bluuebunny/crossref_metadata_2025_split huggingface dataset:
Tokamak Edge Transport Theory
Authors: Hazeltine R. D. , Catto Peter J. 
Published: July 2007

AbstractKinetic equations commonly used to study the plasma core of a tokamak do not allow a balance between parallel ion streaming and radial diffusion and are, therefore, inappropriate in the plasma edge. Standard core tokamak transport orderings allow large divergence‐free flows in flux surfaces, but only weak radial flows. Alternate orderings are required in the edge region where radial diffusion must balance large parallel ion flows to divertor target plates or limiters. Similarly, core transport formulae cannot be extended to the edge region without qualitative alteration. Here we address the necessary changes by considering a large parallel flow ordering appropriate for the scrape‐off layer of a tokamak. By deriving and solving a novel kinetic equation, we construct distinctive transport laws for an impure, collisional edge plasma. We find in particular a surprising form for parallel transport in the scrape‐off layer, in which the parallel flow of particles (and heat) are driven by a combination of the conventional gradients, as well as viscosity terms and new terms involving products of radial derivatives of the parallel mean velocity with density and temperature gradients. The new terms are not relatively small, and could affect understanding of limiter and divertor operation.
3D Fluid Simulations of Turbulence in Detached Scrape‐off‐Layer Plasmas
Authors: Xu X. Q. , Cohen R. H. 
Published: July 2007

AbstractIn this paper, we present 3D simulations of scrape‐off‐layer (SOL) turbulence in detached plasmas. The nonlinear evolution of edge turbulence in the electrostatic limit is described by a four‐field model of the reduced Braginskii equations in a model x‐point geometry, including drives for the (1) conducting‐wall mode, (2) Kelvin‐Holmhotz mode, (3) curvature‐driven mode, (4) ▽xv∥‐driven mode, and (5) axial shear mode. In a detached plasma with Te0/Ted = 50 and Ne0/Ned = 0.2, and without magnetic shear, we find that a conducting‐wall‐like mode is the dominant drive even if turbulence drive from the sheath is small.
Fokker‐Planck Simulation of Electron Transport in SOL Plasmas with ALLA Code
Authors: Batishcheva A. A. , Batishchev O. V. , Krasheninnikov S. I. , Sigmar D. J. , Shoucri M. M. , Shkarofsky I. P. 
Published: July 2007

AbstractThe recently developed Fokker‐Planck code ALLA [1] permits us to simulate the temporal evolution collisional edge plasmas in 1D in space, IV in velocity space. It was used to simulate the SOL parallel electron flow in Alcator C‐Mod (PFC MIT) and TdeV (CCFM) tokamaks. Plasma parameters are taken as „best guesses”︁ from available detachment experimental data [2,3]. We show that the actual electron distribution function is asymmetric in the direction parallel to the magnetic field and its tail starts to departure from the Maxwellian at energies around 2T. This feature can substantially affect probe measurements and modify reaction rates and plasma transport properties.
Effect of the Dynamics of the Impurity Distribution over the Ionization States on the Radiative Plasma Instabilities and Shock Wave Structure
Authors: Krasheninnikov S. I. , Morozov D. Kh. , Sigmar D. J. , Herrera J. J. E. , Soboleva T. K. 
Published: July 2007

AbstractWe show that conventional quasistationary (coronal, or improved coronal) approximation for the energy loss due to the impurity radiation in practice can never be applied to the investigations of the impurity radiation driven instabilities in the edge plasmas due to relatively slow evolution of the impurity population over ionization states. We show that taking into account the effect of the evolution of the impurity population over ionization states results in a very significant change of the growth rates of the radiative driven instabilities and the structure of the shock wave in the radiative plasmas and leads to the strict conditions for the existence of the shock wave.
Simulation of Detachment in ITER‐Geometry Using the UEDGE Code and a Fluid Neutral Model
Authors: Wising F. , Knoll D. A. , Krasheninnikov S. I. , Rognlien T. D. , Sigmar D. J. 
Published: July 2007

AbstractWe have simulated detachment in ITER, with the UEDGE code coupled to a Navier‐Stokes neutral model, for realistic toroidal geometry and plasma parameters, with a heat flux of 100 MW into the outboard SOL. For a fixed fraction of carbon, corresponding to Zeff∼2.5 in the SOL, the power to the outer plate is reduced by more than 90%, the peak plate temperature is reduced to about 1 eV, and the total ion saturation current drops by 85%. Volume recombination plays an important role in removing the current. Bifurcated solutions and marfe‐like structures below the X‐point have been found under certain conditions.
1D Fluid Regime of Plasma‐Neutral Interaction and Divertor Detachment
Authors: Soboleva T. K. , Krasheninnikov S. I. 
Published: July 2007

