Operation of a long-pulse CARM oscillator

Abstract

A long-pulse (1 μs), millimeter-wave cyclotron autoresonance maser (CARM) oscillator experiment has been carried out. A SLAC 5045 klystron gun produced an electron beam at 250–320 kV and 10–20 A; a beam α ≡ p ⊥ p | , 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 28.2–40.1 GHz frequency range in the TE 21 and TE 01 modes near cutoff and, for the first time, a second harmonic upshifted (CARM) TE 11 mode at 74.5 GHz was observed. Using the second resonator, fundamental CARM operation was observed for many parameter settings, and for frequencies ranging from 29 to 32 GHz. Identification of these CARM modes is made by comparison of measured frequencies with uncoupled dispersion theory and measurement of the far-field radiation pattern. Output powers ranged from ∼ 0.1 to 100 kW, resulting in efficiencies of 0.1% to 3%.