Microwave Tube Fault-Current Model for Design of Crowbar Protection

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. An equivalent fuse wire model is utilized for the MWT for the crowbar applications. The paper proposes a model for the dc fault current, the analysis for which is based on Joules Integral energy concept. The model provides 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 nonlinearity of the system due to the multipulse diode rectifier is also considered by introducing a correction factor in the model. This paper shows that the same correction factor can be applied for both dc side parallel- and series-connected rectifier circuits. Both dc fault current and MWT models are verified experimentally. Using the model, a 10 kV, 1 kA crowbar is built to limit the energy in MWT below 10 J.