Showing papers in "Contributions To Plasma Physics in 1992"

Control of the Edge Transport with the Ergodic Divertor

Abstract: The diffusion and the exponential divergence of neighbouring trajectories which characterize the stochastic magnetic field are reviewed The effect of these properties on heat and current transport leads to an increase of the radial heat diffusivity and a reduction of the toroidal electric conductivity The finite connection length to the wall in the perturbed layer determines the regular heat and particle deposition pattern

Kinetic Effects in the Scrape off Layer

Abstract: The scrape off layer (SOL) of a confined plasma (Fig. 1a) is in many respects in a state far from, thermodynamic equilibrium: The energy diffusing outward from the plasma core across the separatrix is condensed in the thin SOL to a strong energy flux along the magnetic field. The recycling of cold neutrals from the limiter or divertor target gives rise to a strong temperature gradient from the hot midplane region to the relatively cold recycling zone. For long mean free path length in the plasma relative to the extension of the recycling zone the thermalization in the recycling zone may be incomplete. The potential gradient length in the sheath in front of the target is small as compared to the mean free path length; the velocity distributions of ions and electrons as determined by the electric field are far from being Maxwellians there. Thus the follwing topics will be discussed in this paper: Parallel transport (electron heat flow, ion viscosity) Recycling Sheath structure (grazing incidence of magnetic field, secondary electrons). Boundary conditions for fluid models Numerical illustrations for these kinetic effects are given from a 1d particle-in-cell code including Coulomb collisions [1]. Not treated are kinetic effects connected with instabilities, perpendicular transport, neutrals and impurities.

Hysteresis and Transition to Chaos in a Thermionic Plasma Discharge

Abstract: We report a period-doubling route to chaos in a modulated unmagnetized low pressure thermionic argon discharge. The bifurcation cascade has many similarities to a noisy logistic map. The non-modulated discharge shows strong hysteresis in the current voltage characteristic. Two distinct discharge states are identified by spatial plasma potential measurements and compared with the classification for higher pressures.

The Application of Moment Equations to Scrape off Layer Plasmas

Abstract: A set of 21 moment fluid equations has been applied to two aspects relevant to scrape off layer plasma transport codes. The first of these is the provision of classical parallel transport coefficients, particularly for impurity plasma ions. Although more general results have been obtained, those only for a low impurity density common ion temperature plasma are presented. The results reflect the coupling between heat and particle fluxes and a coupling of the main ion fluxes into the impurity fluxes. The second application is in response to the problem of nonphysical values for classical viscosities and heat fluxes in regions with large gradients. The basic five moment description of the Braginskii equations has been extended to a full thirteen moment approximation. The new equations are given and some preliminary numerical results from a one dimensional code are discussed.

Theoretische Modelle des katodischen Brennflecks von Bogenentladungen. Teil I: Grundlagen

Abstract: Voraussetzungen, theoretische Grundlagen, Ergebnisse und allgemeine Erfahrungen bei numerischen Modellrechnungen fur quasi-stationare katodische Brennflecken von (Vakuum-)Bogen werden zusammenfassend dargestellt. Die Ergebnisse beziehen sich hauptsachlich auf Kupfer als Katodenmaterial und umfassen alle wesentlichen physikalischen Grosen des Katodenflecks (der Katoden-Oberflache und teilweise des katodennahen Plasmas), sie beschreiben mogliche Existenz-Zustande im Parameterraum. Insbesondere wird auch die Abhangigkeit dieser Ergebnisse von verschiedenen Anderungen der Parameter und Gleichungen untersucht. Mehrere Beispiele werden dargestellt und interpretiert. Der Einflus der Rauhigkeit der Katodenoberflache ist in jedem Fall beriicksichtigt worden. Schlieslich werden die Gul-tigkeits-Beschrankungen des Modells diskutiert, und vor allem wird die grose Bedeutung nichtstatio-niirer (z B. explosiver) Prozesse im Brennfleck betont, obgleich solche Prozesse aukrhalb des Rahmens dieses Modells liegen. Die Arbeit wird in zwei Teilen veroffentlicht. Dieser erste Teil umfast eine Einfuhrung in verschiedene Gesichtspunkte der Brennfleck-Modellierung, die Diskussion physikalischer Prozesse in den Katoden-Brennflecken und die Entwicklung entsprechender Gleichungen, die fur das Modell benotigt werden. Presuppositions, theoretical basis, results and general experiences of numerical model calculations of quasi-stationary cathodic spots in (vacuum) arcs are summarized. The results (mainly related to copper cathodes) comprise the essential physical quantities of the cathode spot surface and (partially) of the cathode spot plasma, i.e. possible states of existence within the space of parameters. Especially, the dependence of these results on changes of the parameters and equations is investigated. Several examples are presented and interpreted. In any case, the surface roughness of the cathode is taken into account. Finally, the limitations of the model are discussed, and the great importance of non-stationary (e.g. explosive) processes within the arc spot is emphasized, though such processes are beyond the scope of this model. The paper is published in two parts. This first part comprises an introduction to several aspects of arc spot modeling, the discussion of the physical processes in cathode arc spots and the development of relevant equations to be used in the model.

