Numerical Studies of the Spectral Evolution of a Narrow - Bandwidth FEL Oscillator *

The design study of an FEL oscillator with a bandwidth of Aw/wo < 10has been carried out. In this conceptual design, a grating rhomb is used to suppress the sideband instability. The spectral evolution of the FEL has been simulated using a one-dimensional, time-dependent FEL code. The system parameters for the simulations are: electron beam energy 120MeV, current I = 50A, electron micropulse length St, = 330ps, optical wavelength A, = 0.5pm, and wiggler period AU, = 2cm. The detailed dependence of the spectral bandwidth on the frequency chirp introduced by the grating rhomb was investigated. A signal with FWHM bandwidth Aw/wo = 4. x 10' was obtained in a simulation using a large chirp parameter r,/6tp = 80, where -r is the time lag of the sideband (with Aw/wo = -0.01) behind the central frequency. *This work was performed for MIT Lincoln Laboratory under Air Force Contract F19628-90-C-0002.