Jj Beulens

A two-dimensional nonequilibrium model of cascaded arc plasma flows

A nonequilibrium model is developed for the prediction of two‐dimensional flow, electron and heavy particle temperatures, and number density distributions in cascaded arcs of monatomic gases. The system of strongly coupled elliptic partial differential equations describing plasma flow is solved by a numerical method based on a control volume with a nonstaggered numerical grid. The model is applied for the computation of both stagnation and flowing argon arc plasmas. The results show that the plasma in stagnation arcs is nearly in local thermal equilibrium (LTE), except very close to the wall, whereas fast flowing arc plasmas exhibit a significant degree of nonequilibrium, both close to the wall and in the inlet region. The results of the calculations are in satisfactory agreement with experimental data, both for the cases of stagnation and flowing argon cascaded arc plasmas.

Carbon deposition using an expanding cascaded arc d.c. plasma

Abstract In this work a strongly flowing cascaded arc burning on an argon-hydrogen mixture is used to dissociate and ionize hydrocarbons which are injected inside a nozzle which is mounted in the anode of the arc. The thermal plasma ( T ≈ 10 000 K , p ≈ 0.5 bar ) will then expand supersonically into a vessel where the pressure can be varied between 0.1 mbar and 100 mbar. In the expansion the cracked hydrocarbons are transported towards a substrate opposing the arc where carbon films can grow. The large number of more or less independent operational variables make it possible to grow any kind of carbon film from graphite to diamond to polymers and amorphous hydrogenated carbon (a-C:H). For the amorphous films growth rates up to 200 nm s -1 on an area of 100 cm 2 were achieved, while for polycrystalline diamond films the rate was 25 μm h -1 , on areas of about 3 cm 2 . Graphite has been grown on top of graphite and steel samples at rates up to 3 mm h -1 . The morphology and film parameters of the grown films were investigated with ellipsometry (a-C:H, refractive index etc.), Raman spectroscopy (diamond, graphite, crystallinity and bond structure), electron microscopy (morphology).

Radiative energy loss in a two-temperature argon plasma

We have calculated the total radiative loss in an argon plasma at wavelengths from 100 nm to 100 μm (zero absorption) as a function of temperature (3000–15,000 K) for several pressures (10-1 × 106Pa) under LTE and non-LTE conditions. The investigated non-equilibrium aspects are deviations of the neutral ground state population with respect to the equilibrium population (partial LTE). A difference between heavy particle and electron temperature is included. When the calculated total radiative loss is divided by the square of the electron density, a curve is obtained which gives the total radiative loss as a function of temperature. The influences of pressure and deviations from LTE on this curve are small and in many cases negligible. Almost all influences of pressure and deviations from equilibrium are incorporated in the electron density. Absolute measurements in an inductively-coupled plasma can be simulated with realistic values of the b factor (Boltzmann decrement).

Fast silicon etching using an expanding cascade arc plasma in a SF6/argon mixture

An expanding cascaded arc is used as a fluorine atom source for fast etching of silicon. Extremely high etch rates up to 1.3 μm/s have been obtained. A reactor parameter study has been performed. The obtained selectivity Si/SiO2 is ∼11 for substrate temperatures of 600 °C, increasing to ∼20 at 100 °C. The etching proces is fully isotropic.

Axial temperatures and electron densities in a flowing cascaded arc: model versus experiment

A Fabry-PCot interferometer is used to measure argon ion line profiles in a strongly flowing cascaded arc plasma. Heavy-particle temperatures, electron densities, and electron temperatures are derived from the Doppler width, the Stark width and the lindcontinuum ratio respectively. The electron temperatures are also obtained from the electrical conductivity of the plasma. The plasma parameters along the arc channel are values averaged over the arc cross section and are determined with an accuracy of 2-5%. Comparison of the measurements with model calculations is also made, to improve understanding of the plasma Drocesses in the arc channel.

