Paper PS2-WeA8
Plasma Diagnostics by a Coaxial Resonant Cavity

Wednesday, October 22, 2008, 4:00 pm, Room 306

Microwave plasma diagnostics were developed mainly in the field of microwave and plasma interaction. Recently, these methods have also been adopted for plasma dischargers. The main advantages of microwave diagnostics are; (a) the capability to measure directly a plasma electron density and, (b) the applicability in a process gas environment. Intended to use in dischargers, new devices (e.g. plasma absorption probe, U-shape probe, plasma microwave interferometer) were developed based on the microwave method in the last decade. However, when these open structural probes are used in a small chamber, radiation from the probe surface cannot be neglected, resulting in random resonances caused by interference with a chamber wall. To resolve this issue, a coaxial resonant cavity is developed, which is open for plasma diffusion, while it is closed with respect to microwave radiation. In vacuum, the cavity is designed to have a sharp resonance in a 2-4 GHz band. When plasma is generated in a chamber, the resonant frequency up-shifts due to a change in the effective dielectric constant. Once the resonant frequency is given as a function of a loaded electron density, the electron density is measured by monitoring a frequency-shift. Preliminary measurements of electron plasma density of Argon and Nitrogen showed good agreements with those of a Langmuir probe. Theoretically, this probe can measure the electron temperature of a plasma, since the electron temperature can be represented as a function of variation in the quality factor of a resonance in the cavity. This possibility will be also studied and discussed in the paper.