A long-pulse, CARM oscillator experiment

Results of a long-pulse, cyclotron autoresonance maser (CARM) oscillator experiment are reported. A Stanford accelerator (SLAC 5045) klystron gun produced a 1 pis electron beam at 250-300 kV and 10-25 A at a repetition rate of up to 4 Hz. A beam α≡β⊥/β 1 , variable from 0 to 1, was produced on this electron beam with a wiggler magnetic field near guide field resonance. The experiments were carried out with two different Bragg reflection resonators designed for the TE 11 mode. Using the first resonator, many harmonic gyrotron modes were observed in the 28-40GHz frequency range in the TE 21 and TE 01 modes and, for the first time, a second harmonic upshifted (CARM) TE 11 mode at 74-5 GHz. Using the second resonator, fundamental CARM operation was observed for many parameter settings, and for frequencies ranging from 29 to 32 GHz. Output powers ranged from ∼01 kW to l00kW, resulting in efficiencies of 01% to 2%. Identification of CARM and gyrotron modes is made by comparison of measured frequencies with dispersion theory and measurement of the farfield radiation pattern. Comparison with theory indicates that the device efficiency is reduced by finite spread in the axial electron beam momentum.