Twenty-kilowatt 890 Mc/s continuous-wave magnetron

The design, construction and performance of a magnetron that has been developed primarily for heating applications are described. Between 20 and 30kW output power can be obtained, at efficiencies of over 80%, near the theoretical limit. This performance is achieved using a 10-segment double-ring-strapped anode, 2in long, in conjunction with a tungsten-helix cathode. A coil round the anode provides the magnetic field. Some initial development difficulties arose from interfering resonances of the cathode structure, and in oscillating valves, overheating parts of the cathode at high magnetic fields. Chokes and cathode modifications for resonance control are described. Two types of output have been tried: a direct feed to coaxial line, and a probe radiating into waveguide. The second type is found more suitable at higher powers. Performance data are presented for fixed fields and for the electromagnet connected in series with the anode¿a mode of operation, which offers some stabilisation of output power against supply-voltage changes. The output of present valves is limited by the anode-dissipation capability and heating of the output window. Further developments are towards improved methods of cooling and the use of lower-loss window materials. The possibilities for higher-power valves developed along similar lines are considered briefly.