A U.S. Department of Energy Office of Science Laboratory Operated by The University of Chicago Argonne National Laboratory Office of Science U.S. Department of Energy I. Vasserman LCLS First Undulator Prototype: Magnetic Measurements

Pioneering Science and Technology Office of Science U.S. Department of Energy 2 Argonne November 14, 2003 Outline Magnetic measurements: key measurements and issues Mechanical stability Temperature effects End phasing Summary

Pioneering Science and Technology Office of Science U.S. Department of Energy 3 Argonne November 14, 2003 Critical Tolerances (per undulator section ; undulator and quadrupole)/Achieved Phase slippage over the length of one undulator section Complex Amplitude of Radiation Trajectory walk-off  x , y 2m2m 0.5  m |A|/|A 0 | –12%0.1%  -2  n 10 deg yy50  m

Pioneering Science and Technology Office of Science U.S. Department of Energy 4 Argonne November 14, 2003 Magnetic Measurements (key measurements and issues) Coil measurements -Moving coil used as reference (especially for horizontal field) Field integrals (no multipole components) Hall probe measurements -Both vertical and horizontal field Fixed gap -Easier to tune field integrals and phase errors (no gap dependence) Small gap (~ 6.4 mm) -Shimming more complicated

Pioneering Science and Technology Office of Science U.S. Department of Energy 5 Argonne November 14, 2003 High magnetic field stability -Very precise measurements needed -Reproducibility of measurements must be << required precision of the field; ΔB eff /B eff ~ 1.5 x (~ 2 Gauss; ~ 1 μm gap change) -Earth field 0.15 Gauss --> 2.0 μm trajectory offset (requirement) -Obtaining the exact field is a challenge -To obtain the real trajectory: environmental field in the tunnel has to be taken into account Magnetic Measurements (key measurements and issues), continued

Pioneering Science and Technology Office of Science U.S. Department of Energy 6 Argonne November 14, 2003 Hall Probe: Horizontal Field Measurements Test of Hall probe horizontal field measurements using Sentron Hall probe done for Undulator A in 1998 showed good agreement with moving coil reference measurements. It means that planar Hall probe effect and cross talk between two sensors is not so big LCLS undulator is longer by 1 m and has stronger vertical field that exaggerates the errors of measurements. A test was done to check the horizontal field readings of a Sentron probe in presence of vertical field Results show that for small perturbations and small horizontal field this probe could be used for tuning but final trajectory measurements should be compared with reference done by moving coil

Pioneering Science and Technology Office of Science U.S. Department of Energy 7 Argonne November 14, 2003 Horizontal Field Nonlinearity vs. Vertical Field Measurements done with 2-axis probe in calibration magnet The angle  ~1 degree was introduced to have X- component of magnetic field B x =B y *  Hall probe sensitivity: 5V/T Cross talk is evident B x =B y * 

Pioneering Science and Technology Office of Science U.S. Department of Energy 8 Argonne November 14, 2003 Table II. Magnetic Measurement Parameters achieved Absolute Hall probe calibration accuracy0.5 Gauss Reproducibility: - Particle beam angle at exit and entrance 2.5 G-cm/0.001 mrad - Displacement at exit and entrance 3400 G-cm/ mm - B eff RMS error 0.15 Gauss - Phase error 0.02 degree

Pioneering Science and Technology Office of Science U.S. Department of Energy 9 Argonne November 14, 2003 Shimming Novel trajectory shims Phase shims Mechanical shims

Pioneering Science and Technology Office of Science U.S. Department of Energy 10 Argonne November 14, 2003 Shims ….phase (flat) and trajectory (side) Both types of shims Trajectory shims

Pioneering Science and Technology Office of Science U.S. Department of Energy 11 Argonne November 14, 2003 Temperature Dependence Accurate measurements of temperature dependence of B eff need to take into account temperature dependence of Hall probe Temperature dependence of recent Hall probes typically <10 -4 / C  -Two of three Sentron probes (S/N 157 and 367 at APS) have temperature dependence close to specified -Result of calibration for third Hall probe (S/N 409 using the APS calibration magnet) shows large deviation from vendor data -(ΔB eff /B eff )/ΔT appears to be 3.0 x /  C if Hall probe temperature dependence is neglected (one calibration file is used); -(ΔB eff /B eff )/ΔT = -5.5 x /  C when Hall temperature dependence is taken into account - ΔT ~ ± 0.3  C will result in ΔB eff /B eff requirement of 1.5 x ) Undulator end-phase corrections will relax the temperature stability requirement

