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

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 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)

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 Δq1 Δq2 Δq3 E2 x,x’ E1 Δy0 Δy1 Δy2 Δy3 Δy4 BPM Offsets Δbi Quad Offsets Δqi E1 < E2

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

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

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

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

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

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

Fit with Linear Quad Constraint BBA Results: 3rd Run Measured Orbit 4.3 – 13.64 GeV Fit with Linear Quad Constraint

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 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 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 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 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

BBA User Interface Fit Options Simulation Measurement Corrections

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)

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 ~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 Apply 1st field integral to corrector coil

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