High-speed plasma measurements with a plasma impedance probe

TL;DR

This paper advances plasma impedance probe techniques by developing a new analytical model incorporating sheath thickness, and achieving MHz-range time-resolved measurements, enabling detailed plasma density oscillation analysis.

Contribution

It introduces a streamlined analytical model for PIP interpretation and techniques for MHz-range high-speed plasma density measurements.

Findings

Validated the model with numerical simulations and measurements.

Achieved 1 MHz time resolution in plasma measurements.

Resolved plasma density oscillations up to 150 kHz at high temporal resolution.

Abstract

Plasma impedance probes (PIPs) are a type of RF probe that primarily measure electron density. This work introduces two advancements: a streamlined analytical model for interpreting PIP-monopole measurements and techniques for achieving $\geq 1$ MHz time-resolved PIP measurements. The model's improvements include introducing sheath thickness as a measurement and providing a more accurate method for measuring electron density and damping. The model is validated by a quasi-static numerical simulation which compares the simulation with measurements, identifies sources of error, and provides probe design criteria for minimizing uncertainty. The improved time resolution is achieved by introducing higher-frequency hardware, updated analysis algorithms, and a more rigorous approach to RF calibration. Finally, the new model and high-speed techniques are applied to two datasets: a 4 kHz plasma density oscillation resolved at 100 kHz with densities ranging between $2 \times 10^{14}$ to $3 \times 10^{15}$ m$^{-3}$ and a 150 kHz oscillation resolved at 4 MHz with densities ranging between $4 \times 10^{14}$ to $6 \times 10^{14}$ m$^{-3}$.