AbstractWe show that self consistent decrease of both plasma flux and neutral ionization in current tokamaks is only possible when neutrals can be treated in a short mean free path approximation. We investigate these fluid regimes of plasma‐neutral interaction with 1D fluid equations employing a neutral viscosity term to treat the neutral interaction with the divertor plate. We have found that plasma flux onto the target starts to decrease at a very low heat flux coming into the hydrogen recycling region, when the temperature near the target drops below 1 eV, which seems lower than observed in the experiments. We conclude that the neutral‐neutral collisions, 2D effects of plasma‐neutral interaction, and plasma recombination processes can play a very important role in divertor plasma detachment.
Discrete‐Ordinates 1‐D Time‐Dependent Kinetic Model for Neutral Particles in the Scrape‐Off Layer
Authors: Sabbadin E. , Zanino R. 
Published: July 2007

AbstractA 1‐D kinetic code has been developed for simulating the transport of neutral particles in the Scrape‐Off Layer using the multi‐energy‐group discrete‐ordinates (SN) solution method. The SN algorithm, usually applied to time‐independent calculations, has been here extended to the time‐dependent case. The code has been coupled with a 1‐D, 1‐fluid, time‐dependent plasma code, and results comparing the diffusion (S2) approximation with the fully kinetic case are presented.
S2: A Finite Element Code for Modelling Transport in Tokamak Divertor and SOL
Authors: Simard M. , Marchand R. 
Published: July 2007

AbstractA symbolic finite element code is used to simulate transport of a single‐species plasma in the edge and divertor of a tokamak. The physical model is based on a fluid description of the ion density, ion parallel momentum, ion and electron pressure. In modelling recycling, transport of neutral density and energy is treated in the diffusion approximation. The system of equations is discretised on an unstructured triangular mesh, thus permitting good spatial resolution near the X‐point and an accurate description of divertor plates of arbitrary shapes. S2′s symbolic interface allows the physical model to be conveniently defined using standard mathematical notation. Comparisons with B2.5 simulation results are presented.
The Effect of Heat Flux Limiting on Divertor Fluid Models
Authors: Day M. , Merriman B. , Najmabadi F. , Conn R. W. 
Published: July 2007

AbstractThe classical plasma electron heat flux can greatly overestimate the physical heat flux when the electron mean‐free‐path becomes long compared to the electron temperature gradient scale length. For fluid modelling of plasmas, a common remedy is to apply an artificial „flux limiter”︁ that keeps the heat flux at a physically reasonable value in this long mean‐free‐path regime. The ad‐hoc limiter is not derived from first principles and it introduces a poorly understood free parameter into the model. We study the effect of this parameter in our divertor plasma models to understand how it influences the computed solution. We investigate regimes of both short and long mean‐free‐path and consistently find large parameter sensitivity. Thus, without additional experimental or theoretical guidance in the choice of flux‐limit parameter, flux limiting does not appear to provide an acceptable basis for predictive plasma fluid modelling.
Parallelization of and Results from Kinetic Edge Plasma Code W1
Authors: Xu X. Q. , Batishchev O. V. , Byers J. A. , Cohen R. H. , Krasheninnikov S. I. , Rognlien T. D. , Sigmar D. J. 
Published: July 2007

AbstractIn this paper, we describe a highly parallelized implementation of the 1D2V collisional PIC‐MCC code (W1 by Batishchev[1]) for Scrape‐Off‐Layer (SOL) plasmas on the Cray T3D using the PVM message‐passing library. The particles are divided equally among the processors, and grid quantities (temperatures, etc) are averaged from all processors. The electric field is solved using the electron momentum equation. The logical sheath potential is calculated in a serial fashion. For a fixed number of particles in the simulations (1 million particles), we have obtained a factor of 31 speed‐up by using 32 processors compared to 1, and a factor of 90 speed‐up by using 128 processors. Through our efforts on Cray T3D with 256 processors we have learned that massively parallel MIMD systems offers a new supercomputing capability for kinetic SOL physics modelling.
Axial Structure of the Field Intensity in Diffusion‐Controlled Surface‐Wave Produced Plasmas
Authors: Aliev Yu. M. , Georgieva M. , Grosse S. , Schlüter H. , Shivarova A. 
Published: July 2007

