Plasma diagnostics with a high-aspect ratio cylindrical probe used as an impedance probe

Abstract

Using a network analyzer to supply a driving rf signal to varying sizes of spherical probes in plasma, we have performed plasma diagnostics using theory based on an assumption of a thin sheath and a collisionless plasma. The rf signal applied to the probe by the network analyzer is small in magnitude compared to probe bias voltages and the instrument returns both real and imaginary parts of the complex plasma impedance as a function of frequency for given bias voltages. This information can be used to determine plasma potential, electron density and temperature, in addition to the electron distribution function.1,2 Further, we have shown that sheath resistance and sheath density profiles are also a product of the method. The theoretical basis of the work indicates that in the thin sheath limit the results should be independent of probe geometry. To test this we have used a 100-1 (length - radius) aspect ratio cylinder which we also sweep as a Langmuir probe. We compare the impedance-based plasma measurements to those of the cylinder when swept as a Langmuir probe using conventional algorithms to reduce the I–V characteristic. This method has general application to either laboratory or space plasma measurement