The Numerical Differentiation of Probe Characteristic

Abstract: In the paper new methods of the computation of second-order derivative of probe characteristic are proposed. All methods are tested on testing data as well as on experimental measured probe characteristic.

Development of Divertor Concept for Optimized Stellarators

Abstract: Development of Divertor Concept for Optimized Stellarators. 1. Stellarator optimization. 2. Configurational aspects. 3. Field line tracing for investigation of diversion properties. 4. Simplified SOL models: one-dimensional estimates and Monte Carlo simulations. 5. Divertor troughs. 6. Discussion and outlook.

Multidimensional and Multifluid Plasma Edge Modelling: Status and New Directions

Abstract: The plasma boundary in magnetic confinement devices is observed in experiments and in computations to contain radial and poloidal variations in plasma potential, strong radial electric fields and large poloidal drift flows in addition to the parallel flow toward the plasma sheath boundaries and the particle recycling from the walls. The global power balence is largely influenced by impurity radiation and thus by impurity transport in the edge region. The behavior of the edge region is related to global confinement in ways not yet fully known and motivates further improvements in edge plasma modelling. General multifluid transport equations are reviewed in the context of the current status of plasma edge modelling. It is assumed that the macroscopic transport in the plasma edge region is suitable for a moments description, or fluid model. Progress is well established in computational treatments of the standard paradigm including classical parallel and anomalous cross-field flux coupled to neutrals in analytic or Monte Carlo steady state computations. Formulation of momentum diffusivities and curvature drift in toroidal coordinates are considered. New directions include defining the dominant influence of drifts and non-ambipolar flows, improving coupled plasma-neutral transport calculations, and treating impurity flow in full multifluid models. Calculation of the plasma potential driving the E × B drift in the scrape-off layer can be extended inside the last closed flux surface by conserving the divergence of the plasma currents. Coupled plasma-neutral fluids using a neutral diffusion approximation show promise for recycling calculations. Impurity transport based on full multifluid treatment along the field lines has shown good agreement with divertor experiments. Advanced topics for the near future include plasma core-edge transport coupling and eventually the direct solution of coupled momentum equations rather than the presently assumed cross-field ‘anomalous’ flux functions. In the edge region, plasma sheath boundary conditions are a significant sink for plasma particles and heat; a small coupling between the momentum components in the plasma edge can be sufficient to drive an ‘anomalously’ large efflux from the core plasma.

Heat transmission through plasma sheath with energetic electrons

Abstract: An electrostatic sheath formation on carbon and molybdenum target plates for plasmas with electrons of two different temperatures is analyzed by a simple formulation including the effect of secondary electron emission from the target plate. A modified Bohm criterion is given in terms of effective electron temperature which defines the ion sound velocity in two component plasmas. Hot electrons and the associated secondary electrons greatly modify the charge balance in the sheath. They change the sheath voltage and consequently the heat transmission characteristics. A critical amount of hot electrons for sharp increase in the sheath voltage is given by a simple formula.

Experimental Observation of Simultaneously Existing Moving and Standing Patterns in a Gas‐Discharge System

Abstract: In a quasi 1-dimensional lateral gas-discharge system we observe experimentally sub- and supercritical bifurcations from a spatially homogeneous to an inhomogeneous distribution of the current density. Among other results we find for the first time the simultaneous existence of a nontrivial nonstationary pattern (travelling waves) with a nontrivial stationary pattern (periodic filaments) in a laterally extended dc-driven gas-discharge system.

Fokker‐Planck Modelling of Edge Plasma Near the Neutralizer Plate in a Tokamak

Abstract: We have modelled the transport of particles and energy through a tokamak SOL near the neutralizer plate. The electron kinetic/ion fluid code FPI and a two-fluid code were employed for this study. Because of the electron time scale, it would lake a very long time to reach steady state with the kinetic code. We have therefore developed a hybrid technique, using both codes in alternation, to reach a full equilibrium compatible with electron kinetics. When it is used in the stand-alone mode, the two-fluid code's results are closest to FPI if a heat flux limiter f=0.2 is used. The electron distribution function is very non-Maxwellian near the plate.

Experimental Data on the Plasma Edge of TEXTOR

Abstract: The optical methods used for plasma edge diagnostics in the tokamak TEXTOR are briefly described. Results obtained in discharges without auxiliary heating are presented. Two topics will be dealt with: The composition of the flux of neutral particles emitted from the limiter and the velocity distribution of these fluxes and - the influence of the low z-impurities, characteristic for TEXTOR, on the plasma edge properties. The significance of the experimental data will be discussed and an outlook on the future work will be given.

Periodically Self-Excited Moving Striations in a Low-Temperature Gas Plasma

Abstract: A new nonlinear effect in a low temperature gas plasma has been found and investigated experimentally. At a critical value of the discharge current in a special glow discharge system the positive column constricts and striations evolve spontaneously. On a large time scale the pattern seems to consist of standing striations. At higher temporal resolution the striations turn out to be moving ones which are modulated by high amplitude oscillations of the total electric current and of the voltage observed at the device. In addition phenomena like period doubling and irregular behaviour occur.

