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Beam-Based Alignment Results. Henrik Loos, for the LCLS Commissioning Team. Undulator Trajectory Requirements. Beam through undulator rms 2 μ m per gain length Undulator with 33 segments total 100 m Not possible with conventional alignment

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Beam based alignment results

Beam-Based Alignment Results

Henrik Loos, for the LCLS Commissioning Team

Undulator trajectory requirements
Undulator Trajectory Requirements

  • Beam through undulator rms 2 μm per gain length

  • Undulator with 33 segments total 100 m

  • Not possible with conventional alignment

  • Use beam based alignment using set of different energies

  • RF BPM resolution < 1 μm

Undulator configuration
Undulator Configuration











Girder Movers

  • Undulator, Quad, BPM, BFW move with girder

  • Beam Finder Wire (BFW) retractable

  • Horizontal translation of undulator

    • Complete retract (80 mm)

    • Undulator K adjustment (± 5 mm)

Beam based alignment principle
Beam Based Alignment Principle

  • BPM offsets unknown

  • Magnetic fields (earth, quad kicks, etc.) unknown

  • Correct field integrals with quad offsets or correctors for dispersion free trajectory at BPM position

  • Trajectory between BPMs remains unknown

  • Measure trajectory at different energies to extrapolate to straight line at infinite energy

  • Fixed undulator quad fields

  • BPM position is BPM offset at infinite energy

Bba measurement schematic
BBA Measurement Schematic

















BPM Offsets Δbi

Quad Offsets Δqi


Bba procedure
BBA Procedure

  • Model beam position (yj) at BPMs as function of initial launch at 1st BPM (xi), quad offsets (Δqi), BPM offsets (Δbi)

  • y = [RxRqRb] [x’Δq’Δb’]’

  • Rxj = Rj1,1:2

  • Rqj = [R1,jend- R1jbeg … Ri<j,jend- Ri<j,j 0 … 0]11

  • Rb = -I

  • Fit solution for y arbitrary to adding linear function to quad and BPM offsets

  • Add constraint equations for quad or BPM offsets

  • 0 = ΣiΔqi and ΣiziΔqi for linear quad offset constraint

  • 0 = Δqi for minimum quad offset constraint

Bba implementation
BBA Implementation

  • Setup accelerator for one energy

  • Calculate response matrix for this energy

  • Measure N orbits at this energy and average

  • Repeat for all energies

  • Generate final matrix with separate launch parameters for each energy and selected constraints

  • Fit quad and BPM offsets and implement

  • Repeat BBA procedure

Bba simulation
BBA Simulation

Simulation Orbits

Simulation Fit Lin. Quad

Bba results 1 st run
BBA Results: 1st Run

  • First test: Energy range only 10 – 13.64 GeV

  • 50 orbits each, averaged

  • BPMs not well calibrated

  • Large oscillation in fit of quad offset, 1mm error bar

  • Assume BPM offset worse than quad offset

  • Apply instead constraint for minimal quad offset

  • Initial position rms 300 μm

Measured Orbit

Bba results 1 st run1
BBA Results: 1st Run

Fit with Linear Quad Constraint

Fit with Min. Quad Constraint

Applied this to BPM offsets

Bba results 2 nd run
BBA Results: 2nd Run

  • Energy range now 7 – 13.64 GeV

  • Still large ~1 mm oscillation on quad offset fit

  • Apply relaxed minimum quad constraint, 100 μm error bar

  • Orbits very similar after correction

  • Position rms ~50 μm after

Measured Orbit

Bba results 2 nd run1
BBA Results: 2nd Run

Fit with Min Quad Scale 20

Measured Orbit after Correction

Bba results 3 rd run
BBA Results: 3rd Run

Measured Orbit 4.3 – 13.64 GeV

Fit with Linear Quad Constraint

Bba results 3 rd run1
BBA Results: 3rd Run

Measured Orbit 4th Iteration

Fit with Linear Quad Constraint

Position rms 2 – 10 μm

Offset Error Bar 10 μm

Bba results best orbit
BBA Results: Best Orbit

  • Carefully calibrated BPMs

  • Energy range 4.3 – 13.64 GeV

  • 4 different energies

  • Undulator launch feedback on

  • Average position rms 1 – 2 μm

  • Betatron jitter ~20 μm

Bba results girder bump test
BBA Results: Girder Bump Test

58um bump @ 13.7 GeV &

-58um BPM offset

-48um bump @ 13.7 GeV

BBA procedure finds both quad offsets and BPM offsets

Typical bba after several months
Typical BBA After Several Months

Observe mostly changes in BPM offsets ~ 10 – 30um

Some quad & BPM offsets in end region of undulator from incremental

orbit corrections (retracting undulators, changing of taper)

Quad alignment measurement
Quad Alignment Measurement

Earth’s field effect

8 mm rms

undulators installed (with m-metal)

Measure quadrupole offset from beam axis

Vary quad magnetic field and fit offset to trajectory kick

Verifies earth field compensation from BBA

Z (m)

P. Emma

Bba user interface
BBA User Interface

Fit Options




Fast linac energy change
Fast Linac Energy Change

  • User interface to run an automated script

  • Block/unblock beam

  • Activate saved klystron configuration

  • Trim saved magnet configuration

  • Toggles feedbacks

  • Enables one BBA run in 10 - 15min (at best), ~2 – 4 h (worst)

Bba undulator taper
BBA & Undulator Taper

  • Orbit effects from undulator motion

    • No earth field shielding with retracted undulator

    • Undulator translation (~80 mm) shifts entire girder by ~10 - 100 um (quad & BPM)

    • Undulator field integral depends on taper

  • Goal

    • Straight trajectory for all undulator translations

  • Strategy

    • Do BBA at design taper, correct quad position

    • Compensate field integral change for different taper with corrector coil

    • Compensate girder shift for retracted undulator with corrector coils and BPM offset

Undulator field integral measurement
Undulator Field Integral Measurement

Apply 1st field integral to corrector coil


  • Achieved

    • BBA procedure successfully implemented

    • Converges to ~1 μm trajectory rms

    • Important to have full energy range

    • Errors on fitted quad offsets decreased from 1 mm to 10 μm with increasing energy range

    • Fast energy switching 15 min BBA possible

    • Complemented by measurement of quad offsets by varying quad strength

  • To Do

    • Fully automate energy change (Interface to energy management, orbit feedback in linac)

    • Study BBA at low charge (< 250 pC)

    • Implement orbit correction from undulator translation

    • Compare girder position from BBA with alignment diagnostic system (ADS)

    • Monitor and study BPM offset drifts