not ill-

conditioned

• We are pursuing tests at other facilities (Fermi in Trieste, ATF2)• We will learn a lot from these tests

• Further tests at FACET should surely be envisaged• Need to speed up the system identification phase

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Extra

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Shift 4 – Sunday – Sectors LI05-11

Test of DFS+WFS followed by WFS only• Iteration 1-7 (including): DFS+WFS

• corresponding to previous plot blow)• Iteration 8-10 (including): drift (gain=0)

• corresponding to previous plot blow)• Iteration:11-18 (including): WFS (setting DFS gain to 0)• Iteration 13: some kind of machine hickup (not identified). Algorithm recovers afterwards• Emittance non measureable in Y – we stopped

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Response 0: nominal orbit

X Y

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Dispersion response: R1-R0

Wakefield response: R2-R0X Y

X Y

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Singular values for X and Y

2 very large singular values – we need to understand what they do represent

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Response 0: rms jitter vs max excitation

38Removed vertical BPM 46

Response 1: rms jitter vs max excitation

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Response 2: rms jitter vs max excitation

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