Harmonic Frequency Locking in the Multifrequency Recirculating Planar Magnetron

This paper presents experimental results that demonstrate the first known instance of harmonic frequency locking in a magnetron. The prototype crossed-field high power microwave source, termed the multi frequency recirculating planar magnetron, consisted of two planar cavity arrays having design frequencies near 1 and 2 GHz, respectively, coupled by smooth-bore electron beam recirculation bends. The magnetron was driven by the MELBA-C modified Marx generator using -300-kV pulses supplying up to 3 kA for approximately 300 ns. Using a 0.17-T axial magnetic field, the harmonic frequency-locked state simultaneously produced microwave pulses of 32 +/- 3 MW at 0.984 +/- 0.001 GHz and 13 +/- 2 MW at 1.970 +/- 0.002 GHz. In addition, changes in the relative phase difference between the magnetron slow wave structures (SWSs) were correlated with changes in the axial magnetic field magnitude. This correlation, along with the results from experiments that tested each individual planar cavity array in isolation, suggested the locking was facilitated through the second harmonic content of the 1-GHz modulated electron beam recirculating from the 1-to 2-GHz SWS. Analysis of shots conducted near 0.17 T showed a +/- 17 degrees shot-to-shot variation in the average relative phase difference between the SWSs, and the average variation in this phase difference during a locked shot was +/- 8 degrees.