Microwave resonator probe for localized density measurements in weakly magnetized plasmas

A simple diagnostic tool for performing spatially resolved density measurements in a large, weakly magnetized, low-temperature plasma is described. The principle is based on a microwave technique to determine the cold plasma dielectric property ε=1−ωp2/ω2, where ωp is the electron plasma frequency. A parallel-wire quarter-wavelength resonator is immersed into the plasma, the resonance frequency (ωres≳ωp) is measured and the density derived from the simple relation ωp2=ω2res−ω2res(ε=1). In contrast to the familiar cavity resonance shift method, the present method is suited for localized density measurements. The spatial resolution is comparable to the size of the resonator (2-mm width×8 mm length, fres=7.45 GHz). The microwave technique is largely independent of sheath and thermal effects and is thus more reliable than probe measurements in nonuniformly rf-heated magnetoplasmas. A comparison with independent density diagnostic tools is presented and additional applications are pointed out.