Study of absolute instabilities in the gyrotron traveling-wave amplifier

Summary form only given. A major obstacle to the development of the gyrotron traveling-wave amplifier (gyro-TWT) has been its susceptibility to oscillations caused by the absolute instability. We present a theoretical study on the absolute instabilities in a TE/sub 01/-mode, fundamental cyclotron harmonic, W-band gyro-TWT recently designed and currently in operation at UC Davis. In this high-order-mode circuit with distributed losses, absolute instabilities assume a variety of forms, including overdrive of the operating mode, fundamental cyclotron harmonic interactions with lower-order modes, and the second cyclotron harmonic interaction with a higher order mode. The device thus provides a good reference case for an examination of the diversity of absolute instability behavior as well as being of great importance for millimeter wave radar. We begin with a study of the field profiles of different absolute instabilities, which exhibit a range of characteristic depending upon the sign and magnitude of the synchronous value of the propagation constant. These profiles in turn explain the sensitivity of oscillation thresholds with respect to the beam and circuit parameters. Significance of the current study to the UC Davis gyro-TWT experiment will be discussed.