Undulator Physics Considerations and Commissioning Heinz-Dieter Nuhn, SLAC / LCLS May 10, 2005 Final Break Length Choice AC Conductivity Wakefields Undulator Tolerance Budget Cradle Component Arrangement and Alignment Commissioning Plan - 1 -

Undulator Break Lengths (Old Strategy) New Strategy Characteristic Lengths Length of Undulator Strongback (Segment): Lseg = 3.4 m Distance for 113 x 2p Phase Slippage: L0 = (3.668 m) 3.656 m Distance for 2p Phase Slippage in Field Free Space: Linc = lu (1+K2/2) = 0.214 m Standard Break Lengths Used Use parameter n to characterize different phase length choices Ln = L0 - Lseg +(n-1)Linc Use 2 Short Breaks Followed by 1 Long Break in n-Pattern 2 – 2 – 4 ([0.482 m – 0.482 m – 0.910 m]) [0.470 m – 0.470 m – 0.898 m] Fine Tuning of Initial Break Length Based on Simulations using Linear Simulation Code, RON Small length increases for first 3 break lengths [0.045 m – 0.020 m – 0.005 m] Total Undulator Length (from beginning of strongback 1 – end of strongback 33): Lund = (131.97 m) 131.52 m Recent GINGER, GENESIS simulations found no significant benefit. - 2 -

AC-Conductivity Wakefield Effects AC-Conductivity aspect of Resistive Wall Impedance Was not included in calculations until 2nd half of 2004 Adds significant dependence on chamber material (Al is better then Cu) Problem Analysis Wakefield Theory Analysis (K. Bane) [K.L.F. Bane, G. Stupakov, “Resistive wall wakefield in the LCLS undulator beam pipe,” SLAC-PUB-10707, October 2004] Start-To-End FEL Simulations (W. Fawley, S. Reiche) Linear Regime Tapering Theory (Z. Huang, G. Stupakov) [Z. Huang, G. Stupakov, “Free Electron Lasers with Slowly-Varying Beam and Undulator Parameters,” SLAC-PUB-10863, December 2004] Reflectivity Measurements Sample Preparation (D. Walters) Reflectivity Measurements (Jiufeng Tu, CCNY) Low-Charge Operating Point Development (P. Emma) - 3 -

Mitigation of AC-Conductivity Wakefield Effects Mitigation Strategy Change Vacuum Pipe Properties Change Surface Material from Copper to Aluminum Change Cross Section from Round to Oblong (10x5 mm) Move to Low-Charge Operating Point (200 pC) Use Tapering in Linear Regime to Enhance Gain (200 – 300 kV/m) Bottom Line Goal photon intensity will be reached or exceeded Overall pulse length shorter for 200 pC operation (~ 50%) - 4 -

Revisiting the Undulator Tolerance Budget Separate budgets exist for undulator tolerances Undulator Field Tuning/Segment Alignment/Optics Matching BBA Temperature Stability Floor Stability A Monte Carlo model is being developed which simultaneously includes all of the above errors Calculates the cumulative phase error with MC statistics Shows the relative importance of different tolerances Next step is to test putative tolerance budgets against FEL code, including beam tolerances. Answer the question: For a give overall tolerance budget, what is the probability that the FEL flux will be above 1012 photons/pulse? - 5 -

Undulator Segment Alignment Tolerance Based on K Tolerance K depends on vertical distance from mid-plane. Canted poles make K also dependent on horizontal position Tolerance Amplitudes Horizontal +/- 180 microns Vertical +/- 70 microns - 6 -

Cradle Component Arrangement and Alignment Problem Characterization Two-Fold Problem for Segment Alignment Initial installation and alignment to a straight line Alignment maintenance in the presence of ground motion Two Strategies under Consideration Cradle Coupling (Train-Link) Upstream-Downstream Beam Position Monitors - 7 -

1: “Monitoring System” Train-link Downstream quad fiducialized to undulator ends BBA facilitates alignment of downstream cradle end and straightens electron beam A combination of measurements using the portable stretched wire device and the portable HLS could be used to determine “loose” end offset Monitoring System WPM and HLS provide real-time cradle position information Info can be used as feed-back for mover system to maintain initial alignment Before any BBA or HLS / Stretched Wire Alignment performed Quad Undulator Strongback After BBA: Quad, BPM and one end of undulator aligned Cradle RF BPM After HLS / Stretched Wire Alignment: Both ends of undulator aligned Beam R. Ruland - 8 -

2: Additional Upstream Monitor Downstream quad and upstream monitor fiducialized to undulator ends BBA facilitates alignment of downstream cradle end and straightens electron beam Absolute zero offset reading of upstream monitor to determine and correct “loose” end offset Considered candidates for Upstream Monitor: 2nd RF BPM or Scan Wire Monitoring System WPM and HLS provide real-time cradle position information Info can be used as feed-back for mover system to maintain initial alignment Before any BBA performed Quad Undulator Strongback After BBA: Quad, BPM and one end of undulator aligned Cradle RF BPM Upstream Monitor After centering of Upstream Monitor: Both ends of undulator aligned Beam - 9 -

