A Study on Enhancing Microwave Power from Virtual Cathode Oscillator for Efficient HPM Generation

Abstract

An introduction of pulsed power technologies to conventional microwave tubes makes the generation of high-power microwaves (HPM) possible. As the output power and frequency are increased, HPM devices expand their applications. These days, HPM devices are studied and used as high-power radar and electromagnetic pulse in military fields, plasma heating sources in the thermonuclear reactor, microwave sources in material processing, etc. A virtual cathode oscillator (vircator) is one of the widely investigated HPM sources which is classified as a space-charge device. When the injected beam current exceeds the space-charge-limiting current, a potential well causing reflection of electrons is developed by the space-charge effects. The potential well is called a virtual cathode. The vircator generates HPM through interactions between electrons and the virtual cathode. An intense electron beam creates a virtual cathode in the drift region after passing through a mesh or foil anode. HPM is generated from the vircator through two mechanisms: an oscillation of the virtual cathode and a coherent reciprocating motion of electrons between the cathode and virtual cathode. Few aspects of vircators, such as their simple structure, low impedance, low requirements for the electron beam, and frequency tunability, make them attractive in constructing HPM systems using them. Due to the simple structure, the modification of the vircator structure is easy compared to other microwave devices. A generation of intense relativistic electron beams is easily possible in the vircator due to its low impedance. Unlike other microwave devices, the vircator requires no external magnetic fields for focusing or additional structures like slow-wave structures (SWSs) for electron bunching. Finally, the frequency of the vircator can be controlled easily by changing the anode-to-cathode gap distance. In spite of these advantages, low efficiency is a critical disadvantage of the vircator. The low efficiency of the vircator needs to be improved to increase its availability. In this paper, three methods to enhance the microwave power from the vircator are presented and investigated through PIC simulations and experiments. First, an open-ended coaxial (OEC) cathode structure is designed to improve the electron beam focusing. The operation features of the vircator when the OEC cathode is used are compared with that of the solid and annular cathode. Second, unlike the existing coaxial vircator, a reflex-triode type coaxial vircator that emits electron beams in the outer radial direction is proposed and investigated by comparing it with the basic axial vircator. Finally, the performance of using a ring reflector in the axial vircator is investigated and the installation conditions are optimized. As a result, compared to the axial vircator, a slight improvement in output microwave power is observed through the use of an OEC cathode, and a large output improvement is observed in the case of the use of a ring-reflector and a reflex-triode type coaxial vircator.