Electron number density measurements in nanosecond repetitively pulsed discharges in water vapor at atmospheric pressure

Florent P Sainct,Christophe O Laux,Erwan Pannier,Deanna A Lacoste,Keiichiro Urabe

doi:10.1088/1361-6595/ab681b

Abstract

This article reports on experiments in a nonequilibrium plasma produced by nanosecond repetitively pulsed (NRP) spark discharges in water vapor at 450 K and atmospheric pressure. The objective is to determine the electron number density in the post-discharge, with spatial and temporal resolution, to gain a better understanding of the discharge development and chemical kinetics. Electron number densities were measured in water vapor from the broadenings and shifts of the Hα and Hβ lines of the hydrogen Balmer series and of the atomic oxygen triplet at 777 nm. For an average reduced electric field of about 150 Td, high electron densities up to 3 × 1018 cm−3 are measured at the cathode, up to 5 × 1017 cm−3 at the anode, and up to 4 × 1016 cm−3 in the interelectrode gap. The high density near the electrodes is attributed to ionization enhancement and secondary electron emission due to the high electric field in the plasma sheath. In the middle of the inter-electrode gap, we show that the electron density mainly decays by electron attachment reactions. The dissociation fraction of water vapor is estimated to be around 2% in the middle of the gap.

Highlights

This article reports on experiments in a nonequilibrium plasma produced by nanosecond repetitively pulsed (NRP) spark discharges in water vapor at 450 K and atmospheric pressure
Atmospheric pressure nonequilibrium plasmas in steam or in water-containing gaseous mixtures are of interest for a wide variety of applications such as the production of hydrogen [1], pollutant abatement in flue gas [2, 3], bio-decontamination [4, 5], surface treatment [6], or plasma-assisted combustion when the discharge is applied in a mixture containing burnt gases [7]
After verifying that the Stark effect was the main source of broadening, this non-intrusive method was applied to determine the electron density in a plasma produced by nanosecond repetitively pulsed (NRP) spark discharges in water vapor with 1% of nitrogen, at 450 K and atmospheric pressure

Summary

Introduction

Atmospheric pressure nonequilibrium plasmas in steam or in water-containing gaseous mixtures are of interest for a wide variety of applications such as the production of hydrogen [1], pollutant abatement in flue gas [2, 3], bio-decontamination [4, 5], surface treatment [6], or plasma-assisted combustion when the discharge is applied in a mixture containing burnt gases [7]. The electron density can be obtained by analyzing the Stark broadening of atomic emission lines [10,11,12]. This is the case at sufficiently high electron densities, where Stark broadening becomes dominant over natural, van der Waals, resonant, Doppler, and instrumental broadenings. After verifying that the Stark effect was the main source of broadening, this non-intrusive method was applied to determine the electron density in a plasma produced by nanosecond repetitively pulsed (NRP) spark discharges in water vapor with 1% of nitrogen, at 450 K and atmospheric pressure.

Methods

Results

Conclusion