An Axial Output Relativistic Magnetron Fed by a Split Cathode and Magnetically Insulated by a Low-Power Solenoid

One of the main difficulties in designing a relativistic magnetron producing high-power microwaves (HPMs) to be compact, is the power source required to feed the solenoid producing the axial magnetic field, which magnetically insulates the electron beam. This is because the diffusion of the magnetic field through the walls of the system is on the millisecond timescale. The latter requires high-power supplies and restricts the magnetron from operating repetitively. Using permanent magnets instead does not make the system sufficiently compact, because of the size and weight of the magnets and does not allow varying the magnetic field. We suggest a simple solution to this problem by cutting longitudinal slits through the entire magnetron anode system. With such slits, the magnetic field penetration is not restricted by the diffusion rate. Thus, one can apply a microsecond-timescale magnetic field produced by a solenoid powered by a considerably smaller power supply. We test this idea by using particle-in-cell (PIC) simulations of a magnetron for the axial output design suggested by Xu et al. (2018) fed by a split cathode (Leopold et al., 2020). With a split cathode, the second major problem with relativistic magnetrons is alleviated—pulse shortening is avoided.

I. Introduction

Relativistic magnetrons have been of interest since the 1970s [1] as high-power microwave (HPM) sources and many ideas were realized to improve their efficiency [2]. These devices are the most promising HPM sources in terms of their microwave generation efficiency and compactness, but some problems need to be solved first. One of the problems that need solution is HPM pulse shortening caused by the explosively formed cathode plasma evolution [3], common to many HPM sources, and which was addressed by ideas such as the transparent cathode [4], the virtual cathode (VC) [5], and the VC with a magnetic mirror [6]. The transparent cathode used a solid cathode made up of a few conducting ribs, placed in front of each resonator, thus reducing the surface of the conductor involved in the explosive emission while increasing the coupling between the microwave field and the drifting electrons. The VC is based on the idea of the squeezed state of an electron beam [7], that is, the low-energy high-density electron charge trapped between two VCs. The disadvantage of producing a VC is that it requires increasing the radius of the tube containing the magnetron. A magnetic mirror is difficult to realize and requires an additional power supply to energize an additional mirror coil. Recently, a novel type of cathode, the split cathode was introduced and tested experimentally [8]. The split cathode consists of a cathode that is placed upstream and outside the magnetron and is connected by an axial rod to a reflector placed downstream from the magnetron. The annular electron beam emitted by the cathode emitter is trapped in the space between the cathode and the reflector and, at the same time, screens the rod from explosive plasma formation. The split cathode is a simple and practical way to realize a VC without the disadvantages mentioned regarding previous VC ideas. The operation of a split cathode as the electron source in a relativistic magnetron was recently experimentally confirmed and revealed that, indeed, using a split cathode mitigates pulse shortening [9].