Temporal evolutions of frequency resolved radiations from a Raman regime free‐electron laser

In pulse power driven free‐electron laser experiments for a diode voltage 300–600 kV, a maximum electron beam current 730 A, with a 110‐ns width of beam voltage pulse and a helical wiggler with a pitch of 2 cm, the temporal evolution of radiation spectra is observed. The frequency variations are explained theoretically by using instantaneous values of beam voltages and currents. In the calculation we take account of the variation in the beam cross section of the beam voltage drop and the wiggler field. The spatial growth rate of the radiation power was also measured. The observed growth rate is 1 dB/cm and the saturation level is 250 kW with the spector peak at 58 GHz.