Undulator K Value Temperature Dependence

LCLS undulator SN27 was used to measure the temperature dependence of its K value. The results are presented. A linear …t gives dK=K=dT = 4:3 10 4 1/C. This result is useful for correcting undulators in the tunnel whose temperature is di¤erent than the temperature used for tuning and …ducialization. 1 Introduction1 The LCLS tunnel has temperatures ranging from 19:6 C to 20:2 C along its length.2 The K values were set and the undulators were …ducialized at 20:0 0:1 C. Using the result presented below that dK=K=dT = 4:3 10 4 1/C, the temperature variations cause the K values to change by up to K=K = +2:2 10 4 to 1:3 10 4 from the …ducialized values. The tolerance limit3 is K=K = 1:5 10 . The temperature of each undulator is measured and the K value is corrected for its temperature dependence. This note discusses measurements which determined the correction factor. The correction is made by horizontally translating the undulator, using the e¤ect of the canted poles which makes the K value vary with horizontal position. Undulator SN27 was used for this study. We also determined whether the trajectories, phase, …eld integrals, etc. change with temperature and this data is presented. In the middle of the test, the air conditioner in our laboratory broke, damaging the undulator. Such damage has been seen previously.4 The temperature in the lab rose by 3 C for a day and a half, causing the damage. We are forced to break the measurements up into two data sets, before and after damage. Results are averaged over the data sets.