Ktal Karel Burm

The isentropic exponent in plasmas

The isentropic exponent for gases is a physical quantity that can ease significantly the hydrodynamic modeling effort. In gas dynamics the isentropic exponent depends only on the number of degrees of freedom of the considered gas. The isentropic exponent for a plasma is lower due to an extra degree of freedom caused by ionization. In this paper it will be shown that, like for gases, the isentropic exponent for atomic plasmas is also constant, as long as the ionization degree is between 5%–80%. Only a very weak dependence on the electron temperature and the two nonequilibrium parameters remain. An argon plasma is used to demonstrate the behavior of the isentropic exponent on the plasma conditions, and to make an estimation of the value of the isentropic exponent of a customary plasma. For atmospheric plasmas, which usually have an electron temperature of about 1 eV, a sufficiently accurate estimate for the isentropic exponent of plasmas is 1.16.

Ionization efficiency in a geometrically pinched cascaded arc

Remote deposition allows separate optimization of the plasma production source and of the deposition process. To improve the ionization performance of the source, an argon cascaded arc plasma is studied by simulations. Improvements of the source performance in ion yield are achieved by constricting the bore of the arc channel near the entrance. Such a geometrical pinch construction leads to a higher neutral density at the arc inlet which results in increased ionization in the cascaded arc. The improved ionization performance is analysed by solving numerically the conservation laws of mass, momentum and energy in a two-dimensional hydrodynamic approximation using a pressure linked algorithm. The results are compared with those of a simplified one-dimensional formulation in order to identify the main mechanisms. The results indicate that, by constricting the bore of the arc channel, a very high ionization degree can be obtained.

An Alternative Quantitative Analysis of Non-Equilibrium Transport Coefficients in Argon Plasmas

To describe plasmas in non-local thermal equilibrium (non-LTE) four parameters need to be used, which are usually besides the pressure and the electron temperature, the electron density and the atom temperature. In the approach presented here it is argued that the use of four other variables is preferable. These four parameters are the total pressure, the ratio of the electron density and the squares root of the total pressure, and two specific non-equilibrium parameters. The non-equilibrium parameters are chosen such that they describe deviations from ionization–recombination equilibrium, and deviations from temperature equilibrium.It appears that the influence of deviations from complete LTE on the transport coefficients is often small when the parameters are scaled with the electron density and the pressure. In this way, the non-LTE transport properties can be estimated by using complete LTE transport coefficients without losing much accuracy.

Plasma expansion in the preshock region

The supersonic expansion of an underexpanding argon plasma from a high density arc source with small dimensions into a low-pressure vessel with large dimensions is studied by an extended one-dimensional nonlocal thermal equilibrium fluid model, called SPIRIT. In an expanding plasma the velocity increases and the pressure, the density, and the temperatures decrease severely. In this article the virtual source model is discussed first, which is a model describing the expanding plasma as originating from a virtual source. The virtual source model includes some viscosity and heat transport in simplified form, but most of the viscosity and heat transport contributions are neglected. The SPIRIT code includes the full energy and momentum balances. The inclusion of viscosity and heat sources may lead to deviations from an adiabatic and/or isentropic expansion. The SPIRIT code can analyze the deviations. When deviations are small, the isentropic expressions from gas dynamics can be used to model expanding plasma t...

Mach numbers for gases and plasmas in a convergent-divergent cascaded arc

For a plasma, flowing through a cascaded arc channel with a varying cross-section, and flowing from a subsonic to a supersonic state, the sonic condition moves downstream and the plasma Mach number at the smallest cross section is less than one, although in case of a transonic isentropic gas flow the sonic condition is found at the smallest cross section. This shift in sonic condition is due to the lack of isentropic behavior of the plasma flow. Sources causing the anisentropy are viscosity, heat and ionization, of which ionization is vital for a plasma. It is found that the plasma Mach number is always lower than the corresponding gas Mach number. A quasi one-dimensional analysis and simulations with a two-dimensional plasma model, which support the analysis, are presented.

Simulations of geometrically pinched argon plasmas using an extended one-dimensional model

The subject of this paper is the modelling of a wall-stabilized cylinder symmetric cascaded arc which is to be used as a high-density plasma source. To enhance the ion flux emerging from cascaded arc argon plasmas the confining wall can be changed into a nozzle geometry. Such pinched geometries increase the degree of ionization and convert the subsonic flow to supersonic. To study arcs with a varying cross section we introduce a one-dimensional non-local thermal equilibrium hydrodynamical model. In this model the various contributions in the momentum and energy balance are integrated over the plasma cross section. The radial profiles of the axial velocity and the electron and heavy particle temperatures are prescribed. The influence of the radial profiles on the flow and on the degree of ionization is studied for various arc geometries. The model is validated against experimental data and previous results.

The isentropic exponent of non-local thermal equilibrium plasmas

For non-equilibrium atmospheric mono-atomic plasmas the isentropic exponent is 1.2 for a large range of ionization degrees. The asymptotic dependence of the non-equilibrium parameters on the electron density is included. The small freedom of deviations from equilibrium expresses that the plasma tends to remain close to its equilibrium.