Overview of recent theoretical studies on ExB multistage depressed collector designs for gyrotrons

The key to increase the gyrotron efficiency to a level significantly higher than 50% is the development of a highly efficient collector system for the recovery of the spent electron beam energy. In this context, a significant effort has been dedicated at KIT during the last years in order to design a reliable, robust and efficient multistage collector system. For this purpose several design approaches have been theoretically and numerically investigated. An overview of the results of these studies will be presented.