Slow processes in startup scenarios of long-pulse gyrotrons

Abstract

A gyrotron startup scenario describes the variation of gyrotron parameters during onset of a pulse. This typically includes variations in beam parameters during the voltage rise, in the course of which the operating mode is excited and then driven to high efficiency operation while suppressing other parasitic modes. For long-pulse gyrotrons, however, additional processes with longer time constants should also be considered as part of the startup scenario. Such slow processes include ion neutralization of the beam space charge, thermal expansion of cavity walls caused by their ohmic heating, and reduction of the beam current due to emission cooling of the cathode. In the present paper, we analyze the importance of these effects on the stability and efficiency of gyrotron operation.