Pioneering Science and Technology Office of Science U.S. Department of Energy 12 Argonne November 14, 2003 Hall Probe Temperature Calibration Coefficient was applied to the curve of deg to coincide with 23.2 degree The shape is close and one coefficient could be used for each temperature

Pioneering Science and Technology Office of Science U.S. Department of Energy 13 Argonne November 14, 2003 Time for undulator to reach thermal equilibrium Temperature response at downstream (D/S) and upstream (U/S) end of prototype core and nearby air in the magnetic measurement laboratory It takes ~one hour to stabilize the room temperature More than 24 hrs is needed for titanium core

Pioneering Science and Technology Office of Science U.S. Department of Energy 14 Argonne November 14, 2003 Mechanical Stability The prototype was removed from the bench and moved twice around APS storage ring. Prototype was aligned and measured at the bench before and after being moved B eff (Gauss) rms (Gauss) of Beff Before move (23.5  C) After move (23.6  C)

Pioneering Science and Technology Office of Science U.S. Department of Energy 15 Argonne November 14, 2003 Undulator End Phasing Full range is ±0.1 mm Implemented into the design Sensitivity to field deviations from one undulator section to the next can be made lower by using the end-gap correction system Remotely controlled at the sub- micron level PZT translator located at the end of the undulator to adjust the magnetic gap of the end section. This adjusts the phasing between undulators

Pioneering Science and Technology Office of Science U.S. Department of Energy 16 Argonne November 14, 2003 Undulator End Phasing (cont’d) End-phase corrections effect on the FEL performance -Calculations of complex amplitude of radiation amplitude -Simulations of beam bunching using code RON Measured phase versus end-gap change - Full range for one end ±0.100 mm is 0.16 period = ± 29°

Pioneering Science and Technology Office of Science U.S. Department of Energy 17 Argonne November 14, 2003 Undulator End Phasing (cont’d): Complex Amplitude of Radiation Complex amplitude of radiation, |A(L)| defines the intensity of radiation and is almost 100% compared to ideal case Ideal case -Regular part of the device: cosine-type field distribution versus z -Ends: from measured data Measured slippage length for 113 periods of phase was m at 6.35 mm gap (K value of 3.729) With two devices in a row, an error of 7x10 -4 in ΔB eff /B eff of the second device could be corrected by applying an end-phase correction of 24.5° from both ends of the device -Complemented by detailed RON simulations (R. Dejus) using random uniform distribution of K eff ; an error of up to ~ 10x10 -4 could be compensated by end phase correction

Pioneering Science and Technology Office of Science U.S. Department of Energy 18 Argonne November 14, 2003 Undulator End Phasing (cont’d): Complex Amplitude of Radiation Absolute value of complex amplitude of radiation versus z for two devices -Second device field changed by 7x Phase correction of 24.5° applied Correcting phase of upstream end of 2 nd undulator is important for maximizing length of vector (absolute value of radiation amplitude)

Pioneering Science and Technology Office of Science U.S. Department of Energy 19 Argonne November 14, 2003 Undulator End Phasing (cont’d): RON Simulations ΔB eff /B eff variation from undulator to undulator for 33 undulators: with and without end-phase 1.2 mm- mrad and 1.5 Ǻ ΔB eff /B eff = {3.5, 7.0, 10.5} x With end-phase corrections applied Curves from top to bottom: 1. Ideal case; 2-4: with corrections; 5-7: no corrections

Pioneering Science and Technology Office of Science U.S. Department of Energy 20 Argonne November 14, 2003 Conclusions Measurements and tuning were done The prototype met all stringent mechanical and magnetic tolerances after a few design changes and magnetic tuning Tuning time is about two days after the exact effective field is set Setting of exact effective field with accuracy better than 1.5x10 -4 is a challenge End-phase corrections of ±29° total range allows compensation of  B eff / B eff of ~ 8.2x10 -4 or ±1.5°C Lessons learned to simplify production - The biggest source of errors is variation of pole heights on assembled device. The tolerances for 1 st prototype were ±0.05 mm -Do not need to measure individual magnet blocks in the half- period fixture with Hall probe with such pole height errors -Mechanical tolerances of ±0.025mm for gap uniformity will facilitate the tuning