Cradle Component Arrangement and Alignment Undulator – to – Quad Tolerance Budget Individual contributions are added in quadrature See R. Ruland Talk for discussion - 10 -

Earth Magnetic Field Effect on Trajectory no Earth’s field – standard errors, after BBA 0.1-Gauss Earth’s field in x-direction – standard errors, after BBA 0.2-Gauss Earth’s field in x-direction – standard errors, after BBA Paul Emma - 11 -

Earth Magnetic Field Compensation Strategy Earth Magnetic Field along Beam Trajectory in Undulator requires compensation. Estimated strength 0.43±0.06 Gauss : (0.18±0.03, -0.38±0.07,0.08±0.05) Gauss based on Measurements by K. Hacker. (see LCLS-TN-05-4) Compensation Strategy: Position the Undulator on Magnetic Measurement Bench in same direction as in Undulator Tunnel. Add correction field (Helmholtz Coils), if necessary. Compensate Earth Field Component in Undulator in Shimming Process Scheduling Issues : Undulator Hall Beneficial Occupancy occurs 2 months after 33rd undulator is received. Undulator Hall Magnetic Field can not be measured before tuning of most of the undulator segments is complete Risk that field found in undulator hall is different from field used during shimming. Tolerance for error field is 0.1 G. - 12 -

Earth Magnetic Field Compensation Adjustable Shim Concept Risk arises from the lack of precise knowledge of the earth field in the tunnel at the time of undulator segment tuning. Considering mitigation strategy based on use of a small number of precisely adjustable shims along each undulator. One extra shim per segment will reduce phase error by factor 4. Shims could be installed before undulator tuning, but adjusted before undulator installation when field errors have been determined. Quad BPM Undulator Quad BPM Undulator Quad BPM Trajectory w/o Shim Shim Position Trajectory w/ Shim - 13 -

Undulator Commissioning Plan Overview Pre-Beam Checkouts Conventional Alignment Control System Checkout Undulator Motion Control Checkout Magnet Polarity Checkout Commissioning with Beam LTU Commissioning to Tune-Up Dump First Beam through Undulator Vacuum System (All Undulator Magnets Rolled-Out; Quads could initially be turned off) BBA Commissioning with Undulator Magnets Rolled-Out First Beam through Undulator Magnets BBA Commissioning with Undulator Magnets inserted XTOD Diagnostics Commissioning (with one or more Strongbacks inserted) Spontaneous Radiation Characterization up to full energy and charge range FEL Radiation Characterization at 15 Angstrom FEL Radiation Characterization stepwise towards shorter Wavelengths Transition to Operation - 14 -

Undulator Radiation Protection Considerations Undulator Radiation Protection Greatest MP Concern during Commissioning Sources for Undulator Radiation Damage Upstream of Undulator Beam Halo (Emittance, Energy Spread)  Collimators Energy Errors  Collimators Steering Errors  Collimators Power Supply Failures  Collimators, Interlocks Inside Undulator Chamber Alignment Error  Single Shot Steering Errors  Interlocks (SP, BPM, Radiation) OTR Screen  Restricted Use (Automatic Monitoring and Interlocks) Wire Scanner  Restricted Use (No Problems Expected) Power Supply Failures  Interlocks (Radiation) - 15 -

FEL Measurements Desirable measurements as function of position along undulator : Intensity (LG, Saturation) Spectral distribution Bunching Total energy Pulse length Spatial shape and centroid Divergence Saturation Exponential Gain Regime Undulator Regime 1 % of X-Ray Pulse Electron Bunch Micro-Bunching - 16 -

Alternative: End-Of-Undulator Diagnostics Characterize x-ray beam at single station down stream of undulator Solid Attenuator Gas Attenuator Direct Imager Indirect Imager Spectrometer Turn-Off Gain at Selectable Point Along Undulator by Introduction of trajectory distortion Roll-out of individual undulator segments - 17 -

Measurement of SASE Gain with Trajectory Distortion Quadrupole Displacement at Selectable Point along Undulator GENESIS Simulations by Z. Huang - 18 -

Measurement of SASE Gain Using Rollout Undulator Segments can be removed by remote control from the end of the undulator. They will not effect radiation produced by earlier segments. - 19 -

Conclusions Break lengths structure simplified and finalized. AC conductivity risk can be mitigated. (Al, Oblong Cross-Section, Gain Tapering) Fine tuning of undulator tolerance budget is underway. Cradle component arrangement issues are being addressed. Mitigation for insufficient knowledge of earth field component inside undulator hall is under investigation. The undulator commissioning plan for the LCLS is under development. x-ray diagnostics located down-stream of undulator. - 20 -

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