Rudolf Hrach

Mechanisms of methane decomposition in nitrogen afterglow plasma

Abstract Chemical reactions initiated by the dissociation of methane in the nitrogen flowing afterglow have been studied by the computer modelling. The input experimental data were obtained from the microwave CH4/N2 plasma. The modelling of methane decomposition was based on a macroscopic kinetic approach. The 24 neutral and excited species were introduced: electrons, hydrocarbons, radicals, neutral and excited gases, and nitrogen containing species. Between these species 61 chemical reactions were studied. It was found that in the flowing afterglow conditions, where the energy of excited particles is reduced, the limited amount of reactions is really important. With the aid of this simplified model of 18 chemical reactions the yield of stable products and the detailed kinetics of their creation in the dependence on activity of individual species were studied. Special attention was devoted to the study of reaction kinetics in dependence on the afterglow time in accordance with experimental data.

Kinetics of reactions in CH4N2 afterglow plasma

Abstract In this paper chemical reactions initiated by the decomposition of methane in the nitrogen flowing afterglow are studied by computer modelling with the input data from microwave plasma. The model consists of 61 reactions between 24 kinds of species. The simulation technique is based on a macroscopic kinetic approach which provides the final concentrations of stable products—C 2 H 6 , C 2 H 4 , C 2 H 2 , HCN, H 2 together with CH 4 and N 2 . It was found that the stiffness of the set of equations describing the decomposition process can be monitored by the C and H mass balance.

KINETICS OF REACTIONS IN CH4/N2 AFTERGLOW PLASMA : A SIMPLIFIED MODEL

Abstract Processes in CH 4 \N 2 microwave plasma are studied by computer experiment based on experimental data derived from the nitrogen flowing afterglow discharge. By means of a macroscopic kinetic approach, the original model consisting of 72 chemical reactions between 35 active species in the discharge is analysed in detail, the main attention being devoted both to the initial stages of methane decomposition and to the yield of stable products together with the most important reactions for the production of individual species. From these reactions the new simplified model is formulated.

Energy angular distributions of electrons emitted from MIM systems

Abstract The distribution of electrons emitted from AlAl 2 O 3 Au sandwich cathodes has been studied. The measurements were performed with the half-spherical collector divided into several concentric zones. In addition to the usual angular and energy distributions of emitted electrons, the total energy distribution into single collector zones was measured at various leakage voltages and temperatures. The experimental energy angular distributions suggest that the dominant scattering process in the aluminium oxide is the interaction of hot electrons with optical phonons and traps.

A DC probe diagnostics for fast electron temperature measurements in tokamak edge plasmas.

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. It provides simultaneous measurements of electron temperature and parallel ion current density with high frequency at the same point in space. We describe ongoing work to characterize the ion flows within the probe, and to calibrate the diagnostics using 2D kinetic simulations.

Simulation of thin film growth

A short review is presented of various models used for computer simulation of the initial stages of the nucleation and growth of metals on dielectric substrates. Attention is devoted to both the algorithms used and the results obtained. In the second part of the paper, a simple atomistic model is proposed and some new possibilities for such kinds of model are shown.

Self-consistent modelling of plasma-solid interaction

In the paper the transport of electrons and ions in plasma is studied by computer simulation. The main goal of the modelling was to establish an exact form of energy distributions of charged particles near the metal substrates immersed into plasma and to analyse changes in their distributions during the transport through both the presheath and the sheath for various voltage biases and substrate geometries. Further results concern the probe characteristics both in inert gas and chemically active plasma and the distribution of potential and space charge in the vicinity of metal electrodes. As a simulation technique the combination of molecular dynamics and Monte Carlo methods was used. The movement of charged particles in self-consistent electric field was studied under the assumption of internal symmetry of the problem, which enabled to reduce the number of necessary coordinates to four (either 1d3v or 2d2v).

Emission of electrons from mim systems: Energy distributions

In this part of our study we measured energy distributionsN(E) andN(EX) of electrons emitted from sandwich cathodes, and their basic temperature (at 300, 200 and 80 K) and voltage dependences. From the distributionN(E) at 80 K we calculated the mean free path of electrons in cathode and the effective temperature of distribution. The anomalous emission (beginning at small leakage voltages) was observed and its energy distribution measured, too. From the graphs it can be seen that some electrons acquire energy 2–5 eV during their transition through the sandwich cathode.

Tunnel probes for measurements of the electron and ion temperature in fusion plasmas

We have developed tunnel probes for localized measurements of the electron and ion temperature in the edge plasma region of smaller tokamaks and stellarators. A normal tunnel probe for Te measurements consists of a metallic tunnel of 5 mm diameter and 5 mm length, with the axis parallel to the magnetic field. One side is open, the other side is closed by a metallic backplate, isolated from the tunnel. If both electrodes are biased negatively, ions flow into the office, and their current is distributed between the tunnel and the backplate. The ratio of the two ion currents is a function of Te. With an additional diaphragm in front of the orifice, the probe becomes ion sensitive, since the electrons are prevented from the tunnel because of their smaller gyroradius, but ions can still reach it. In this way, the perpendicular ion temperature can be derived. By segmenting the tunnel axially into two parts, also an approximate measure for the parallel ion temperature can be found.

Emission of electrons from mim systems: Discussion of processes in the cathode

Experimentally derived emission characteristics — the transfer ratio, angular distribution, energy distributionsN(E) andN(Ex) and energy-angular distribution of emitted electrons — are discussed on the basis of our theoretical model of the sandwich cathode. It was found that electrons during their transfer through the cathode are scattered in the dielectric layer mostly by interactions with optical phonons and traps and in the top metal electrode by collisions with conduction electrons. Comparison of our model with experimental results gave us approximate values of mean free paths and further cathode parameters.