Kh. Tarnev

Magnetic filter operation in hydrogen plasmas

A fluid-plasma model description of the operation of a magnetic filter for electron cooling in gas-discharge plasmas is presented in the study. Directed to the use of weak magnetic fields in the sources of negative hydrogen ion beams for additional heating of fusion plasmas, hydrogen discharges have been considered. The numerical results obtained within a 2D-model are stressed. The 1D-model presented aims at showing the main trends whereas the results obtained within the 3D-model, also developed, confirm the 2D-model description. The models outline the importance of the transport phenomena: electron-energy and charged-particle fluxes. Reduction of the thermal flux across the magnetic field together with thermal diffusion and diffusion, acting in combination, is the basis of the electron cooling and of the spatial distribution of the electron density. Effects due to the (E × B)-drift and the diamagnetic drift form a fine spatial structure of the plasma-parameter variations.

Electron release in the afterglow of a pulsed inductively-coupled radiofrequency oxygen plasma

A pulsed inductively-coupled radiofrequency plasma in oxygen is investigated by means of time-resolved microwave interferometry in a wide pressure range from 0.5 to 200 Pa. In the afterglow a peak of the electron density is observed. The effect is maximum for pressures around 50 Pa. The time-resolved measurements of the electron density are interpreted in the framework of a fluid model. This model points out the significance of negative ions. The overall electron density is comparatively small. Attachment and detachment processes nearly balance during the power-on phase. But when the power is switched off, the electron temperature drops very quickly. This means that the production of new negative ions is inhibited so that the negative ions are destroyed by collisions. These reactions quickly set free electrons in the afterglow and are the reason for the observed peak in the electron density after switching off the power.

A small radius hydrogen discharge: An effective source of volume produced negative ions

Free-fall regime maintenance of hydrogen discharges is analyzed based on numerical solutions of a set of equations involving the balance equations of the charged particles [electrons, the three types of the positive ions (H+, H2+, and H3+), and negative H− ions] and of the neutral species (hydrogen atoms H and vibrationally excited molecules), the momentum equations of the positive ions, the electron energy balance equation, and the Poisson equation, all together 25 differential equations. The obtained results for varying discharge radius show strong accumulation of the negative ions in the on-axis region of the discharge when the discharge radius is small, which leads to a concept for a design of a volume-production based source as a matrix of small radius discharges. The variation in the negative ion density with changing gas pressure and electron density at the discharge axis is also analyzed.

Matrix of small-radius radio-frequency discharges as a volume-production based source of negative hydrogen ions

Based on experience from a work--both theoretical and experimental one--on negative hydrogen ion beam sources studied regarding fusion applications, a novel design of a rf source with volume production of the ions is proposed. The suggestion is for a source constructed as a matrix of small-radius tandem discharges (with magnetic filters largely extended over the discharge length), inductively driven (by a single coil, for the whole matrix) and with a single aperture extraction from each of them.

Two-dimensional fluid model of a two-chamber plasma source

This study presents a two-dimensional fluid-plasma model developed for describing the cw regime operation of a tandem plasma source, consisting of a driver and an expansion plasma volume of different sizes. The moderate pressure range considered (tens to hundreds of milliTorr) allows a description within the drift‐diffusion approximation, as employed in the model. Argon discharges maintained in a metal gas-discharge vessel are treated. The discussions stress charged-particle and electron-energy fluxes as well as the spatial distribution of their components. The main conclusions are for (i) different electron and ion fluxes resulting in a net current in the discharge; (ii) a radial ion flux prevailing over the axial one and an axial electron flux prevailing over the radial one; (iii) ion motion determined by the dc electric field and drift‐diffusion electron motion influenced by thermal diffusion; (iv) plasma maintenance in the expansion plasma chamber due to charged-particle and electron-energy fluxes from the driver; (v) importance of the convective flux in the electron-energy balance; (vi) electron-energy losses for sustaining the dc electric field in the expansion plasma volume strongly predominating over the losses through collisions and (vii) electron cooling accompanied by a strong drop in the plasma density and in the potential of the dc electric field, due to the plasma expansion in a bigger volume. In general, the results show that the gas pressure range usually considered to be governed by ambipolar diffusion shows up in a different regime: a regime with a dc current, when the discharge is in a metal chamber with different dimensions in the transverse and longitudinal directions.

Effects of plasma-density inhomogeneity and collisions and their relation to maintenance of waveguide discharges by Trivelpiece-Gould modes : I. Resonance absorption

This study aims to contribute to the analysis of the mechanisms of wave-energy absorption and electron heating in waveguide discharges sustained by propagating Trivelpiece-Gould modes. The discussion is based on a derivation of the dispersion law and the wavefield distribution in cylindrical waveguides with magnetized collisional plasmas with a diffuse boundary. Through resonance absorption associated with the transformation of Trivelpiece-Gould modes into upper hybrid volume waves, the plasma-density radial inhomogeneity affects the wave behaviour. The conditions of the existence of resonance absorption are analysed and its role in the maintenance of wave-produced gas discharges in external magnetic fields is discussed.

