High efficiency relativistic magnetron with diffraction output operating with a virtual cathode

Abstract

In simulations using the particle-in-cell (PIC) code MAGIC [1] in [2] we found that a magnetron with diffraction output (MDO) with a transparent cathode is capable of operating with electronic efficiency exceeding 70%, which was recently verified in experiments [3]. The transparent cathode is transparent to the operating TE-modes in magnetrons and consists of separate longitudinal emitters periodically placed about the cathode radius. The strong azimuthal electric field of the operating wave acts on electrons drifting around the cathode to provide rapid conversion of electron potential energy to electromagnetic energy. In addition, the periodic structure of this cathode promotes the interaction of electrons with the slow wave structure. Furthermore, this cathode operates as a wiggler since the self-magnetic field around each emitter creates an additional interaction of the rotating electron flow in a periodic magnetic field. It was our motivation to replace this physical cathode with a virtual cathode (VC) that is transparent to any wave. For this we insert an additional anode section containing a solid cylindrical cathode axially upstream of the magnetron interaction space. The radius of this new anode section is less than the radius of the MDO anode block, such that the input electron beam current exceeds the sum of the space-charge-limiting current and the anode current in the interaction space, a condition that supports the creation of a VC at the location where a physical cathode would normally be placed. Since in [2] we found the optimal design of an MDO, here in our PIC simulations we used the same MDO configuration. We found that an MDO with VC operates in the π-mode with applied voltage 460 kV with axial magnetic field B = 0.325 − 0.37 T. When the uniform field B = 0.36 T in the interaction space the electronic efficiency (that is related to the anode current) of stationary generation achieves 72%, whereas the total efficiency (accounting for leakage current that does not participate in the interaction) is about 24%. In order to increase the total efficiency the uniform magnetic field can be enlarged up to the widest cross section of the horn antenna. Then the total efficiency increases up to 35%, but the electronic efficiency decreases to 50%.