Undulator Physics: Considerations and Commissioning

1. Undulator Physics Considerationsand CommissioningHeinz-Dieter Nuhn, SLAC / LCLSMay 10, 2005 • Final Break Length Choice • AC Conductivity Wakefields • Undulator Tolerance Budget • Cradle Component Arrangement and Alignment • Commissioning Plan - 1 -

2. 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 choicesLn = 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 -

3. 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 -

4. 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 -

5. 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 -

6. 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 -

7. Cradle Component Arrangement and AlignmentProblem 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 -

8. 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 -

9. 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 -

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

11. 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 -

12. Earth Magnetic Field CompensationStrategy • 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 -

13. Earth Magnetic Field CompensationAdjustable 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. Undulator Quad BPM Undulator Quad BPM Quad BPM Trajectory w/o Shim Shim Position Trajectory w/ Shim - 13 -

14. 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 -

15. 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 -

16. 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 BunchMicro-Bunching - 16 -

17. 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 -

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

19. 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 -

20. 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 -

21. End of Presentation - 21 -