Fully ionized nanosecond discharges in air: the thermal spark

Abstract

The formation and decay of the thermal spark generated by a single nanosecond highvoltage pulse between pin electrodes are characterized in this study. The influence of air pressure in the range 50 -1000 mbar is investigated at 300 K. By performing short-gate imaging and Optical Emission Spectroscopy (OES), we find that the thermal sparks exhibit an intense emission from excited electronic states of N + , in contrast with non-thermal sparks for which the emission is dominated by electronic transitions of N2. Spark thermalization consists of the following steps: (i) partial ionization of the plasma channel accompanied by N2 emission, (ii) creation of a fully ionized filament at the cathode characterized by N + emission, (iii) formation of a fully ionized filament at the anode, (iv) propagation of these filaments toward the middle of the interelectrode gap, and (v) merging of the filaments. The formation of the filaments, steps (ii) and (iii), occurs at subnanosecond timescales. The propagation speed of the filaments is on the order of 10 4 m/s during step (iv). For the 1-bar condition, the electron number densities are measured from the Stark broadening of N + and Hα lines, with spatial and temporal resolution. The electron temperature is also determined, from the relative emission intensity of N + excited states, attaining a peak value of 48,000 K. In the post-discharge, the electron number density decays from 4×10 19 to 2×10 18 cm -3 in 100 ns. We show that this decay curve can be interpreted as the isentropic expansion of a plasma in chemical equilibrium. Comparisons with previous experiments from the literature support this conclusion. Expressions for the Van der Waals and resonant broadenings of H, Hβ, and several lines of O, O + , N and, N + are derived in the appendix.

Ionization Mechanism in a Thermal Spark Discharge

“…The electron number density, measured by Stark broadening of N + , reached ne = 9×10 18 cm -3 , representing an ionization of 36%. Such high electron number densities and temperatures were also measured by other teams in pin-to-pin configurations [10][11][12][13][14] and nanosecond Dielectric Barrier Discharges [15,16].…”

“…In a previous work [13], we compared our results to the literature and demonstrated that, in pin-to-pin configurations, thermal sparks are generated in five stages:…”

Improvement of lean blow out performance of spray and premixed swirled flames using nanosecond repetitively pulsed discharges

“…The temperature in the plasma T gas,plasma ≈ 3500 K, higher than the adiabatic flame temperature (T ad < 2100 K), and the dissociation of O 2 indicate that this discharge is neither an NRP-corona nor an NRP-glow discharge [13]. n e is between the values reported for nonequilibrium NRP-sparks [12], and for thermal sparks [21]. As neither N + emission (500 nm) nor an electron continuum is observed in the spectra, the NRPspark is necessarily of the nonequilibrium type.…”

“…A 6-meter long coaxial cable (75 Ω impedance) connects the generator and the electrode. A high voltage probe (Lecroy PPE20kV) and a current probe (Pearson 6585) are placed midway on the cable and are used to monitor the deposited energy on an oscilloscope (Lecroy HDO 6104) [21]. Typical voltage and current curves are shown in Fig.…”

“…This energy is deposited during subsequent reflections at t = 90 ns and t = 180 ns. For detailed descriptions of the oscillograms, see [11,21].…”