Abstract
We report observations of frequency upshifts in a free electron laser amplifier operating in the Raman regime. The FEL is driven by a mildly relativistic electron beam (750 M 100 A, 30 ns) subjected to a combined axial magnetic field (2-12 kG) and a helical wiggler field (500 - 1500 G; 3.3 cm periodicity). The input into the FEL amplifier is Provided by a high Power magnetron (-100 kW) tuned to a frequency of 33.39 GHz. The frequency spectrum of the output power is determined by a heterodyne technique. A crystal rectifier is used as a mixer for the 33-GHz FEL radiation and for radiation from a variable-frequency 23-GHz local oscillator. The beat wave is amplified by a TWT amplifier and sent into a filter bank, composed of thirteen channels adjacent in frequency, each 80 MHz wide. Thus, the radiation frequency is detennined on a single-shot basis. It is found that in the Group I regime of FEL operation the output frequency is upshifted by -100 MHz, but no upshift is observed in either the group 11 Or the reversed field configurations. The results are consistent with calculations that take account of fine dependent FEL effects of the pulsed electron beam.
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