A Novel Diagnostic Technique for Cold Atmospheric Pressure Plasma Diagnosis

Placing the Langmuir probe directly into a Cold Atmospheric Plasma (CAP) jet for plasma diagnosis will not yield proper result especially while the jet is operated at low frequencies. This is due to the very large breakdown voltage required to run the jet at low frequencies in ambient pressure conditions. This results in creation of additional filament discharges in the presence of the conductive probe in the plasma. Such discharges would consequently alter the plasma parameters that the probes are intended to measure. Also, excessive collisions of charged particles at atmospheric pressure drastically restrict the current magnitude that might be captured by probe. Here we propose a technique of extracting the plasma plume from the jet, taking it to a vacuum system through a tiny orifice for Langmuir probe diagnosis. Simulations were conducted to determine the feasibility of establishing and maintaining a low pressure within an open chamber, while minimizing the pressure differential at the orifice. Based on simulation results, a chamber was constructed, and the experimental validation of the results were performed. Various probe designs have been explored to get an accurate I-V curve due to the low current produced by the extracted plasma sample. A double Langmuir probe configuration was utilized. The probe analysis incorporated the influences of sheath collisions, fluid velocity, recombination losses and pressure differential in order to compute the plasma number density and electron temperature under the working condition of 1–5 watt plasma power at 12 kilohertz, with a low flow rate of 5slm.