HIGH-POWER SPIKE PULSES EMITTED FROM A MICROWAVE FEL

''Spiking'' is observed on the high power (approximately 10 MW) emitte d from our 24 GHz FEL oscillator. The features of the spike are simila r to those reported from our 1.5 mm wavelength FEL, but the spike FWHM is approximately 450 ps. Spikes may occur randomly or in a sequence w hich is not the mode-locked period of the resonator. The slippage of t he FEL is varied by changing the diameter of the drift tube (cylindric al waveguide); for 17 mm diameter, the slippage should be nearly zero since the electron axial speed is the same as the wave group velocity for the TE11 mode. The absence of slippage rules out the sideband inst ability, but spiking occurs both with and without slippage. Comparison with a numerical model and a solitary-wave theory for spiking are pre sented. Measurements of the FEL spectrum are reported under conditions of multimode oscillation, feedback with a tuned reflector, and inject ion of a ''seed'' signal; the latter two methods result in a more cohe rent spectrum; nevertheless, spiking persists.