Laser Absorption Measurements of Electron Density in Nanosecond-ScaleAtmospheric Pressure Pulsed Plasmas

TL;DR

This paper demonstrates a method for measuring electron density in nanosecond atmospheric pressure plasmas using continuous-wave laser absorption, showing high densities and decay dynamics consistent with Stark broadening data.

Contribution

It introduces a time-resolved laser absorption technique for electron density measurement in low-energy air plasmas, validated against Stark broadening results.

Findings

Electron densities reach up to 7×10^{19} cm^{-3}.

Electron density decays to 1/e in about 50 ns.

Laser absorption measurements agree with Stark broadening data.

Abstract

We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as $n_\text{e}=7\times10^{19}$ cm$^{-3}$, and decay to $1/e$ of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[$^5S{^o}$]-3p[$^5P$] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics.