Ph.D Thesis

Ph.D StudentTal Queller
SubjectInvestigation of Plasma in Pulsed High-Current Electron
Sources and High-Power Microwave Devices
DepartmentDepartment of Physics
Supervisors Full Professors Krasik Yakov
Professor Emeritus Felsteiner Joshua
Full Thesis textFull thesis text - English Version


High-voltage pulsed vacuum diodes are being used for a variety of applications such as high-power lasers, relativistic high-power microwave tubes, x-ray sources, material processing and others. The key element in high-power diodes is a cathode suitable for generation of a high-current relativistic electron beam. The latter determines the efficiency of laser pumping, microwave tubes operation and other technological processes.  

 High-Power Microwave (HPM) sources are a class of slow-wave structures in which the energy exchange is between the relativistic high-current electron beam and the radiation field. The pursuit for hundreds of megawatts radiation levels had encouraged researchers to shift toward the relativistic devices where generation of pulsed electron beams with peak power above 108 W can be achieved. Throughout the years, different relativistic microwave tubes have been developed in order to establish this mechanism. Both the relativistic magnetron and VIRCATOR are related to the microwave generators. The source of electrons in these microwave devices is explosive emission plasma whose parameters determine the operation efficiency of these devices.

One of the purposes of this research was to investigate the parameters of the plasma during the operation of a relativistic magnetron in order to find a correlation between the measured plasma parameters and the output RF power.

The double-gap Virtual Cathode Oscillator (VIRCATOR) was first introduced almost ten years ago in order to improve the commonly used vircator operation efficiency whose value does not exceed several percent. In the present research the problem of pulse shortening was addressed, and specifically the role of plasmas in the delay in the microwave pulse initiation and its early termination was studied. It was shown that the plasma formed on the foil separating the two cavities plays a key role in the RF pulse termination and that the role of reflected electrons in the initiation of RF generation is much more significant in comparison to the Virtual Cathode (VC) formation.

A promising new high-current plasma electron source is suggested and investigated. This cathode is a multi-capillary carbon-epoxy cathode, and various cross-sectional configurations of this cathode were studied. This pulsed electron source was found to be able to produce a high-current density, long duration, uniform and reproducible electron beam, with an arbitrary cross-section and had a long life-time. The plasma formation mechanism on the surface of this cathode was investigated by characterization of the cathode plasma parameters.