Secondary electron emission modeling for simulated multi-stage depressed collector operation using MICHELLE

Electron beam collection systems variously employed in O-type fast-wave and slow-wave devices such as gyrotrons, gyroklystrons, klystrons, and TWTs must often be carefully designed to avoid potentially undesirable consequences of secondary electron production at electrode surfaces. Some high performance device applications may experience added noise power resulting from secondary electron current returning to the RF interaction space. In high average power slow-wave devices, this returned current might contribute intolerable levels of additional thermal dissipation power on thermally stressed RF circuit structures. For more efficient operation of the amplifier system, multiple electrode geometries, set at prescribed "depressed" voltages, are often used to recover energy from "spent" beams exiting from the RF interaction space. However, the emission of secondary particle currents can alter the expected efficiency improvements by refocusing and redistributing the collected currents toward electrodes with the higher potential values, producing additional thermal stress in the process. An advanced, three-dimensional electron beam design tool, called MICHELLE, is currently undergoing development by a team lead by SAIC under ONR sponsorship. As part of this new effort, an algorithmic representation of a comprehensive model of secondary emission is being developed for this computational tool in order to achieve an accurate beam collection design capability.