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 Recent Measurements of the LCLS Prototype Undulator: Canted Pole Gap Design and Hall Probe Center Calibration LCLS UNDULATOR SYSTEM REVIEW March 3 - 4, 2004

Pioneering Science and Technology Office of Science U.S. Department of Energy 2 Argonne March, Outline Magnetic measurements vs. X (horizontal motion) of canted pole gap design (3 mrad cant) -Effective magnetic field -RMS phase error -Trajectory Fringe Fields Fine adjustment of effective magnetic field End gap correction (is it needed)? Hall probe center calibration Summary

Pioneering Science and Technology Office of Science U.S. Department of Energy 3 Argonne March, Measured slope of 6.6 Gauss/mm agree with calculations (~ 5.7 Gauss/mm for 3 mrad cant) Alignment accuracy needed for  B/B ~ 1.5x10 -4 ~ 2 Gauss -> 0.3 mm Effective magnetic field

Pioneering Science and Technology Office of Science U.S. Department of Energy 4 Argonne March, RMS phase error No significant dependency on X An RMS phase error of ~ 6.5 degree is an upper limit for near- perfect (~100%) performance

Pioneering Science and Technology Office of Science U.S. Department of Energy 5 Argonne March, Horizontal trajectory (averaged over period length) at 14.1 GeV Trajectory vs. X well behaved and well within the tolerance requirement of 2  m maximum walk-off from a straight line Operational range is ±1.2 mm for ±1.0°C temperature compensation

Pioneering Science and Technology Office of Science U.S. Department of Energy 6 Argonne March, Fringe fields at X=65 and 100 mm Fringe fields with new shims are close to earth field for X=100 mm. (Earth field contribution to trajectory shift has to be corrected.)

Pioneering Science and Technology Office of Science U.S. Department of Energy 7 Argonne March, Fine adjustment of effective magnetic field (Procedure to tune the field) 1. Select spacers with thickness step ~ 15 µm to set the effective field in the range of ±30 Gauss (1 µm in gap corresponds to ~ 2 Gauss in field). 2. Set horizontal position of spacers to adjust the effective field to ~ ±6 Gauss (spacers are wedged with 3  m/mm cant) 3. Set horizontal position of the undulator as a whole so that the effective field is in the range ±2 Gauss (  B/B ~ ±1.5x10 -4 ) (This step is required to save time and to provide better accuracy) 4. The undulator horizontal position could be remotely controlled during operation to compensate for in-tunnel temperature variations (motion of ±1.2 mm for ±1°C needed). Such option is available, if quadrupoles are separated from undulator sections

Pioneering Science and Technology Office of Science U.S. Department of Energy 8 Argonne March, End gap correction (is it needed)? Slippage length L = u (1+K 2 eff /2) -> change in slippage period in free space is the main contributor to break length errors (if K eff is adjusted to compensate for particle energy loss) Tolerance criterion for phase error between undulator segments is ~ 10 . It corresponds to  L/ L~ u * K eff *  K eff /L= 10/360, from where  K eff /K eff ~ 1.6% for minimum break length. Real break length error depends on actual particle energy loss and break length chosen. This error will accumulate over distance. Computer simulations are required to examine this case. Remotely-controlled end gap correction for final part of undulator line may be useful. (Or adjustment of break lengths can be done in advance, if reliable energy loss data will be available.)

Pioneering Science and Technology Office of Science U.S. Department of Energy 9 Argonne March, Magnetic needles for alignment Only one needle is required for alignment in the X direction One more needle has to be added at Y=0 for alignment in the Y direction

Pioneering Science and Technology Office of Science U.S. Department of Energy 10 Argonne March, Hall probe center calibration in X Accuracy of calibration < 50 µm (limited by encoder resolution) one needle used

Pioneering Science and Technology Office of Science U.S. Department of Energy 11 Argonne March, Hall probe center calibration in Y Accuracy of calibration < 5 µm. This procedure will be used to define distance of needle (By=0) from device magnetic center one needle used

Pioneering Science and Technology Office of Science U.S. Department of Energy 12 Argonne March, Summary A 3 mrad pole gap cant was chosen and tested, using wedged spacers between the aluminum base plates and the titanium core Magnetic measurements show good agreement with calculated change of the effective magnetic field versus X (horizontal motion). No significant change of the RMS phase error versus X was measured, hence no degradation of FEL performance is expected. Additional flexibility of gap correction is provided by shifting the spacers in the X direction so that the effective magnetic field can be set to the required tolerance (  B/B ~ 1.5x10 -4 ). This can be achieved even at X=0 by moving the spacers.

Pioneering Science and Technology Office of Science U.S. Department of Energy 13 Argonne March, Summary, cont’d Additional advantage of the canted pole gap design is the easy of introducing a tapered field by moving the ends of the device in opposite directions in X during tuning. (This is impossible for other fixed-gap devices.) A disadvantage of this option is the necessity to provide a separate support for vacuum chamber Estimate of particle energy loss shows that remotely-controlled end corrections are not needed for the regular part of the undulator line. Computer simulations will be required to investigate the necessity to apply this correction to the final part of the undulator line. Magnetic needle tests confirmed that this feature is an effective alignment tool. The canted pole gap design looks very promising and appears to be the best choice to proceed for fine adjustment of the effective magnetic field