Frequency-agile relativistic magnetrons

ABSTRACT We are developing a family of frequency agile relativistic magnetrons to continuously cover the bands from1 to 3 GHz. We have achieved tuning ranges of > 33 %. The magnetrons have been operated repetitively in burst mode at rates up to 100 pps for 10 sec. Power is extracted from two resonators, and is in the range of 400— 600 MW, fairly flat across the tuning bandwidth. We are using a network of phase shifters and 3-dB hybridsto combine the power into a single arm and to provide a continuously adjustable attenuator.Keywords: relativistic magnetron, frequency agile, high power microwaves1. INTRODUCTIONMicrowave oscillators are, in general, smaller, lighter and less complex than amplifiers. They do,however, suffer from narrow bandwidth and lack of phase control required for efficient combining of outputsfrom a number of "identical" sources. The latter limitation has been ameliorated by the development of phase-locked oscillators, where the sources are locked to an external signal1 ,2 or to each others The formerlimitation has been addressed in conventional microwave devices (<100 MW) with methods for mechanicallyor electrically tuning the oscillators' frequency, producing sources that have narrow instantaneous bandwidth,but that are tunable, or "agile," over a broader bandwidth4 . These methods, have not been previously appliedto HPM sources, and many are not applicable due to the high DC and RF fields and the intense space charge ofthe electron beam.In this paper, we discuss our development of a family of frequency agile relativistic magnetrons and thewaveguide transmission system designed to connect the magnetrons to antennas. The tuning mechanism iscompany proprietary, patent pending, and is therefore not described.As Figure 1 indicates, we have achieved an increase of greater than a factor of two in the tuningbandwidth and two orders of magnitude in the peak power, compared with conventional magnetrons.2. MAGNETRON DESIGNThe basic magnetron geometry, without the tuning mechanism, is shown in Figure 2, below. It is a 10resonator rising sun magnetron (RS10), with alternating vane and rectangular cavities. Microwaves areextracted through irises in two of the vane resonators. The resonators are terminated axially by endcaps atboth ends. The cathode has "pin" enhancements over approximately 70% of the anode length, centered axiallyabout the anode midplane.The magnetron is constructed completely of 304L stainless steel. The anode is electron discharge machinedfrom a single billet of material and then thoroughly cleaned and vacuum baked. [We have found that thisapproach provides a suitable middle ground in cleanliness; we typically achieve vacuums of io8 Torr with