THE EFFECT OF TRANSVERSE-MODES IN A WAVE-GUIDE RESONATOR ON THE RESONANCE CONDITION OF A COMPTON FEL

In a free electron laser where the resonator has a wave-guide structur e the phase velocity of the light has a significant impact on the wave length of the output radiation. This effect is well known in Raman-typ e FEL's, It is however less known that this effect can also play an im portant role in Compton-type FEL's. This effect has been experimentall y verified in our 6 MeV Compton-type FEL, having a wave-guide resonato r with a hole in the outcoupling mirror. Successive transverse modes, which have distinct phase velocities, show up at different wavelengths in the output spectrum. The difference appeared to be as large as 5% of the resonance wavelength. In normal lasers this difference is not m ore than about 0.1%.