Lock-in Interferometry for Time-resolved Electron Density Measurements at Atmospheric Pressure

As part of our work on understanding plasma-based remediation of contaminated water, we are developing an interferometer to measure the electron density in atmospheric pressure plasmas driven by nanosecond pulses (NSP). Interferometry is widely used in low-pressure plasmas but is less common for atmospheric pressure plasma (APP). The index of refraction of APPs is not entirely dominated by the electron density and interpretation is often complicated by neutral gas heating. We use a pulsed light source and low-jitter timing electronics to simplify interpretation and recover stroboscopic time-dependence in the plasma discharge. Because the ionization fraction is low (10^-4) and the physical size of the plasmas is small (mm), the interferometric phase shift is also small (microradians). We use lock-in detection and an integer-ratio combination of pulse rates to recover the small signal in spite of significant EMI from the plasma. We will describe the current status of our diagnostic and present preliminary time-resolved electron density measurements.