Characterization of ALS undulator radiation—High K, taper, and the near field effect

The results of the measurements of spectral and angular distribution of the U5.0 (5 cm period) and U8.0 (8 cm period) Advanced Light Source (ALS) undulators are described. The spectra of on‐axis undulator radiation were measured for various values of the deflection parameter K. In particular, the high K case was studied, representing the region between the undulator and wiggler limits. Good agreement in harmonic peak flux and width is found between the measurements and the computations that include the real magnetic field, the electron beam energy spread, and the electron beam emittance. The effect of the undulator taper on the spectral characteristics of the radiation was analyzed. At small taper (∼0.5% gap variation) significant reduction in peak brightness and flux of the fifth harmonic was observed with small effect on the fundamental. The near field effect in the undulator radiation was studied by measuring the off‐axis spectral and angular distributions of the radiation. The broadening and the fine ...