Radial density profiles in a poloidal divertor modelled by diffusion across a region with variable connection length

Abstract: A simple model to describe the diffusion of particles in the scrape-off layer of a poloidal divertor tokamak is presented. This model takes into account the basic magnetic characteristics of an open poloidal divertor and provides analytical expressions for the plasma density profiles at the divertor target, that are used to fit the experimental measurements in JET.

Minimum Entropy Production Principle Applied to Decontaminating Regime in a Stochastic Layer in Presence of a Radial Electric Field

Abstract: The Ergodic Divertor of TORE SUPRA induces a stochastic layer at the plasma edge by creating a resonant magnetic perturbation which destroys the magnetic surfaces. A radial electric field directed outwards is then established to confine the electrons along the open magnetic field lines. This electric field tends to deconfine ions. One may then expect that it strongly influences the neoclassical ion transport. We use a variational principle of minimum entropy production to study the transport of impurity ions in the stochastic layer. In low collisionality regime, a decontaminating outwards flux of impurity ions is found, contrasting with the neoclassical inwards flux which produces the deleterious accumulation of impurity ions in the nH profile.

Influence of the Argon Pressure at dc Magnetron Sputtering on Contaminations in CrSi Thin Films

Abstract: This paper deals with the influence of process parameters, especially of argon pressure, power, and target-substrate spacing on impurity content and properties of Cr-Si thin films deposited by dc magnetron sputtering in a non-heated high vacuum apparatus The results confirm the well-known fact that in such a unit by means of a magnetron pure films can be produced without any difficulties However, this has proved to be possible only in the case of low argon pressure being preferentially employed in magnetron sputtering Boosting the argon pressure yields drastic film contaminations by oxygen and hydrogen, which are caused by a reinforced release of adsorbats from the apparatus walls under these operating conditions A high argon pressure effects simultaneously a change in the film microstructure resulting together with the contaminants in modified electrical properties

Towards Finite Element Modeling of the Tokamak Scrape-Off Layer

Abstract: Finite elements provide a natural and flexible method for two-dimensional fluid modeling of the tokamak scrape-off layer. Here we present the first results obtained with the FELS code [1] on an unstructured grid, for the case of a toroidal limiter.

Monte-Carlo Calculations of Electron Transport Coefficients in H2. First Simulations of the Townsend-Huxley Experiment

Abstract: The aim is to get additional evidence to assist in resolving the discrepancy between swarm-derived, theoretical and beam-measured vibrational cross sections for the case v = 01. Good agreement between Monte-Carlo and experimental transport data is obtained with the swarm-derived cross sections. However, some simulations relevant to the Townsend-Huxley experiment call into question the accuracy of the characteristic energies used in swarm analyses. Possible reasons of the observed discrepancies between Monte-Carlo and experimental data and expected implications even on the accuracy of the cross sections in H2 are discussed.

Pressure Dependence of the Mean Electron Energy in the Bulk Plasma of an RF Discharge in Argon

Abstract: The pressure dependence of the mean electron energy 〈ϵ〉 was investigated in discharges in Ar experimentally by sampling the plasma potential with an ion mass spectrometer, and theoretically by solving the electron Boltzmann equation for the bulk plasma. Both results show a decrease in 〈ϵ〉 with increasing pressure in the regime between 0.2 and 40 Pa although this behaviour is much more pronounced in the experimental results. The results are compared to those of single-probe measurements of other authors. In one case they are in good agreement [10], while there is a discrepancy in another case [11]. Possible reasons for this behaviour are discussed. The advantage of the mass spectrometric sampling method is, that it is absolutely non-invasive, so that also in sensitive processes no contamination will occur.

On the nonlinear processes during generation of current drive

Abstract: The manifestations of the induced scattering effects in the generation process of current driven by lower-hybrid (LH) waves are studied taking into account the radiative-resonant interactions. The influence extent of the LH wave modulational instability on current drive is estimated. It is shown that the induced scattering of LH waves on plasma particles leads to a change of the LH waves spectrum and through this leads to an essential influence on fast electrons generation rate and the steady-state current drive. The modulational instability of LH waves can provide a “spectral gap” filling in the case of sufficiently strong LH wave pumping.

Simulation of Tokamak Divertor Plasmas including Cross-Field Drifts

Abstract: The effects of plasma currents and cross-field drifts on single null tokamak divertor operation are simulated using a fully implicit 2-D fluid code. Equations solved are those for particle continuity, parallel momentum, electron energy, ion energy, electrostatic potential, and neutral gas diffusion. The core and scrape-off layer regions are separated by a magnetic separatrix, both of which are included in the simulation. The core plasma is poloidally periodic, and the inner and outer private flux regions are properly connected. The code utilizes a fully implicit method-of-lines scheme to advance the variables in time with a Krylov technique or a direct Newton iteration with a numerical Jacobian. Results are presented on the effects of currents and cross-field drifts for DIII-D single-null parameters.