Abstract
We report observations of frequency chirping and phase of a free electron laser amplifier operating in the Raman regime. The FEL is driven by a mildly relativistic electron beam (750 kV, 25 ns) subjected to a combined axial magnetic field and a helical wiggler field. The input into the FEL amplifier is provided by a high power magnetron tuned to a frequency of 33.39 GHz. Phase and frequency shifts are measured both as a function of time and interaction length. It is found that in the Group I regime of FEL operation the output frequency is upshifted by ∼ 100 MHz, but smaller upshifts are observed in the Group II or the Reversed Field configurations.
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