Numerical optimization of a multistage depressed collector with secondary electron emission for an X-band gyro-BWO

A three-stage depressed collector was previously designed and simulated to recover the kinetic energy of the spent electron beam in an X-band gyrotron backward wave oscillator (gyro-BWO) by using the 3-D particle-in-cell code MAGIC. The geometry of the depressed collector was optimized using a genetic algorithm to achieve the optimum overall recovery efficiency for specific parameters of the spent beam. In this paper, secondary electron emissions were simulated, and a few emission models were compared to investigate the effects of the secondary electrons on the overall recovery efficiency and the backstreaming of the electrons from the collector region. The optimization of the shape and dimensions of each stage of the collector using a genetic algorithm achieved an overall recovery efficiency of more than 80% over the entire operating regime of the Gyro-BWO, with a minimized backstreaming of 1.4%. The heat distribution on the collector was calculated, and the maximum heat density on the electrodes was approximately 195 W/cm2, hence avoiding the generation of ¿hot spots¿