High-Resolution Undulator Measurements Using Angle-Integrated Spontaneous Radiation

The Linac Coherent Light Source (LCLS) is a fourth-generation light source. Its proper operation requires a stringently controlled undulator field. The tolerance for the field parameter K is less than 1.5 × 10^-4for all thirty-three undulator segments totaling 112 meters. The fluctuation of the electron energy (∼ 0.05%) presents a serious challenge to measurement techniques based on electron or x-ray beams. We show that the steep spectral feature in the angle-integrated undulator radiation can be used to measure the field of the undulator one segment at a time. With simultaneous measurement of electron beam charge and energy shot-by-shot, the K-parameter can be determined with required accuracy. We also propose a differential measurement technique that makes use of the radiation intensities from two undulator segments. When the x-ray beams emitted from the two undulator segments are separated but allowed to pass through the same monochromator, the two beam intensities will change almost identically with the change of many electron beam parameters. As a result, the intensity difference becomes a very sensitive and reliable measure for the difference in the fields of the two undulators. Numerical simulations show that a resolution in the range of ΔK/ΔK/ΔK/K ∼ 10^-5can be achieved.