Sampling of Cold Atmospheric Pressure Plasmas into a differential pumping arrangement for Langmuir Probe Diagnostics

Cold atmospheric plasmas (CAP) are non-thermal gaseous discharges that are generated in ambient (atmospheric) pressure conditions. Here we present a novel diagnostic method where a small volume of CAP is taken from atmospheric pressure (10³ mbar) into a low-pressure environment (1 mbar) using a differential pumping arrangement. Once the plasma sample is brought into a low-pressure region, it can be characterized using conventional plasma diagnosis tools such as the Langmuir probe. However, when a differential pumping stage is used to take a sample of atmospheric plasma, the pressure gradient existing in the flow (from 10³ mbar to 1 mbar) will modify the density of the plasma, which in turn will corrupt the probe and analyzer data. So, to solve this issue, the number density of the flow should be estimated at every point inside the vacuum system. Also, the feasible dimensions of the vacuum chamber to achieve the desired vacuum needs to be fixed before actual fabrication. To solve the above-mentioned issues, the gas flow inside the chamber was simulated using COMSOL multi-physics software. High Mach number flow module which uses Navier Stroke’s equations for compressible flow has been adopted. In the experimental part, we present the Langmuir probe measurement of CAPs sampled into the vacuum system. The ion number density was successfully measured using the IV data acquired from the Langmuir probe and was found to be increasing with the power ranges we have considered in our experiments.