Wj Goedheer

Hybrid Monte Carlo‐fluid model of a direct current glow discharge

A self‐consistent hybrid Monte Carlo‐fluid model for a direct current glow discharge is presented. The Monte Carlo part simulates the fast electrons while the fluid part describes the ions and slow electrons. Typical results of the model include collision rates of the fast electrons, energy distributions of these electrons, fluxes and densities of the different plasma species, the electric field and the potential distribution, all as a function of position from the cathode. The influence of the negative glow on the calculations in the cathode dark space is studied. Moreover the influence of three‐dimensional scattering instead of forward scattering and the incorporation of side wall effects is investigated. Calculations are carried out for a range of voltages and pressures in order to study their influence on the calculated quantities. Comparison was made between total electrical currents calculated in the model and experimentally measured ones to check the validity of the model.

Extreme hydrogen plasma densities achieved in a linear plasma generator

A magnetized hydrogen plasma beam was generated with a cascaded arc, expanding in a vacuum vessel at an axial magnetic field of up to 1.6T. Its characteristics were measured at a distance of 4cm from the nozzle: up to a 2cm beam diameter, 7.5×1020m−3 electron density, ∼2eV electron and ion temperatures, and 3.5km∕s axial plasma velocity. This gives a 2.6×1024H+m−2s−1 peak ion flux density, which is unprecedented in linear plasma generators. The high efficiency of the source is obtained by the combined action of the magnetic field and an optimized nozzle geometry. This is interpreted as a cross-field return current that leads to power dissipation in the beam just outside the source.

One-dimensional fluid model for an rf methane plasma of interest in deposition of diamond-like carbon layers

A one-dimensional (1D) model for a methane rf plasma consisting of 20 species (neutrals, radicals, ions, and electrons) is presented. The equations solved are the particle balances, assuming a drift-diffusion approximation for the fluxes, and the electron energy balance equation. The self-consistent electric field is obtained from the simultaneous solution of Poisson’s equation. The electron–neutral collision rates are expressed as a function of the average electron energy. These expressions are obtained from the solution of the Boltzmann equation using the Lorentz approximation. The results presented in this article are limited to the alpha regime, hence no secondary electrons are considered. In total, 27 electron reactions (vibrational excitation, dissociation, and ionization) have been included in the model, as well as seven ion–neutral reactions and 12 neutral–neutral reactions. The 1D fluid model yields, among others, information about the densities of the different species in the plasma. It is found...

PLASIMO, a general model: I. Applied to an argon cascaded arc plasma

A non-LTE argon cascaded arc plasma is studied and modelled with the general plasma simulation program PLASIMO. The structure of PLASIMO is flexible and transparent, so that apart from the study given in the present paper several other multicomponent stationary plasmas in a wide pressure range ( to 1 bar), from local thermal equilibrium (LTE) to non-LTE, and with different energy coupling mechanisms can be simulated as well. The modular structure is divided into three main parts: the transport part which forms the heart of the model, the plasma configuration part, and the composition part. The latter two parts define the input parameters for the transport part and are controlled by the PLASIMO user. The three parts are again divided into separate modules. The strong modularity makes PLASIMO easy to handle and easy to adjust or expand. Results of PLASIMO applied on the cascaded arc are compared with experimental data and show reasonable agreement. The influence of the boundary conditions on the simulation results is discussed.

Hybrid Modeling of a Capacitively Coupled Radio Frequency Glow Discharge in Argon: Combined Monte Carlo and Fluid Model

A hybrid model has been developed for a capacitively coupled rf glow discharge in argon, employed as a spectroscopic source in the field of analytical chemistry. The cell is a rather small cylinder with a very small rf-powered electrode (only 5 mm in diameter). The typical working conditions applied for analytical spectroscopy are a gas pressure of 6 Torr and incoming power of 10 W. The hybrid model consists of a Monte Carlo model for the electrons and a fluid model for the electrons and argon ions. The latter model also contains Poissons equation, to obtain a self-consistent electric-field distribution. The input values for the model are the gas pressure, the discharge power, the cell geometry and the collision cross sections. The typical calculated results include the rf and dc bias voltage, the electrical current at the rf electrode, the potential distribution, the density of argon ions and electrons, the electron energy-distribution function and information about the collision processes of the electrons. These results are presented throughout the discharge cell and as a function of time in the rf cycle. Moreover, we have investigated how many rf cycles have to be followed with the Monte Carlo model before a periodic steady state is reached.

Optimization of the output and efficiency of a high power cascaded arc hydrogen plasma source

The operation of a cascaded arc hydrogen plasma source was experimentally investigated to provide an empirical basis for the scaling of this source to higher plasma fluxes and efficiencies. The flux and efficiency were determined as a function of the input power, discharge channel diameter, and hydrogen gas flow rate. Measurements of the pressure in the arc channel show that the flow is well described by Poiseuille flow and that the effective heavy particle temperature is approximately 0.8eV. Interpretation of the measured I-V data in terms of a one-parameter model shows that the plasma production is proportional to the input power, to the square root of the hydrogen flow rate, and is independent of the channel diameter. The observed scaling shows that the dominant power loss mechanism inside the arc channel is one that scales with the effective volume of the plasma in the discharge channel. Measurements on the plasma output with Thomson scattering confirm the linear dependence of the plasma production on...

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.

The plasma inside a dust free void:hotter, denser, or both?

The existence of a dust-free void, as often observed in dusty plasmas with small particles, or in dusty plasmas under micro-gravity conditions, requires a maximum of the ionization inside the void. Enhanced optical emission inside the void has indeed been observed. The extra losses of plasma on the dust has to be compensated for by extra ionization, which means that the electron temperature must rise. Inside the void there is no depletion of electrons, so a rise in electron temperature is not immediately obvious. It was therefore proposed that the relatively high electron density in the void with respect to the surrounding dusty region, where the electrons are depleted, causes the higher ionization inside the void. Different observations and models have until now not given a decisive answer, however. Using a global model, we predict that a homogeneous dusty plasma without a void should have an increased electron temperature, but at a reduced electron density. A dusty plasma with a fully formed void should have both an increased electron temperature and density in the void. A fully self-consistent two-dimensional model agrees with these conclusions and also shows that the void is a complex system, which is heated by the dust on the outside, but has most of the ionization on the inside.

Magnum-psi, a new linear plasma generator for plasma-surface interaction studies in ITER relevant conditions

The Magnum-psi project (magnetised plasma generator and numerical modelling for plasma-surlface interaction) aims at the fundamental study of plasma-surface interaction processes in conditions relevant to the ITER divertor. A linear plasma generator has been constructed in which a hydrogen ion flux of 10 23 particles/m 2 s to a surface can be realised, at a temperature of around 1 eV. A longitudinal magnetic field of I 0 cm and a controlled temperature in the range 0.1-10 eV.

Investigation of growth mechanisms of clusters in a silane discharge with the use of a fluid model

A one-dimensional fluid model was developed to study the formation of dust nanoparticles in a low-pressure capacitively coupled radio-frequency silane (SiH/sub 4/) discharge. In this model, the particle balances, the electron energy balance, and the Poisson equation are solved self-consistently. A set of 66 species, including neutrals, radicals, ions, and electrons, are incorporated. A total of 111 reactions are described in the model, comprising electron impact reactions with silanes, neutral-neutral, and ion-neutral reactions. Plasma conditions are typically those used for the deposition of amorphous silicon (a-Si:H). The model includes the formation of silicon hydrides (Si/sub n/H/sub m/) containing up to 12 silicon atoms. Anion-induced chain reactions are considered to be the main pathway leading to cluster formation.