High-current, long-pulse gyrotron-backward-wave oscillator experiments.

Abstract

Experiments have been conducted on cyclotron resonance maser interactions to generate high power microwaves employing a long-pulse electron beam accelerator, MELBA (Michigan Electron Long Beam Accelerator), with electron beam parameters of: V = 0.6-0.9 MV; Current = 1-50 kA; and pulselength = 0.5-1.5 $\mu$s. To generate high power and long-pulse microwaves, the interaction cavity and magnetic field were designed to excite the TE$\sb{11}$ fundamental mode gyrotron backward wave. Two types of electron beam are employed. One is an annular electron beam of $\sim$150-250 amps which is extracted through an apertured-mask anode (24 five mm holes). The other is a solid electron beam of 1-2 kA which is extracted through a two inch diameter aperture. An S-Band, Vlasov-type antenna (with an efficiency of $\sim$10%) is used to detect the backward wave power at the diode end of the interaction cavity. A series of high pass waveguide, low pass coaxial and cylindrical cavity frequency filters are employed to show the existence of the gyro-backward-wave, as well as show the magnetic tunability of the gyrotron-backward-wave. In the solid beam case (1-2 kA), about 300-800 kW of extracted microwave power was detected in the waveguide detection system, implying that approximately 3-8 MW (efficiency $\sim$1-2%) of power is generated from the gyro-BWO device. The pulselengths for the solid beam case were from 300-600 ns (essentially the total flat-top voltage pulselength) over a frequency range of 4.5-6 GHz. The annular beam generated approximately 10-80 kW extracted microwave power for voltages of 600-650 kV (device efficiency of $\sim$0.1-1%) and about 100-300 kW of extracted microwave power (device efficiency of 1-2%) for voltages of 850-900 kV. Pulselengths for the 600-650 kV interaction were from 100-600 ns for a frequency range of 4.5-6 GHz, and pulselengths for the 850-900 kV interaction were from 40-100 ns (which corresponds to the initial overshoot of the voltage pulse) for a frequency range 4.5-6 GHz.