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

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 H α and H β 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.

Fully ionized nanosecond discharges in air: the thermal spark

“…The measurements clearly show that the main broadening effect is due to the Stark effect. This approach was already used by Sainct et al [11] to confirm that, in their work, the measured H broadening widths were due to the Stark effect. Based on our measurements, we find that the lines at 519 nm are well fitted using Eq.…”

The role of excited electronic states in ambient air ionization by a nanosecond discharge

“…The different states of non-equilibrium NRP plasmas (corona, glow, and spark) have been described by Pai et al [2]. However, other works [7][8][9][10][11][12][13][14] have shown that nanosecond discharges can also produce fully ionized thermal plasmas. This type of nanosecond discharge, called the thermal spark, is in thermal equilibrium [7] and differs from the NRP spark described by Pai et al [2].…”

On the Stark Effect of the O I 777-nm Triplet in Plasma and Laser Fields

“…The error bars indicate estimated uncertainties of our calculations. The results presented above are based on the assumption of a stationary diagonal density matrix, see Equation (6). This is, however, not necessarily the case with nearly resonant laser pumping of sufficient magnitude.…”

“…Notably, in that study a plasma filament was formed by a femtosecond Ti:sapphire (λ = 805 nm) laser pulse in air. On the other hand, a very recent experimental study of Sainct et al [6] found a fair agreement in the density determined from the Stark broadening of the O I 777-nm line and that of hydrogen Balmer-α and -β in pulsed discharges at atmospheric pressure. The typical electron density in this case was 1-2 orders of magnitude higher than in [4,5], though.…”