Development of a surface wave probe to examine intermediate pressure plasmas

Examination of intermediate pressure (1-5 Torr) plasmas is difficult because often the collision frequency is on the order plasma frequency. In this poster, we will examine how surface waves can be used to measure plasma density, temperature, and collision frequency in discharges at 1-5 Torr. Surface waves can be excited along the sheath of a small antenna by coupling electromagnetic waves into the plasma via the electron plasma frequency. This coupling can be understood by examining the dielectric constant of the plasma. When electron-neutral collision becomes non-negligible the dielectric constant becomes a function of both the plasma frequency and the collision frequency. Typically, the collision frequency is small and thus is approximated as zero. However, a simple analysis of typical electron thermal velocity, electron-neutral collision cross-section, and neutral density shows above 1 Torr, the momentum transfer collision frequency is on the order of the plasma frequency. Thus, in the regime of 1 Torr and above, an attempt to measure the plasma density with interferometry or a single wire probe such as the Langmuir probe is futile. However, with the simultaneous use of three sensitive plasma absorption probes, it is possible to measure not only the density and temperature but also the collision frequency of the plasma. In this poster, we will examine the basic models needed to accomplish this task as well as preliminary measurements from plasmas in the 0.5 to 2.5 Torr regime.