An electron plasma wave absorption diagnostic

We test a prototype electron plasma wave absorption diagnostic in a low density Argon DC gas discharge plasma with multi-dipole confinement. A primary difficulty with microwave probe diagnostics near the electron plasma frequency is the fact that the probe fields typically sample principally the spatial region dominated (and perturbed) by the material probe. In addition, the probe impedance is normally dominated by the evanescent surface-wave component which produces a significant capacitance with a dielectric that changes sign at the plasma frequency - and thus passes through zero. We attempt to avoid these difficulties by using a two-part probe that is designed to cancel the surface wave component and permit the excitation and detection of electron plasma waves across a volume of plasma well outside of the plasma sheath. In principle, the electron plasma wave absorption signal can provide information on the electron density, temperature, and flow. We present preliminary results of transmission signals in an unmagnetized plasma with density near 10^9 cm^-3.