Using rf impedance probe measurements to determine plasma potential and the electron energy distribution

Abstract

Earlier work has demonstrated the usefulness of a network analyzer in plasma diagnostics using spherical probes in the thin sheath limit. 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 sheath resistance, sheath density profiles, and a technique for measuring electron temperature. The present work outlines a method for finding plasma potential and the electron energy distribution within a limited energy range. The results are compared to those using conventional Langmuir probe techniques. The rf method has general application to diverse areas of plasma investigations when the plasma is uniform and probe dimensions are much less than the size of the plasma. These applications include laboratory and space environments.

Cutoff probe using Fourier analysis for electron density measurement

“…In addition, by virtue of this method, the cutoff frequency point can be measured without a network analyzer and the speed of cutoff probe measurement was enhanced more than a million times without loss of measurement accuracy compared to the previous one. This new method is expected to have applications not only in the fast measurement of absolute electron density with a cutoff probe, but also to other previously developed diagnostics where a network analyzer is used, specifically a hairpin probe, 21,22 and an impedance probe, [23][24][25] by replacing the network analyzer with a nanosecond impulse generator and an oscilloscope. …”

Advances in Impedance Probe Applications and Design in the NRL Space Physics Simulation Chamber

“…It is possible to estimate the plasma potential p from the minimum in Re(Z (ω, V p )) versus probe voltage at low frequency (ω < ω p /3) as shown in [19] dRe(Z (ω, V p )) dV p…”

“…Note also that the higher frequency resonance is unaffected. Subsequent work demonstrated that the plasma potential could be identified from sweeping the probe dc bias at a fixed ac frequency, as long as that frequency met the criterion that ω pi < ω < ω pe (r 0 ), where ω pe (r 0 ) is the plasma frequency at the probe surface [19]. Eliminating ω pi allows us to avoid ion plasma oscillations and assuming that ω < ω pe (r 0 ) eliminates any resonant energy losses associated with collisionless damping [16].…”

Magnetic Field and Geometry Effects on Finding Plasma Potential with a Cylindrical Impedance Probe

“…In that work we determined electron density and temperature (n e , T e ), plasma potential, φ p , and, the electron energy distribution, f(ε), using the same impedance probe techniques developed for spherical geometry and presented in studies using probes of varying sizes. [2][3][4][5] It is well known that n e , T e determined using Langmuir techniques to analyze the IV characteristic of a cylindrical probe can be affected by the presence of a magnetic field. Magnetic field effects are a function of probe geometry, bulk plasma parameters and, the magnitude of the field itself.…”