Design of an asymmetrically biased triple Langmuir probe and accompanying diagnostics tool

Abstract

A refined triple Langmuir probe design is described for use in a glow discharge device, which creates plasma by applying a large bias voltage across a neutral gas. The goal is to design a Langmuir probe which can measure the plasma temperature, density, and floating potential to within an order of magnitude while minimizing plasma perturbation. The probe functions in a plasma temperature range of 1-10 eV. First, an overview of the relevant theory is provided, followed by the design assumptions and a derivation of the working regime of the Langmuir probe. This working regime dictates the appropriate branch of Langmuir probe theory whose equations can be used to design the probe and extract the plasma electron temperature, density, and floating potential. Second, the probe’s radius, length, and electrode spacing are derived using the applicable branch of Langmuir probe theory. The derived probe radius, length, and electrode spacing are 0.18 mm, 3 mm, and 55 mm, respectively. Third, an overview of the electrical design used to measure the triple probe voltages and currents is described. Finally, a discussion of the limitations and future work is provided, with methods listed to improve the specificity of the relevant theory and the accuracy of the probe measurements.