The Injection Locked Magnetron (ILM) has been used as a source of coherent power in a number of radar systems. It has a number of characteristics that make it particularly suitable to heating applications, above competing tube types used in radar applications.
Power available is dependent upon the frequency of operation, ranging from several hundred watts at Ku-band to tens of kilowatts at L-band. Operation could be either cw or pulsed, but at higher power levels pulsed operation would be required. Designs for tubes at 1 GHz and 15 GHz will be presented.
ILMs could be made at any frequency from 800 MHz to 20 GHz, beyond which the magnetron becomes either too large to be practical or too small to fabricate. Locking bandwidths are typically 0.5%, sufficient to be able to guarantee the tube remains locked at the required operating frequency. A tuner would increase the tube life and compensate for frequency drift effects.
The tube life is dependent on the desired frequency of operation, ranging from several hundred hours at high frequency to an order of magnitude higher at low frequency. Adding a tuner will increase this several fold. ILMs are inherently rugged and may be run in any orientation, having been developed for airborne radar applications. Tubes are operated directly into a circulator and consequently can operate into high VSWR.
The magnetron is a low cost tube, hence its use in domestic microwave ovens. ELMs have a similar part count and complexity. Efficiency varies from approximately 45% in Ku-band to 80% at L-band. Operating voltages are lower for ILMs than other vacuum tubes, at 1 GHz a 100 kW system would require 20 kV and for a 15 GHz 200 watt system, a 2 kV power supply would be required. These would run in the region of 10% duty cycle, 1 kHz PRF. Tubes would have integral rare earth magnets and require cooling water. The required quantity of tubes would be the most significant cost driver.