Undulator Physics Requirementsand AlignmentHeinz-Dieter Nuhn, SLAC / LCLSApril 7, 2005 • Final Break Length Choice • Mitigation of AC Conductivity Wakefield Effects • Undulator Tolerance Budget Considerations • Cradle Component Arrangement and Alignment • Earth Magnetic Field Compensation • Radiation Damage Calculations
Undulator Break LengthsOld Strategy • Characteristic Lengths • Length of Undulator Strongback (Segment):Lseg = 3.4 m • Distance for 113 x 2p Phase Slippage:L0 = 3.668 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] • Fine Tuning of Initial Break Length • Suggested by N. Vinokurov based on Simulations by R. Dejus and N. Vinokurovusing 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
Undulator Break LengthsGINGER Simulation Summary • As undulator gets closer to construction phase lock-down of segment spacing is required. • R. Dejus requests checking of RON results with nonlinear FEL simulation codes before break length distances are being frozen. • Phase correction scheme was tested recently by Bill Fawley and Sven Reiche using non-linear FEL codes, GINGER and GENESIS, respectively. • With canted poles, phase corrections can be implemented with K adjustments rather than break lengths adjustments. • The simulations used changes in break length.
Undulator Break Lengths GINGER Simulations Time Domain Results • GINGER Simulations for three different applications of the Vinokurov/Dejus correction • Reduced [-0.045 m, -0.020 m, -0.005 m] • Nominal • Increased [+0.045 m, +0.020 m, +0.005 m] • Increased lengths produce slightly more power but no significant change in gain length. William Fawley
Undulator Break Lengths GENESIS Simulations Time Domain Results • GENESIS results comparable to those from GINGER. Sven Reiche
Undulator Break Lengths GINGER Simulations Spectrum • During linear regime uncorrected break pattern gives best results. • Towards end of undulator no significant effect of corrections • General outcome: No need for break in regular break pattern. • New break pattern will consist of 22 short and 10 long break lengths only. William Fawley
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 • Suggested by N. Vinokurov based on Simulations by R. Dejus and N. Vinokurovusing 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
Mitigation of AC-Conductivity Wakefield Effects • Change Vacuum Pipe Properties(See Bane talk) • Change Surface Material from Copper to Aluminum • Change Cross Section from Round to Oblong (10x5 mm) • Move to Low-Charge Operating Point(see Emma talk) • Use Tapering to Enhance Gain(see Huang talk)
Revisiting the Undulator Tolerance Budget • Separate budgets exist for undulator tolerances • Undulator Field Tuning • Segment Alignment • BBA • 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? Jim Welch
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
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
Cradle Component Arrangement and AlignmentProblem Description • BBA will only correct alignment of quadrupoles. • Undulator segment alignment is not affected. • Additional alignment strategy needed. Horizonal Before BBA Quad After BBA Vertical Before BBA After BBA
Cradle Component Arrangement and AlignmentSolution 1: Cradle Coupling (CLIC) • Coupling will be adjusted on appropriately designed setup in MMF. • Quadrupole motion during BBA will through coupled cradle motion. • Cradles will be aligned in the process. Horizonal Before BBA Quad After BBA Vertical Before BBA After BBA
Cradle Component Arrangement and AlignmentSolution 2: Downstream Monitor • Monitor downstream of the undulator, fiducialized to the strongback, will be used to correct undulator alignment after BBA. • Monitor could be RF Cavity BPM (either the one used for BBA or additional) or a pair of wire scanners. • Use of BBA BPM for the strongback alignment restricts freedom in BPM positioning. Horizonal Before BBA Monitor Quad After BBA Vertical Before BBA After BBA
Cradle Component Arrangement and AlignmentWinning Candidate • Monitor (wire scanner) upstream of the undulator fiducialized the strongback to control undulator alignment after BBA . • RF Cavity BPM for BBA mounted next to quadrupole. Horizonal Quad BPM Before BBA Beam Monitor After BBA Vertical Before BBA After BBA
Cradle Component Arrangement and AlignmentUndulator – to – Quad Tolerance Budget Individual contributions are added in quadrature See R. Ruland Talk for discussion
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 if necessary. • Compensate Earth Field Component in Undulator in Shimming Process • Scheduling Issues
Earth Magnetic Field CompensationSchedule Issues • Milestone Dictionary 07/03/2006 Delivery of Undulator 1st Articles to MMF (MS2_UN010)07/28/2006 27% Production Undulators Received (MS3_UN015)07/28/2006 MMF Qualified & Ready to Measure Prod Undulators (MS3_UN005)08/28/2006 MMF Qualified & Ready to Measure Prod Undulators (MS2_UN005)10/17/2006 50% Production Undulators Received (MS3_UN022)01/03/2007 75% Production Undulators Received (MS3_UN027)03/09/2007 Undulator Production Unit (33) Received (MS3_UN029)05/03/2007 Undulator Facility Beneficial Occupancy (MS3BO_035)07/02/2007 Undulator Facility Beneficial Occupancy (MS2BO_035)07/18/2008 Start Undulator Commissioning (1st Light ) (MS3_UN025)08/18/2008 Start Undulator Commissioning (1st Light ) (MS2_UN025) • 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.
