Electron Temperature Derived from Measurements of Complex Plasma Impedance
Abstract
In earlier work, using a network analyzer, we have shown the existence of collisionless resistance (CR) in the sheath of spherical probe when driven by a small RF signal. For any position in the sheath, the CR was shown to be inversely proportional to the plasma density gradient there; but to exist only when the applied frequency corresponds to the plasma frequency (omega sub pe), or density, at that position. We have recently begun a study of the low-to-intermediate frequency response of the probe to the RF signal. At sufficiently low frequencies, the CR is beyond cutoff, i.e., since the earlier work shows that the existence of CR depends on the plasma density gradient, there is a cutoff which is proportional to the applied bias level and which will occur at the plasma frequency at the surface of the probe. Since electron density at the probe surface decreases as a function of applied (negative) bias, the CR will extend to lower frequencies as the magnitude of negative bias increases. Therefore, to eliminate both CR and ion current contributions, the frequencies we consider are much greater than the ion plasma frequency (omega sub pi), but less than the plasma frequency at the probe surface (omega sub pe (r sub 0)), where r sub 0 is the probe radius. We show that, in this frequency regime, the complex impedance measurements made with a network analyzer can be used to determine electron temperature. We present an overview of the theory used along with comparisons to data sets made using three stainless steel spherical probes of different sizes in different experimental environments and different plasma parameter regimes. We compare the results of the temperature measurements to those made by conventional Langmuir probe sweeps; the method shown here requires not curve fitting as is the usual procedure with Langmuir probes when a Maxwell Boltzmann electron distribution function is assumed.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 20, 2008
- Accession Number
- ADA489386
Entities
People
- D. D. Blackwell
- D. N. Walker
- Richard F. Fernsler
- W. E. Amatucci
Organizations
- United States Naval Research Laboratory