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Beam-Based Alignment Results

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Beam-Based Alignment Results

Henrik Loos, for the LCLS Commissioning Team

- 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

In/Out

BFW

Undulator

Quad

Corr

RF BPM

K

Beam

Scan

Girder

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)

- 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

Δb0

Δb1

Δb2

Δb3

Δb0

Δq1

Δq2

Δq3

E2

x,x’

E1

Δy0

Δy1

Δy2

Δy3

Δy4

BPM Offsets Δbi

Quad Offsets Δqi

E1<E2

- 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

- 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

Simulation Orbits

Simulation Fit Lin. Quad

- 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

Fit with Linear Quad Constraint

Fit with Min. Quad Constraint

Applied this to BPM offsets

- 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

Fit with Min Quad Scale 20

Measured Orbit after Correction

Measured Orbit 4.3 – 13.64 GeV

Fit with Linear Quad Constraint

Measured Orbit 4th Iteration

Fit with Linear Quad Constraint

Position rms 2 – 10 μm

Offset Error Bar 10 μm

- 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

58um bump @ 13.7 GeV &

-58um BPM offset

-48um bump @ 13.7 GeV

BBA procedure finds both quad offsets and BPM offsets

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)

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

Fit Options

Simulation

Measurement

Corrections

- 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)

- 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

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