Cyclotron autoresonance maser (CARM) oscillator experiment at 28 GHz

Experimental results on a long pulse, high frequency, cyclotron autoresonance maser oscillator (CARM) are reported. An electron gun produces a 1 μs, 450 kV, 80 A electron beam with good beam quality. A bifilar magnetic wiggler operated near the guide field resonance followed by an adiabatic compression of the guide field is used to impart transverse velocity (β⊥) to the beam. A capacitive probe, located after the adiabatic compression, is used to measure the mean parallel velocity of the beam, and using energy conservation the beam pitch (α = β⊥/β‖) is derived. The measured α values are significantly higher than the theoretical values. The interaction region is defined by a Bragg resonator designed to oscillate in the TE11 mode at 27.8 GHz (diffractive quality factor QD = 300). A cold test measurement of the resonant frequency of the Bragg resonator is consistent with the designed frequency. A rf power level of 1.9 MW (5.2% efficiency) has been measured in the TE11 mode at the designed frequency; however, significant mode competition is observed between TE11 mode and TM01 mode. Identification of these CARM modes is made by comparison of measured frequencies with cold test frequencies, uncoupled dispersion theory and measurement of the far field radiation pattern. Operation in the TE11 CARM mode occurred at a Doppler upshifted frequency of 2.9 times the relativistic cyclotron frequency.