Observations of multipactor in magnetrons

Abstract

The process of back-bombardment of a magnetron cathode is considered as a single-surface multipactor. Experimental evidence is given, showing that this back-bombardment is concentrated in lines on the cathode surface, whose intensity is controlled by the RF voltage, and position by the dc voltage and magnetic field. The operating equations of a series-field magnetron are given, and experimentally the performance is shown to follow the Langmuir-Childs, Hull, and Hartree curves in sequence (in distinction to a conventional fixed-field magnetron which operates only near the Hartree voltage). A semiquantitative explanation is given of the bombardment lines, and their dependence on the voltage. A cathode with grooves at the position of the bombardment lines is found to receive much less back-bombardment, and to have a more favorable variation of the residual bombardment as a function of output loading. Multipactor between vane tips of a very lightly loaded magnetron was observed through a large glass window, and was seen to be the cause of a vane tip melting problem. Axial multipactor discharges between vane tips and the pole pieces were also seen; being parallel to the magnetic field, these saturate at a low power level and do no harm. Finally, an axial electron discharge was seen, which appeared to have some of the characteristics of a multipactor, but is not really understood. This discharge destroyed the tube after a few minutes, ending the investigation.