Atomic and Molecular Emission Spectroscopy on an Expanding Argon/Methane Plasma

A supersonically expanding cascaded arc plasma in argon is analyzed axperimentally by emission spectroscopy. The thermal cascaded arc plasma is allowed to expand through a conically shaped nozzle in the arc anode into the vacuum vessel. In the nozzle monomers (CnHv) are injected. The monomers are dissociated and ionized by the argon carrier plasma, and transported toward a substrate by means of the expansion. Emission spectroscopy is used to obtain temperatures and particle densities. By varying external parameters (e.g., arc power, gas flow rates) plasma parameters can be linked with (e.g. parameters (e.g., refractive index).

Plasma deposited carbon films as a possible means for divertor repair

Abstract Fast deposition of graphitic carbon layers by an expanding cascaded arc plasma was studied as a means for in situ repair of graphite erosion damage in the next step fusion reactor NET/ITER. Amorphous graphite was produced at rates of hundreds of nanometers per second on several square centimeters with an argon-hydrocarbon plasma. Crystalline graphite was produced at rates of 10–50 nm s −1 on several square centimeters by means of an argon-hydrogen-hydrocarbon plasma. Relations between the deposition parameters, morphology (from scanning electron microscopy) and Raman spectra were determined. Using laser thermal shock testing, the erosion resistances of the best crystalline coatings were determined at about 2 MJ m −2 (in a 10 ms pulse).

Axial Temperatures and Electron Densities in a Flowing Cascaded arc

Since about 1985 a cascaded arc is used as a particle source in the deposition machine described by Kroesen [1a,1b]. This method of deposition showed to be very fast and efficient to grow amorphous carbon films (a–C:H), varying from graphite and diamond to polymers [1,2]. The most important difference of this method, with respect to R.F. techniques, is that the three most important functions of a deposition process, as there are dissociation/ionization, transport and deposition are spatially separated. The dissociation takes place in a cascaded arc burning on argon. The temperatures in the arc are about 10000–12000 K. At the end of this arc hydrocarbons are injected which are then dissociated and ionized effectively. At the end of the arc the plasma expands supersonically into a vacuum vessel. That means that the plasma cools down and the formed hydrocarbon fractions are transported towards the substrate, where an amorphous carbon film can grow. The quality of the films depend mainly on the amount of energy available for each injected carbon atom. The behavior of the refractive index as a function of this energy could be a confirmation that in our deposition method the carbon ions rather than radicals govern the deposition process [1,3,4]. Therefore the cascaded arc is investigated numerically and experimentally in order to improve the ionization efficiency. The conservation laws for mass, momentum and energy for both the electrons and the heavy particles are solved 2 dimensionally by a control volume numerical method with a non, staggered grid. By Fabry Perot interferometry heavy particle temperatures, electron temperatures and electron densities as a function of the axial position in the cascaded arc are measured. The obtained numerical results are compared to the experimental data, obtained by the optical Fabry Perot diagnostics.

Optical Diagnostics for High Electron Density Plasmas

Nowadays high electron density plasmas are, beside their fundamental interest, widely used for many applications, e.g., light sources and plasma processing. The well known examples of high electron density plasmas can be found among the class of thermal plasmas as, e.g., the Inductively Coupled Plasma (ICP) and the Wall Stabilized Cascaded Arc (WSCA). Usually the pressure of the plasma is high, i.e., sub atmospheric to atmospheric. Other examples are the plasmas generated in tokamaks for fusion purposes and the recently exploited plasmas for etching and deposition devices such as the Electron Cyclotron Resonance plasmas. For the plasmas mentioned, the electron density is typical in the range of 1018 to 1023 m−3, and the electron velocity distribution is close to a Maxwellian velocity distribution.

Thermal discharges: experiments and diagnostics

In plasma processing commonly a distinction is made between low pressure (or low (ion) temperature) plasmas and thermal plasmas1. The transition between these two classes is gradual. Plasmas cover a wide spectrum in electron density (1015/m3–1023/m3) and ionization degree (10−7–1). In low pressure plasmas2 as RF—discharges the ionization degree is usually small and these plasmas are characterized by an abundance of molecular fragments and large ambipolar fields. The high electron density thermal plasmas have a high ionization degree and nearly full dissociation and a high heavy particle temperature. In this paper we will focuss on these plasmas.