Computer simulation of a high-power magnetron and the possible implications for RF pulse shortening

The main objective of our investigation is the phenomenon of RF pulse shortening at high microwave output power. For that purpose, we have used the geometry of an experimental L-band ten-vane rising-sun magnetron developed by English Electron Valve Ltd. (EEV); in its standard form, the tube is designed to generate 100 MW, 1-/spl mu/s pulses at 100 pps, with an overall efficiency of 60%, the voltage and the magnetic field being, respectively, equal to 180 kV and 0.13 T. Varying the above parameters over a range of values in order to increase the output power and then changing the radius of the cathode while keeping the radius of the anode constant, we were able to indicate how the phenomenon of pulse shortening could occur. In our case, the duration of the RF pulse is very much shorter than the time required for the plasma to fill the whole of the interaction space and, thus, act as a short circuit. Assuming that the movement of plasma inside the tube can be simulated by the corresponding change in the cathode/anode separation, it is suggested that the process of explosive emission and/or anode degassing under bombardment of high-energy electrons very quickly reduces the anode/cathode ratio to a value below that required for the cutoff condition; the tube then stops oscillating and starts conducting like an ordinary magnetic diode. This, in our opinion, strongly contributes to the process of pulse shortening. Other interpretations of this phenomenon are also discussed in this paper.