1. Beam-Based Alignment Results
Henrik Loos, for the LCLS Commissioning Team

2. 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

3. Undulator Configuration
In/Out
Scan
Beam
Undulator
Quad
RF BPM
Girder
BFW
Corr
Girder Movers K
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)

4. 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

5. BBA Measurement Schematic
Δq1
Δq2
Δq3 E2
x,x’ E1
Δy0
Δy1
Δy2
Δy3
Δy4
BPM Offsets Δbi
Quad Offsets Δqi
E1 < E2

6. BBA Procedure
Model beam position (yj) at BPMs as function of initial launch at 1st BPM (xi), quad offsets (Δqi), BPM offsets (Δbi)
y = [Rx Rq Rb] [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 = Σi Δqi and Σi zi Δqi for linear quad offset constraint
0 = Δqi for minimum quad offset constraint

7. 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

8. Simulation Fit Lin. Quad
BBA Simulation
Simulation Orbits
Simulation Fit Lin. Quad

9. 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

10. BBA Results: 1st Run Fit with Linear Quad Constraint
Fit with Min. Quad Constraint
Applied this to BPM offsets

11. 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

12. Measured Orbit after Correction
BBA Results: 2nd Run
Fit with Min Quad Scale 20
Measured Orbit after Correction

13. Fit with Linear Quad Constraint
BBA Results: 3rd Run
Measured Orbit 4.3 – GeV
Fit with Linear Quad Constraint

14. BBA Results: 3rd Run Measured Orbit 4th Iteration
Fit with Linear Quad Constraint
Position rms 2 – 10 μm
Offset Error Bar 10 μm

15. BBA Results: Best Orbit
Carefully calibrated BPMs
Energy range 4.3 – GeV
4 different energies
Undulator launch feedback on
Average position rms 1 – 2 μm
Betatron jitter ~20 μm

16. BBA Results: Girder Bump Test
58um 13.7 GeV &
-58um BPM offset
-48um 13.7 GeV
BBA procedure finds both quad offsets and BPM offsets

17. 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)

18. Quad Alignment Measurement
Measure quadrupole offset from beam axis
Vary quad magnetic field and fit offset to trajectory kick
Verifies earth field compensation from BBA
Earth’s field effect
8 mm rms
undulators installed (with m-metal)
Z (m)
P. Emma

19. BBA User Interface
Fit Options
Simulation
Measurement
Corrections

20. 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 min (at best), ~2 – 4 h (worst)

21. Orbit effects from undulator motion
BBA & Undulator Taper
Orbit effects from undulator motion
No earth field shielding with retracted undulator
Undulator translation (~80 mm) shifts entire girder by ~ 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

22. Undulator Field Integral Measurement
Apply 1st field integral to corrector coil

23. Summary Achieved To Do 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