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 mechanisms affecting the relativistic magnetron (RM) performance and causing so-called RF pulse shortening. This paper will show how the development of new cathode technologies has led to a significant enhancement of the RM efficiency and power. We have conducted a series of experiments with various cathodes intended for use in RM´s. A primary objective in this research was to determine how the cathode geometry and type of emission surface would influence major characteristics of the L-band high-power RM in a rising-sun configuration. In these experiments, the magnetron operated at a fixed frequency of 1.3 GHz, voltage of 100-500 kV, total electron current of 2-8 kA, and total microwave peak power of 100-700 MW depending on operating conditions and type of cathode used. It was found that the geometry (smooth cylindrical, series of disks, pins) and the type of cathode emission surface (stainless steel, velvet, carbon fibers) affected the magnetron performance. This process resulted in a variation of the maximum microwave power of ~30%. The cathode end caps, which have been mostly abandoned after transition from classic to RM´s, were shown to be able to increase the microwave power and RM efficiency by ~80% without facilitating the pulse shortening effect. This result was achieved through the implementation of cathode design principles that are compatible with the operation of RM´s. A maximum total efficiency of 24% was achieved with a velvet cathode with end caps, determined as the ratio of peak power to input electrical power