Streamer-to-spark transition initiated by a nanosecond overvoltage pulsed discharge in air

Abstract

This study is focused on the streamer-to-spark transition generated by an overvoltage nanosecond pulsed discharge under atmospheric pressure air in order to provide a quantitative insight into plasma-assisted ignition. The discharge is generated in atmospheric pressure air by the application of a positive high voltage pulse of 35 kV to pin-to-pin electrodes and a rise time of 5 ns. The generated discharge consists of a streamer phase with high voltage and high current followed by a spark phase characterized by a low voltage and a decreasing current in several hundreds of nanosecond. During the streamer phase, the gas temperature measured by optical emission spectroscopy related to the second positive system of nitrogen shows an ultra-fast gas heating up to 1200 K at 15 ns after the current rise. This ultra-fast gas heating, due to the quenching of electronically excited species by oxygen molecules, is followed by a quick dissociation of molecules and then the discharge transition to a spark. At this transition, the discharge contracts toward the channel axis and evolves into a highly conducting thin column. The spark phase is characterized by a high degree of ionization of nitrogen and oxygen atoms shown by the electron number density and temperature measured from optical emission spectroscopy measurements of N + lines. Schlieren imaging and optical emission spectroscopy techniques provide the time evolution of the spark radius, from which the initial pressure in the spark is estimated. The expansion of the plasma is adiabatic in the early phase. The electronic temperature and density during this phase allows the determination of the isentropic coefficient. The value around 1.2-1.3 is coherent with the high ionization rate of the plasma in the early phase. The results obtained in this study provide a database and the initial conditions for the validation of numerical simulations of the ignition by plasma discharge.

On the arc transition mechanism in nanosecond air discharges

“…One can see from Table 1 that the transition to the arc increases the electron number density and the gas temperature. It has been demonstrated recently [12] that this transition occurs for nanosecond plasmas in less than 5 ns. We note that spectra before the transition are dominated by molecular emission (in air, the second positive system of N2), while the spectra after the transition are dominated by a broadband continuum emission and ion lines of N + .…”

Experimental characterisation of the thermal energy released by a Radio-Frequency Corona Igniter in nitrogen and air

“…The plasma assisted ignition is based on the non-equilibrium low-temperature plasma and is characterised A C C E P T E D M A N U S C R I P T by streamers emission. Under this regime the heat loss to the electrodes is negligible [26], contrary to standard spark igniter. The energy is moved efficiently to the rotational, vibrational and electronic excited states of the medium molecules [27,28,29].…”

Hydrodynamic effects induced by nanosecond repetitive pulsed discharges

“…Values measured for all the considered conditions are within the range 1.5-3.5 mJ per pulse in the burst. It should be noted that for all the considered conditio ns a transition streamer arc described in [38] occurs within a nanosecond or so. Figure 3 demonstrates images of total emission before and after streamer-arc transition for a single nanosecond spark initiated in ambient air by a 10 ns pulse of 30 kV.…”

“…Radial profiles of the discharge before and after the transition to the arc are presented in figure 4. One of the main peculiarities of the transition to the arc is the temperature rise up to 40000 K at nanosecond scale (see [38] for more details). This provides high level temperature gradients and as a consequence shock waves and hydrodynamic motion of the gas in the area around the electrodes.…”