Flow dynamics and invasion by background gas of a supersonically expanding thermal plasma

Abstract

The transport of neutral argon atoms in an expanding thermal argon/hydrogen plasma is studied by means of laser-induced fluorescence spectroscopy around 811 nm, on the long living Ar[4s] atoms. Although the Doppler shifted laser-induced fluorescence measurements are performed on argon atoms in the metastable Ar * ( 3 P 2 ) and resonant Ar * ( 3 P 1 ) states, it is argued that in the plasma jet the velocity distribution function of these Ar[4s] atoms images the velocity distribution functions of the ground-state argon atoms. From the results it is inferred that the velocity behaviour of the supersonically expanding argon gas can be predicted from the momentum balance, and the temperature from the adiabatic relation between density and temperature. However, the adiabatic constant is found to be 1.4 ± 0.1, smaller than the adiabatic constant of a neutral argon gas expansion which is 5 3 . Both in the axial and in the radial directions the velocity distributions measured in the shock region show clear departures from thermodynamic equilibrium. From the radial velocity distribution it is concluded that background gas invades the supersonic part of the expanding plasma jet. The results on temperature and velocity in the subsonic region show that the radius of the plasma jet hardly increases after the stationary shock front, indicating that the flow pattern is geometrically determined.

Inflow and shock formation in supersonic, rarefied plasma expansions

“…Plasmas 12, 102303 ͑2005͒ mation, in which freely expanding, cold particles are converted into slow, warm particles, is further supported by the axial profiles of the density ratio, temperature, and velocity of both components, which can be determined from the measured vdfs. 27 Such a change from a supersonic to a subsonic flow is usually observed in shocks. 13,14 The supersonic flow adapts to the downstream conditions over a finite length in the order of one to several mean free paths.…”

“…Three different gases have been used to create a plasma expansion: argon, hydrogen, and nitrogen. In argon, a flow of 50 scc/ s ͑standard cubic centimeter per second͒ and a current of 40 A is used, leading to a pressure in the arc of around 6.2ϫ 10 4 Pa. 27 In hydrogen, a flow of H 2 of 50 scc/ s is used and a current of 60 A is applied, leading to a pressure of around 1.2ϫ 10 4 Pa in the arc. 28 In nitrogen, a diameter of the arc channel of 3 mm instead of 4 mm was used and a current of 55 A.…”

“…19,20 In fact, in the Ar fluorescence experiments a few percent of H 2 has been admixed to lower the ionization degree and thus the Ar * metastable density to avoid saturation. 27 In the nitrogen plasma the recombination via molecular processes is less efficient. 29,34 Hence the ions survive longer and the ionization degree remains relatively high.…”

“…35 Finally, Rayleigh scattering is used to measure the neutral densities of both argon atoms and hydrogen molecules. 15,27 The Rayleigh scattering is performed using a 230 nm laser beam, of which the details are given in Ref. 36.…”

Multi-scale Modelling of the Two-Dimensional Flow Dynamics in a Stationary Supersonic Hot Gas Expansion

“…Although several studies have been devoted to supersonic expansion of gas jets in a low pressure environment [3,4,5,6], full understanding of the processes governing the flow has not been reached yet. In particular, it is still not completely clear whether, due to rarefaction effects, the barrel shock still protects the supersonic part of the flow from the invasion of background molecules, i.e.…”