On the inertia term in the momentum equation in the free-fall regime of discharge maintenance

The study, being on two-dimensional modelling of low pressure discharges, suggests an approach to the nonlinear inertia term in the momentum equation of the positive ions needed to be accounted for in the free-fall regime of the discharge maintenance. On the basis of conclusions that the inertia term acts in the wall sheath, where the ions fly perpendicularly to the walls, it is shown that (i) the parallel – to the walls – velocity component can be neglected, and (ii) the rest of the convective derivative can be determined by using the energy conservation law in the collisionless case. In a way, the inertia term acting as a retarding force is joined to the momentum loss term by introducing effective collision frequencies. The validity of the procedure is proved in a model of a low pressure argon discharge by comparison with the exact solutions for the two-dimensional spatial distribution of the discharge characteristics (ion velocity, electron density and temperature and DC electric field and its potential). The conclusion is that (i) ignoring the velocity component that is parallel to the walls does not cause deviation from the exact solution, and (ii) the approximation of using the energy conservation law in the collisionless case is good enough.

Free-standing microwave excited plasma beam

A free-standing plasma beam can be generated in a large volume process chamber as a continuation of a surface wave excited plasma column bounded within a dielectric plasma chamber. This phenomenon occurs if the threshold values of the gas flow and microwave power are reached. The typical values are quartz plasma chamber diameter of 20–40 mm, microwave power of 1–1.5 kW at 2.45 GHz, argon flow of 500–6000 sccm and pressure of 50–500 Pa. The length of the free-standing beam can reach many tens of centimetres. The mechanism of free-standing beam creation and an explanation for its shape and properties is proposed, based on analysis of the charged particle balance in the beam. Crucial for this explanation is the gas flow in the free-standing beam zone. The results of a flow dynamic simulation are presented. The spatial distributions of neutral particle velocities are used as input for the model, which predicts the electron concentration profiles. Good agreement of the modelled and measured electron concentration distributions in radial and axial directions of the free-standing beam is obtained.

Surface-wave produced discharges in hydrogen: I. Self-consistent model of diffusion-controlled discharges

A fluid-plasma model of diffusion-controlled hydrogen discharges sustained in the field of propagating surface waves is presented in this study. The self-consistent description of the discharge structure achieved provides results for the inter-related variations of the discharge characteristics: the electron concentration ne, the concentrations of the three ionic species (H+, H2+ and H3+), the concentrations of the two neutral gas components (H and H2), the electron temperature, the power ? absorbed on average by an electron, the gas temperature, the wavenumber and the space damping rate ? of the wave. Wave behaviour in radially inhomogeneous collisional plasmas is taken into account because it provides the proper description of diffusion-controlled discharges. The general mechanism of nonlocal heating of the electrons in the wave field is considered. The model is extended to comparatively low gas pressures (p?0.2?Torr) by introducing effective mobilities of the ions in which, besides the ion?neutral elastic collisions, the production and destruction of ions by collisions are included. Based on these effective mobilities, the ambipolar diffusion coefficients of the charged particles are specified. The most important reactions that contribute?under the gas-discharge conditions considered?to the production of charged particles and hydrogen atoms as well as to the electron-energy and gas-energy balances are involved in the model. It is shown that in hydrogen discharges, the (??ne)-relation, which besides the (??ne)-relation ensures a self-consistent description of the axial structure of surface-wave-sustained plasmas, in general, stems from the dependence of ? on the concentrations of the neutral gas components (H, H2) and their relation to the concentrations of the ions (H+, H2+ and H3+). The results obtained using the model are discussed in the context of experiments showing a peculiar behaviour of the axial structure of hydrogen discharges compared to discharges in other gases.

Effects of plasma-density inhomogeneity and collisions and their relation to maintenance of waveguide discharges by Trivelpiece-Gould modes: II. Numerical analysis

The effects of collisions and radial inhomogeneity of the plasma density on the behaviour of the azimuthally-symmetric Trivelpiece-Gould modes (including the waveguide resonance and the wave resonance absorption at the upper hybrid frequency) are treated in this study in terms of their relation to gas-discharge production in an external magnetic field. Numerical solutions for the wave-field patterns and for the propagation properties of the waves are presented and their changes are discussed with a varying averaged plasma density, for various plasma-density profiles. First modifications due to collisions in homogeneous plasmas are presented for different values of the external magnetic field. Then, for a given magnetic field, the combined effects of collisions and Bessel-type radial profiles of the plasma density are emphasized. After which, for a given profile, the influence is discussed of varying the magnetic field. It became evident that, for low collision frequencies and a cyclotron frequency less than the wave frequency, the plasma-density profile shape has a strong impact on the results and that the wave properties, as the average density is varied for dominantly diffusive charged particle losses, are badly represented by wave properties in uniform plasmas. The behaviour discussed here should be useful in understanding the structure of diffusion-controlled discharges maintained in the fields of propagating Trivelpiece-Gould modes.