Development of the step tunable 140/110 GHz 1 MW gyrotron for fusion

Electron-cyclotron heating and current control in plasma set-ups require gyrotrons with a level of CW power up to 1 MW at frequencies 80-170 GHz and efficiency not less than 50%. The output window was the key factor that limited the achievable pulse duration in gyrotrons. The CVD diamond single disk window is capable of full 1 MW CW operation. Recent results generated interest in the use of frequency tunable gyrotrons for ECRH plasma experiments. In this paper we discuss the main problems of the step tunable gyrotron and present the design of a step tunable 110/140 GHz 1 MW gyrotron. The 80 kV, 40 A beam is produced by a magnetron injection gun (MIG) with an average perpendicular-to-parallel velocity ratio of 1.2 and a transversal velocity spread below 25%. The microwave circuit has a TE/sub 22.81/-TE/sub 19.61/ cavity, which operates at 140 GHz and 111.5 GHz respectively. Electron beam parameters were optimized for efficient generation of the modes. These modes were chosen from the condition of the gyrotron CVD window transparency at the operation frequencies. The highly efficient quasi-optical mode converter was elaborated for operation with two different cavity modes.