Effect of cathode end caps and a cathode emissive surface on relativistic magnetron operation

Cathode plasma expansion into a vacuum gap is one of the major physical mec hanisms affecting the relativistic magnetron (RM) performance and causing s o-called RF pulse shortening. This paper will show how the development of n ew cathode technologies has led to a significant enhancement of the RM effi ciency and power. We have conducted a series of experiments with various ca thodes intended for use in RM's, A primary objective in this research was t o determine how the cathode geometry and type of emission surface would inf luence major characteristics of the L-band high-power RM in a rising-sun co nfiguration, In these experiments, the magnetron operated at a fixed freque ncy of 1.3 GHz, voltage of 100-500 kV, total electron current of 2-8 kA, an d total microwave peak power of 100-700 MW, depending on operating conditio ns and type of cathode used. It was found that the geometry (smooth cylindr ical, series of disks, pins) and the type of cathode emission surface (stai nless steel, velvet, carbon fibers) affected the magnetron performance, Thi s process resulted in a variation of the maximum microwave power of similar to 30%. The cathode end caps, which have been mostly abandoned after trans ition from classic to RM's, were shown to be able to increase the microwave power and RM efficiency by similar to 80% without facilitating the pulse s hortening effect. This result was achieved through the implementation of ca thode design principles that are compatible with the operation of RM's, A m aximum total efficiency of 24% was achieved with a velvet cathode with end caps, determined as the ratio of peak power to input electrical power.