Vacuum Resonant-Cavity Dielectrometer for in Situ Measurevents of Complex Permittivity in a Simulated Space Environment

Abstract

The instrument consists of an invar cylindrical cavity of variable length, suitably excited in the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">014</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">015</sub> modes by a fixed X-band frequency of 9.28 GHz. The inside surface of the cavity is silver plated. Disc-shaped specimens are placed on top of a micrometer-driven noncontacting plunger that forms one of the end-plates. The other end-plate is a vacuum-tight flange with a window to permit particulate or ultraviolet radiation to enter the cavity and bathe the specimen. Signal coupling hoses are placed along the cavity side wall on the same plane, at a distance 3 λ/4 from the top end-plate. Input ports are two tectangular slots machined lengthwise along the major axis of the cavity and coupled 180 degrees out of phase at the points of maximum magnetic field. The output port is an orifice located halfway between the input slots. A side-arm vacuum flange below the cavity connects to a sputter-ion vacuum pump. Another similar flange provides an insertion port for the specimens. The shaft of the plunger is isolated from the chamber by a stainless steel bellows. To facilitate specimen insertion, the plunger-micrometer assembly is lowered by a rack and pinion. Precise repositioning is achieved by two conical indexing plugs. Relative dielectric constant of a specimen is obtained from the shift in cavity resonant length when the specimen is inserted. Loss tangent is obtained from the shift in half-power resonance bandwidth. Dielectric measurements under vacuum have been made with the specimen irradiated with ultraviolet light.