Lcls undulator alignment and motion review beam based alignment bba paul emma slac oct 21 2005
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LCLS. LCLS Undulator Alignment and Motion Review Beam-Based Alignment (BBA) Paul Emma, SLAC Oct. 21, 2005. Brief Review of Method Simulations Drift Tolerances. Motivation. For SASE FEL at 1.5 Å : Electron trajectory through undulator needs to be straight to <5 m m over ~10 m,

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Brief Review of Method Simulations Drift Tolerances

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Lcls undulator alignment and motion review beam based alignment bba paul emma slac oct 21 2005

LCLS

LCLS Undulator Alignment and Motion ReviewBeam-Based Alignment (BBA)Paul Emma, SLACOct. 21, 2005

  • Brief Review of Method

  • Simulations

  • Drift Tolerances

Paul Emma, SLAC


Motivation

Motivation

  • For SASE FEL at 1.5 Å:

    • Electron trajectory through undulator needs to be straight to <5 mm over ~10 m,

    • Traditional survey methods inadequate here,

    • BPM data acquired for several beam energies (14, 7.0, 4.5 GeV) can resolve this level,

    • Beam-based, energy-variation method is sensitive to all fields, not just misaligned quadrupole magnets

Paul Emma, SLAC


The method

j

ith BPM

DE = 0

quad offsets and/or pole errors

DE < 0

The Method

  • BPM readings, mi, written as sum of upstream kicks + offset, bi

  • Kicks are sensitive to momentum, pk, while offsets, bi, are not

bi > 0

s

Paul Emma, SLAC


The method1

...The Method

  • Reference line defined by incoming x0, x0 launch conditions

mi

linear only if Cij independent of p

offset = -bi

1/p

p

(15 GeV/c)-1

(7.0 GeV/c)-1

(4.5 GeV/c)-1

Paul Emma, SLAC


Schematic layout before bba cartoon example

quads

~300 mm

BPMs

LINAC

best final trajectory

UNDULATOR

(132 m)

steering elements

Schematic layout before BBA (cartoon example)

Undulator misaligned w.r.t. linac axis with uncorrelated and correlated* (‘random walk’) component

initial incoming launch error

x0

x0

* suggested by C. Adolphsen

Paul Emma, SLAC


Final trajectory after bba cartoon example

Final trajectory after BBA (cartoon example)

Beam is launched straight down undulator, with possible inconsequential kink at boundary

LINAC

dispersion generated is insignificant

Quadrupole magnets moved onto straight line and BPM offsets subtracted in software, while undulators track quadrupoles

Paul Emma, SLAC


Input errors used for simulation

100

200

100

200

0.04

0.5

Input errors used for simulation

Paul Emma, SLAC


Initial bpm and quad misalignments w r t linac axis

+ Quadrupole

positions

 BPM offsets

Initial BPM and quad misalignments (w.r.t. linac axis)

quad positions

BPM offsets

Now launch beam through undulator

Paul Emma, SLAC


Initial trajectory before any correction applied

+ Quadrupole

positions

e trajectory

oBPM readback

Initial trajectory before any correction applied

‘real’ trajectory

quad positions

BPM readings

Paul Emma, SLAC


After weighted steering prior to bba procedure

+ Quadrupole

positions

e trajectory

oBPM readback

After weighted steering – prior to BBA procedure

Paul Emma, SLAC


After 1 st pass of bba 13 6 gev

+ Quadrupole

positions

e trajectory

o BPM readback

After 1st pass of BBA (13.6 GeV)

sx 44 mm

Dj 3322°

sy 33 mm

Paul Emma, SLAC


Steering coils used for small final corrections

Steering coils used for small, final corrections…

Use steering coils for final iterations (quad move equivalent down to 0.5 mm)

BPM

BPM

7 mm

Paul Emma, SLAC


After 3 rd pass of bba 13 6 gev

+ Quadrupole

positions

e trajectory

o BPM readback

rms beam size: 36 mm

After 3rd pass of BBA (13.6 GeV)

sx 3.2 mm

Dj 98°

RON (FEL-code) simulation shows Lsat increased by <1 gain-length;

R. Dejus, N.Vinokurov

sy 2.5 mm

Paul Emma, SLAC


Run bba on 25 different random seeds

Run BBA on 25 Different Random Seeds

1-mm BPM resolution +

100-mm initial BPM & quad offsets

x & y mover distrib.

Djx,y

Paul Emma, SLAC


Fel code evaluation of bba simulation results

FEL code evaluation of BBA simulation results...

FEL Saturation Power at 1.5 Å

FEL Saturation Length at 1.5 Å

B. Fawley, H.-D. Nuhn, S. Reiche, PE

Paul Emma, SLAC


Alignment drift during bba procedure

Alignment drift during BBA procedure…

  • Quadrupole and BPM alignment may change during BBA procedure.

  • One iteration of BBA procedure will require <1 hr after full development and experience.

  • Simulate alignment drift during BBA, using 2.5-mm quad and BPM (separately) changes, which are uncorrelated 

Paul Emma, SLAC


2 5 m m uniform quad and bpm changes during bba procedure

+ Quadrupole

positions

e trajectory

o BPM readback

2.5-mm uniform quad and BPM changes during BBA procedure

Dj 167°

REQUIRE:

< 2 mm quad/BPM stability over 1 hr

Paul Emma, SLAC


Allow 5 m m uniform quad and bpm drift over long term 24 hrs

Allow 5-mm uniform quad and BPM drift over long term (24 hrs)

10 mm quad and BPM drift

After beam-based alignment

MICADO steering applied

Tolerance set at 5 mm over 24 hrs

Paul Emma, SLAC


Summary

LCLS

Summary

Alignment can be achieved at adequate level using beam-based technique, given that…

  • BPMs resolve trajectory to 1-2 mm rms

  • Quad positions and BPM readings ‘drift’ <2 mm over 1-hr procedure

  • Trajectory is stable to <20% of beam size (already demonstrated in FFTB)

  • BBA procedure repeated no more than once per week given 5 mm drift tolerance

Paul Emma, SLAC


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