Laser absorption measurements of electron density in nanosecond-scale atmospheric pressure pulsed plasmas

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 e = 7 x 10 19 cm(-3) and decay to 1 / e of their peak values over a period of about 50ns following plasma formation using a 20 mJ, 10ns 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[ 5 S o]-3p[ 5 P] electronic transition in atomic oxygen at 777nm. 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.

Automated summary

This study reports time-resolved measurements of electron number density in low-energy nanosecond-scale laser-produced sparks in atmospheric pressure air using continuous-wave laser absorption. The plasma electron number densities decay rapidly over time, and are shown to be reasonably consistent with Stark broadening measurements of electronic transitions in atomic oxygen.