Oscillator priming and preoscillation noise in a gyrotron

Abstract

Phase control is achieved in a pulsed gyrotron oscillator both by applying an external priming signal directly to the oscillator and by applying the signal to a prebunching cavity. A pulse-to-pulse phase jitter of greater than 2.5 degrees is achieved in the gyrotron at drive-to-oscillator power ratios of -36.6 dB (drive signal-to-noise power ratio of 36 dB) in the direct injection case and -71 dB (drive signal-to-noise power ratio of 22 dB) in the prebunched case. A lumped-element theory is compared to the experimental results. The theoretical description seems valid when the drive frequency is within about 5 MHz of that of the oscillator. Preoscillation noise in the gyrotron is about 10 mu W, larger than expected from either shot noise thermal noise but in the vicinity of spontaneous cyclotron emission. Convective radio-frequency noise growth is investigated. No evidence of the electrostatic cyclotron instability was seen. All growth observed is attributed to the gyroklystron amplification mechanism. However the noise growth per unit length is not as large as that of a narrow-band drive signal. >