Numerical and experimental study on a pulsed-dc plasma jet

Abstract

A numerical and experimental study of plasma jet propagation in a low-temperature, atmospheric-pressure, helium jet in ambient air is presented. A self-consistent, multi-species, two-dimensional axially symmetric plasma model with detailed finite-rate chemistry of helium-air mixture composition is used to provide insights into the propagation of the plasma jet. The obtained simulation results suggest that the sheath forms near the dielectric tube inner surface and shields the plasma channel from the tube surface. The strong electric field at the edge of the dielectric field enhances the ionization in the air mixing layer; therefore, the streamer head becomes ring-shaped when the streamer runs out of the tube. The avalanche-to-streamer transition is the main mechanism of streamer advancement. Penning ionization dominates the ionization reactions and increases the electrical conductivity of the plasma channel. The simulation results are supported by experimental observations under similar discharge conditions.

The effect of working gas impurities on plasma jets

“…The plasma channel filled with electrons is often used to show the discharge structures of plasma jet. 12 For the highest purity helium (99.99999%) without consideration of air intrusion, the discharge near the dielectric surface over the metal electrode creates a high-density plasma (10 18 /m 3 ) during the rising phase of applied voltage, and the increasing voltage broadens the high density plasma region. Figure 1(a) represents the plasma at 42.5 ns (2 ns before the discharge stopped).…”

“…Figure 1(a) represents the plasma at 42.5 ns (2 ns before the discharge stopped). The high density plasma region is squeezed by the expanding sheath 6,12 to form a channel along the tube axis, and the peak electron density in the channel is 7.4 Â 10 18 /m 3 . For the other three cases (helium purity 99.99%, 99.9%, and 99%), plasma jets with a ring-like hollow shape are observed at 60 ns [ Figures 1(b)-1(d)].…”

“…The numerical model used is an extension of a previously described model 6,12 and incorporates the photoionization source term based on the three-term exponential Helmholtz model developed by Bourdon et al 14 The threeterm exponential Helmholtz model is derived from the widely used integral model by Zheleznyak. 15 The photoionization of air serves as the starting point where the rate of photoionization of O 2 (O 2 þ hv !…”

“…The calculated helium and air mole fraction distribution were found to be the same as those previously reported. 12 Power is applied at a thin electrode embedded within the dielectric tube. The thickness of the powered electrode is 0.2 mm and the distance from the nozzle is between 1.5 mm and 3.5 mm.…”

“…Air entering the discharge from the nozzle and impurities from the gas supply system were reported to significantly influence the density and lifetime of He ( 3 S 1 ) metastables. Two dimensional models developed by Boeuf and Pitchford, 10 Naidis, 11 and Liu et al 6,12 captured the ringshaped plasma bullet at the position where air molar fraction is about 1% in the mixing layer. Breden and Raja 13 added 1% air, 1% pure nitrogen (N 2 ), and 1% pure oxygen (O 2 ) impurities to the helium background in their model.…”