Mode competition and control in a 170 GHz gyrotron cavity for ECRH application

The present paper is devoted to the study of mode competition and control in a 170 GHz gyrotron cavity for potential application in electron cyclotron resonance heating (ECRH) of plasmas. An extensive analysis has been carried out for coupling coefficient, start oscillation current, voltage depression, and limiting current for the candidate modes in the gyrotron cavity resonator. The study reveals that the main competing modes for TE28,8- come from TE25,9+ and TE27,8-. The optimum beam position for TE28,8- mode is found to be at 8.28 mm. To avoid excitation of competing modes, it has been found that slight increase in electron beam radius from 8.28 mm to 8.40 mm avoids the excitation of all the competing modes. Based on the considerations of coupling coefficient, start oscillation current, voltage depression, and limiting current; the TE28,8- mode is selected as the operating mode.