Low pressure plasmas for high power microwave sources

Abstract

This thesis describes an investigation of the use of low pressure plasmas for the generation of high power microwaves. Previous research has shown that the efficiency of a high power microwave ("HPM") source such as a BWO is enhanced by the introduction of a low pressure plasma into the oscillator cavity. The principle aim of this thesis is to extend the use of low pressure plasmas to the whole HPM system. Electron beams with current densities of the order of 20 A cm-2 can be generated in a cold cathode glow discharge at low gas pressures. Results are presented which show the effects of magnetic fields and electrode spacing on the I-V characteristics of a DC glow discharge electron gun. A glow discharge electron gun with an operating voltage of 350 kV has been designed and tested. A new kind of RP plasma cathode is proposed in which electrons are drawn from an RF discharge in a low pressure gas. An analysis of the production of an annular RF plasma cathode using a microwave-excited helical slow-wave structure is presented. Experimental results show that the RF plasma cathode yields electron current densities an order of magnitude higher than does a solid cathode. Examples of the implementation of the RF plasma cathode in a number of components of an HPM system are given. The propagation of electromagnetic waves in plasma-loaded waveguides of circular cross-section has been modelled. Numerical solutions are presented for the case of slow-waves in a longitudinally-magnetised plasma waveguide. Propagation below the cut-off frequency of the waveguide is generally possible and, according to the configuration, the propagating waves may be used for plasma generation or for RF power transmission. A new kind of high power microwave waveguide switch, based on the properties of plasma waveguides, is proposed. The design of new kind of magnetron, the "Glow Discharge Inverted Magnetron" ("GDIM"), is presented. The GDIM is an inverted magnetron with the resonant structure located on the cathode. The resonant cavities are used as a source of glow discharge electron beams, which gives high power operation without requiring relativistic voltages.