Isaac Vasserman Magnetic Measurements and Tuning 10/14/ I. Vasserman LCLS Magnetic Measurements and Tuning

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Outline Magnetic measurements: key measurements and issues Fine adjustment of the field Tuning: trajectories, phase errors and break length Temperature effects Hall probe specifics Matching the devices K eff, Phase Summary

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Critical Tolerances (per undulator section )/Achieved Trajectory walk-off  x , y 2  m 0.5  m |A|/|A 0 | –12%0.1%  -2  n for one section 10 deg0.5 deg Phase slippage Complex radiation amplitude

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Magnetic Measurements (key measurements and issues) Coil measurements Moving coil used as reference (especially for horizontal field) Stretched wire (for vertical 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 (~ 6.8 mm) Shimming is more complicated

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ 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) Field offset error 0.15 Gauss --> 2.0 μm trajectory offset (equal to straightness requirements) 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

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Table II. Magnetic Measurement Parameters Achieved Absolute Hall probe calibration accuracy0.5 Gauss Reproducibility: - First field integral2.5 G-cm (absolute accuracy is ~40G-cm) - Second field integral3400 G-cm - B eff measurement RMS 0.15 Gauss - RMS phase error 0.02 degree

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Canted device cross section Cant is exaggerated Created by wedged spacers between aluminum base and titanium core Wedged spacer Titanium core

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Measured slope of 6.6 Gauss/mm agrees 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 for 3-mrad canted device

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ 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 (for spacers 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)  m/mm cant will be used in production, and the numbers shown above should be corrected accordingly

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Shimming Novel trajectory shims Phase shims Mechanical shims

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Trajectory shims Horizontal trajectory shims Vertical trajectory shims Vertical trajectory shims are non- adjustable magnetic blocks attached to the pole sides Horizontal trajectory shims are magnetic screws inserted into holes of the holder with large adjustment range depending on the distance to the pole side

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Measurement and tuning steps Alignment of device Alignment of Hall probe vertical position horizontal position Measurements and fine adjustment of the field Tuning: trajectories, phase errors and break length Tuning: matching the devices

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Measurement and tuning steps (cont) Alignment of Hall probe starts with alignment of horizontal field sensor so that the vertical component contribution is ~ 10 G (angle set with.001 accuracy) Vertical position of the vertical field sensor is adjusted by scanning it in the vertical direction. Fine adjustment is done by scanning along the Z- axis to define the center line where K eff has a minimum

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Measurement and tuning steps (cont) All tuning has to be done in a good field region of ±5 mm to allow remote tuning of K eff and phasing. Vertical position of the horizontal field sensor is adjusted by scanning it in the vertical direction to find where the measurement results are close to the reference measurements done by moving coil/or stretched wire. Trajectories are corrected by applying appropriate trajectory shims at the locations where trajectory angle correction is needed to provide the required trajectory straightness.

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Hall Probe: Horizontal Field Measurements Test of Hall probe horizontal field measurements using a Sentron Hall probe shows good agreement with moving coil reference measurements, which means that the planar Hall probe effect and cross talk between the two sensors are not so large. Results show that, for small perturbations and a small horizontal field, this probe could be used for tuning, but final trajectory measurements should be compared with a reference done by moving coil or stretched wire (see FEL2004 proceedings, TUPOS51). A few examples related to trajectory and phase-error tuning of the LCLS prototype are shown in the next slides.

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Electron Energy = GeV

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Phase correction Local field distortions do not increase phase errors as much as continuous long-range field distortions like linear or parabolic taper. Next slide shows the maximum field distribution for the LCLS prototype. Due to the large pole-to-pole gap variation, the RMS field error of this distribution is ~90 Gauss. Nevertheless the device performance is close to 100%.

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Magnetic field with taper RMS phase error is sensitive to taper: a) B y =B 0y -0.29*z; F=13.0 deg b) B y =B 0y +0.19*z; F=6.1 deg RMS field error is not so important and is close to each other in both cases. Nonlinear components of the taper have to be taken into account (see slide 20). a) b)

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Phase errors for prototype Very sensitive to linear taper: a) With linear taper, b) adjusted taper a) b)

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Simulation of phase errors for prototype Each half of the device can be assumed in the first approximation to being linearly tapered.

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Temperature Dependence Accurate measurements of temperature dependence of B eff need to take into account the temperature dependence of the Hall probe. Temperature dependence of recent Hall probes typically <10 -4 / C  Two of three APS Sentron probes (S/N 157 and 367) have a temperature dependence close to that specified Result of calibration for S/N 409 Hall probe (using the APS calibration magnet) shows large deviation from vendor data and close to prototype field temperature dependence with opposite sign (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 Temperature change of ΔT ~ ± 0.3  C results in ΔB eff /B eff change of 1.5 x 10 -4

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Hall Probe Temperature Calibration Coefficient was applied to the curve of deg to coincide with 23.2 deg The shape is close so one coefficient could be used for each temperature

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ 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

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Undulator segments alignment Alignment is crucial to the project to provide the overlap of particle beam and radiation Magnetic needles were proposed to find the reference between magnetic center of undulator segment, defined by the Hall probe, and some reference base points

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Magnetic needles for alignment One needle is enough for alignment in the X direction One more needle has to be added at Y=0 for alignment in the Y direction

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Magnetic needle location definition in X Accuracy of calibration < 50 µm (limited by encoder resolution) Needle is in vertical position one needle used

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Magnetic needle location definition in Y The zero in Y is the vertical magnetic center of the undulator. B y =0 at the vertical position of the needle Accuracy of calibration < 5 µm. This type of graph will be used to determine distance of needle from device magnetic center one needle used Needle is in horizontal position

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Hall probe in zero field chamber Hall probe measurement absolute accuracy is limited by zero drift and noise

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Phasing of the devices Break length correction can be done using phase shims at the ends of the devices 3*2 phase shims (upper and lower jaws) will be initially installed at each end to allow for tuning of the phase in both directions by adding (to decrease the phase) or removing (to increase the phase) phase shims Another way of tuning the phase between devices is shown in the next two slides

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ |A(z)| vs. arg(A(z)) in polar coordinate system. A is complex radiation amplitude Phase between devices is not optimal. Performance is 97.5%. Close to requirements with no break-length correction.

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Correction of K eff to compensate for phase Performance is 99.2%, close to perfect. No break length correction. Small slippage inside the devices does not affect the performance

Isaac Vasserman Magnetic Measurements and Tuning 10/14/ Summary Details of undulator tuning are well understood Main challenge is very tight time schedule All software and hardware should be ready well in advance to have time for debugging, teaching of the responsible personnel and providing the necessary expertise