Design of an Interferometer to Measure Electron Density in Atmospheric Pressure Plasma

As a cost-effective way to measure electron density in atmospheric pressure plasmas, we explore optical interferometry using a continuous wave laser and lock-in detection. We present a Nomarski-type geometry designed to minimize sensitivity to inter-path vibrations and allow for future measurements with pulsed light. Typical electron densities of atmospheric pressure plasma jets range down to approximately 10^{12}/cc, which corresponds to a phase shift of only a few microradians in 632.8nm light. In order to increase our sensitivity to this small change, we drive the plasma with oscillating high voltage at 40 kHz and use that as our reference signal for phase-sensitive detection. We present the current design, preliminary results, and plans for upgrading the diagnostic in the future.