High-frequency dielectric resonance probe for the measurement of plasma densities

Recent theory and experiments show that the resonance value of the high-frequency signal of a metallic probe can only be used to measure the plasma density accurately if the detailed structure of the sheath is known. This paper describes a new resonance probe which makes use of the fact that a system consisting of a vacuum cylinder embedded in an infinite plasma has an eigenfrequency equal to ω_e/√2 and independent of the radius of the cylinder. It is shown that the resonance frequency of the probe-and thus the plasma density found-depends only slightly on the sheath. There is very good agreement between theory and experiments when the density obtained by this method is compared to the values measured by the electron and ion characteristics of the Langmuir probe. Plasma densities can be measured over a wide range by this method. General considerations on the difference between the resonant (or antiresonant) behavior of a metallic probe and that of a dielectric structure are given.