Microwave tube fault-current model for design of crowbar protection

Abstract

Many applications that use high energy plasma are realized using Microwave tubes (MWT) that operate at peak power in the range of hundreds of MW and frequency in GHz. One failure mode of the MWT is due to the excess energy in the tube during internal arcing events. Crowbar is used to protect the MWT by diverting the energy during fault. To compute the energy released into the MWT, the dc fault current model and the MWT model are essential. The paper utilizes a model for the MWT for crowbar applications as an equivalent fuse wire. The paper proposes a model for the dc fault current, the analysis for which is based on joules integral energy concept. The model gives flexibility to choose a range of practically observed reactance to resistance ratio (X/R) of transformer and also allows the use of a range of dc current limiting resistances that are utilized in the High Voltage (HV) power supply circuits in Microwave applications. The non-linearity of the system due to the multipulse diode rectifier is also considered by introducing a correction factor in the model. Both dc fault current and MWT models are verified experimentally. Using the model a 10kV , 1kA crowbar is built to limit the energy in MWT below 10J.