Millimeter-wave graphite antennas for diagnostics of plasmas in large fusion devices

Abstract

The investigate plasmas in large reactor-type fusion devices, e.g., ITER, various microwave diagnostics such as reflectometry, interferometry, radiometry in the frequency range between 20 and 200 GHz, the at ordinary and extraordinary waves are being developed [1]. Their antennas and reflectors, facing the plasma, must retain their operating characteristics unchanged over a long period of service under exposure of plasma fluxes and different radiations of a thermonuclear plasma: fluxes of neutrons, gammas and electromagnetic radiation. These factors can bring about the deterioration of heat conductivity, the change in the material structure, a heating up to 1000 °C with the result that the mechanical and radiotechnical propertiesof antennas change. To avoid changes in antenna performance (radiation pattern (RP), wave polarization, absolute wave amplitude) and to preclude the ingress of heavy impurities into the plasma due to erosion of metal microwave components we have proposed to use carbon-graphite (CG) antennas, reflectors, waveguide sections as an alternative to metallic ones [2].

We have investigated horn, spherical antennas, and also plane and shaped reflectors in the 20 to 105 GHz band. Since pyramidal horns are in most common use, their sizes were optimized in the basic parameters, i.e., antenna beam half-width, amplitude near the principal axis, level of signal depolarization.