Spatial evolution of the plasma kernel produced by nanosecond discharges in air

Abstract

This work presents an experimental investigation of the hydrodynamic effects induced by nanosecond and conventional spark discharges. The energy deposited in sparks in the short breakdown time (~1 mJ/mm) induces hydrodynamic effects that redistribute the energy over a large volume (~1 cm 3) surrounding the initial plasma channel. This process influences the subsequent formation of the ignition kernel and the initiation of combustion. The experimental results presented in this paper were obtained with a set of synchronized diagnostics including Schlieren, OH Planar Laser Induced Fluorescence and electrical measurements of the energy deposited in the plasma. It is shown that the motion of the gas excited after the discharge breakdown depends not only on the total deposited energy but also on the dynamics of the energy input in the plasma. Finally, the effects of nanosecond sparks are compared with those of conventional sparks used for internal combustion engines. We show that, with 20 times less energy, the nanosecond spark produces a twice bigger excited gas volume than the conventional spark. This is because the energy deposited by the nanosecond spark during the breakdown stage is three times higher than for the conventional spark.

Ionization Mechanism in a Thermal Spark Discharge

“…For the same reasons, a 3 rd pulse is detected at t = 150 ns and deposits another 0.25 mJ. This sequential energy deposition was already observed in previous work [13,17,18] and could explain the "ringing" observed in Ref. [19].…”

“…Note that the steep increase of the electron number density (more than one decade) at t = 4 ns corresponds to the filament formation. For t > 10 ns, the electron density slowly decays to 5×10 18 ) Time (ns) We note that the measurements from the Stark broadening of Hα and N + are in excellent agreement. The electron number densities measured with the continuum emission and the intensity of N + and O + lines agree within the uncertainties of each technique for t > 12 ns.…”

“…This heating was associated with the emission of N + lines. 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].…”

Gas dynamics and vorticity generation in laser-induced breakdown of air

“…In this scenario, a perfectly symmetric torus will be obtained (with no third lobe observed) since the vortex pair will have equal strength. Such kernel dynamics are sometimes encountered in nanosecond pin-to-pin discharges when the energy is distributed homogenously inside the electrode gap [24,38,39]. Moreover, while the present focus is on NIR laser sparks, the mechanism described here can also be used to explain the hydrodynamics observed in visible and ultra-violet laser induced plasma.…”

Ground-State Atomic Nitrogen Measurements using fs-TALIF in High-Pressure NRP Discharges

“…During the first 10 µs, a very sharp decrease is noted. This is believed to be caused by the hydrodynamic effects that were previously observed in the discharge at early times which makes the plasma expand very quickly as it cools down 23 . Between 10-60 µs the number density stays relatively constant throughout.…”