A grating spectrometer for Raman free electron laser diagnostics

Abstract A grating spectrometer is developed for Raman free electron laser (FEL) diagnostics. Spectral measurements of a Raman FEL by a microwave grating spectrometer are presented. The aluminum reflection gratings with groove constant d = 7.5 mm, d = 3.5 mm and blaze angle ϑ = 30° are used. The spectrometer provides spectral coverage in the ranges of 6–10 mm and 2.2–4.8 mm with a resolution of 0.3 mm for an angle of incidence α = 60°. The experimental results show that the central frequency of the spectra decreases with increasing the wiggler magnetic field, which is in accordance with the theoretical predictions concerned with waveguide mode effects on the collective FEL. The spectrum of an FEL oscillator with distributed feedback cavity was measured. 3 mm FEL radiation using a small-period wiggler is obtained.