Abstract
The free electron laser (FEL) oscillator is known to excite parasitic sideband wavelengths in addition to the fundamental wavelengths at high intracavity power. When a tapered wiggler is used, the sidebands reduce extraction efficiency. We studied an FEL cavity with one of the mirrors ruled as a diffraction grating. The diffraction grating deflects the light in the cavity selectively according to the wavelength: the sidebands can be eliminated by deflection to off-axis of the cavity. Because the grating mirror must be mounted with an angle, the laser pulse is stretched after diffraction. The pulse stretching reduces optical intensity. We studied the effect of pulse stretching numerically using a one-dimensional FEL code FELP. (FELP is a one-dimensional free electron laser code written by B.D. McVey, MS-E531, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.) Using this code, the free electron laser performance is calculated and compared with different line densities of grating for the following three wigglers: an untapered wiggler, a 12% wavelength tapered wiggler, and a wiggler tapered in wave number by 30%. The wavelength aperture caused by the grating also is studied as a function of line density.
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