Earth Magnetic Field Effect on Trajectory 0.1-Gauss Earth’s field in x-direction – perfect system, quads on, no steering 0.1-Gauss Earth’s field in x-direction – perfect system, after BBA Paul Emma
Earth Magnetic Field Effect on Trajectory 0.1-Gauss Earth’s field in x-direction – standard errors, after BBA no Earth’s field – standard errors, after BBA Paul Emma
Earth Magnetic Field Effect on Trajectory 0.2-Gauss Earth’s field in x-direction – standard errors, after BBA 0.1-Gauss Earth’s field in x-direction – standard errors, after BBA Paul Emma
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. • Will not affect definition of magnetic center of undulator (Standard Undulator Axis, SUSA, [see PRD 1.4-001 4.7]) Undulator Quad BPM Undulator Quad BPM Quad BPM Trajectory w/o Shim Shim Position Trajectory w/ Shim
Radiation Damage Calculationsfrom Inserted Screen • Question: Can OTR Screen be used in the LCLS undulator without causing significant radiation damage to the magnets? • Problem Setup and Initial FLUKA Simulations by A. Fasso (To be published as SLAC RADIATION PHYSICS NOTE)
Radiation Damage Calculationsfrom Inserted Screen • Longitudinal Distribution of Dose Deposited in Magnets (3) Alberto Fassò
Radiation Damage Measurements on NdFeB Fast-Neutron Fluence most likely source of damage. On-set of field change at 1014 n/cm2. Change in Intrinsic Remnant Induction from Fast –Neutron Irradiation in 252Cf Spectrum[Fig 16 from J. Alderman, P.K. Job, R.C. Martin, C.M. Simmons, G.D. Owen, J. Puhl, “Radiation-Induced Demagnetization of Nd-Fe-B Permanent Magnets,” APS Report LS-290 (2000)]
Radiation Damage Calculationsfrom Inserted Screen • Longitudinal and Vertical Distribution of Neutron Fluence (6a) [cm] [cm] [cm] [cm] n/cm2/electron n/cm2//day Maximum at 1013 n/cm2/day Alberto Fassò
Radiation Damage Calculationsfrom Inserted Screen • Damage clearly observed for neutron fluences of the order of 1014 n/cm2 byJ. Alderman, P.K. Job, R.C. Martin, C.M. Simmons, G.D. Owen, and J. Puhl, “Radiation-Induced Demagnetization of Nd-Fe-B Permanent Magnets,” APS Report LS-290 (2000) • Integrated levels of neutron fluences of 1014 n/cm2 would be reached after 10 days for 120 Hz, 1 nC, when keeping a 100 micron thick screen continuously inserted. • Integrated radiation doses can be strongly reduced under controlled operation at 10 Hz, .1 nC, with a 1-micron thick screen. This increases time to reach integrated fluence levels at continuous use to more than 300 years. • The planned occasional use of OTR screens should not present any problem.
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. • System for radiation damage calculations has been set up for FLUKA. Initial result look supportive for use of OTR screens.