Excitation of backward waves in beam tunnels with saw-teeth wall profiles in gyrotrons

Summary form only given. High-power long-pulse millimeter-wave gyrotrons are used for electron cyclotron resonance plasma heating and current drive in many plasma installations around the world. There are many problems that may hinder the operation of those devices. One of the problems that may reduce the efficiency of operation is excitation of backward waves in the beam tunnels of gyrotrons. There are known several methods of suppression of backward waves studied experimentally. Also the effect of resistive coating of the walls on the conditions of backward wave excitation was studied theoretically1. Here we adopt the approach developed in [1] to study how modification of the beam tunnel wall profile affects the mode stability. We consider beam tunnels with a saw-teeth wall profile and determine start currents of backward waves. We also plan to confirm our studies with numerical simulations using MAGY2.