AbstractSurface‐wave gas discharges sustained under the conditions of a diffusion controlled regime are studied on the basis of the fluid plasma model. The self‐consistent behaviour of the plasma density and of the field intensity along the discharge length is obtained. It is shown that the nonlinearity in the particle balance equation due to stepwise ionisation determines the plasma density in terms of the field intensity and ensures self‐consistent behaviour of the wave ⟺ discharge characteristics in a diffusion controlled regime.
Doppler Broadening and Magnetic Field Effects on the Balmer Lines Emitted at the Edge of a Tokamak Plasma
Authors: Hey J. D. , Korten M. , Lie Y. T. , Pospieszczyk A. , Rusbüldt D. , Schweer B. , Unterberg B. , Wienbeck J. , Hintz E. 
Published: July 2007

AbstractProfiles of the Balmer lines Da(Ha), Dβ(Hβ) and Dγ(Hγ) have been measured in the scrape‐off layer and within the edge of the TEXTOR (upgrade) plasma, under Ohmic conditions and with neutral‐beam injection. Each line profile shows a strong Zeeman effect in the vicinity of line centre, and a marked central dip when mainly the σ components are observed. The line core evidently originates from cold atoms in the edge plasma, excited in the course of molecular dissociation, while the broad pedestal on which the core rests is radiated by excited atoms produced through charge‐exchange recombination of deuterons (protons), transported outwards from the much hotter plasma interior, and by atoms heated directly by collisions with the deuterons (protons). Core temperatures of about 0.5 eV and less are obtained from line profile analysis.
The Interaction of the Supersonic Plasma‐Jet with the Substrate in the RF Plasma‐Chemical Reactor
Authors: Šícha M. , Hubička Z. , Tichý M. , Novák M. , Soukup L. , Jastrabík L. , Behnke J. F. , Kapička V. , Kapoun K. , Šerý M. 
Published: July 2007

AbstractAlthough the plasma‐chemical processes and properties of the deposited thin film can strongly depend on the interaction of the supersonic plasma jet channel with the substrate, up to now the study of this phenomenon has not yet been done. We decided to study this interaction by means of CCD camera. The presented results indicate that the light intensity recorded close to the substrate can be used as a measure of the intensity of plasma‐chemical deposition processes on the substrate surface.
Anomalous Electric Fields Inside a Dense Plasma of a Current Sheet
Authors: Frank A. G. , Gavrilenko V. P. , Ispolatov Y. A. O. , Kyrie N. P. , Oks E. 
Published: July 2007

AbstractFor the first time there have been analyzed profiles of electric‐field‐sensitive na spectral lines of the Li‐like impurity ions CIV, NV, OVI emitted by a hot dense plasma of a plane current sheet. Theoretical profiles of these lines have been calculated with an allowance for Anomalous Electric Fields (AEF). By comparing the experimental and theoretical profiles of the lines it has been established for the first time that in the plane current sheet there were developed AEF of rms strengths F0 ≈ (100–120) kV/cm in the time interval from τ = −0.2 μs to τ = 0 (the time τ was counted from the instant of the maximum intensity of the spectral line OVI 5291 Å). These results have been backed up by a similar analysis of the experimental profiles of the spectral line HeII 4686 Å of the primary gas of the discharge. The AEF reached these strengths at the same instant of time, τ = −0.2 μs, when electron and ion temperatures reached their maxima: Te ≫ 100 eV, Ti ≈ 300 eV. The instant τ = −0.2 μs corresponds to a transition from the metastable stage to the explosive phase of the current sheet evolution. At this time the ratio of the AEF energy density to the thermal energy density of the plasma was § ≡ F20/[8π/NeTe + NiTi)] ≈ 10−3, which exceeded a similar ratio for the thermal wave field by three orders of magnitude.
Measurement of Excited States Density and the VUV‐Radiation in the Pulsed Xenon Medium Pressure Discharge
Authors: Kindel E. , Schimke C. 
Published: July 2007

AbstractTime resolved absolute density measurements of the 1s5 (metastable) and 1s4 (resonance) level in the positive column of a pulsed xenon gas discharge for gas pressures between 1 and 40 Torr and currents between 100 and 300 mA are presented. The densities ranged from 1 × 1011 to 5 × 1012 cm−3 and were found to be ten times larger in the afterglow than in the active part of the discharge. The enhanced radiation obtained in the afterglow in the near infrared regions as well as in the VUV region is caused by a dissociative recombination and formation of excimer states.
Dissociative Excitation of Tetramethylsilane (TMS) and Hexamethyldisiloxane (HMDSO) by Controlled Electron Impact
Authors: Kurunczi P. , Koharian A. , Becker K. , Martus K. 
Published: July 2007

AbstractThe optical emission spectra in the wavelength region 200–800 nm produced by electron impact on the silicon‐organic molecules TMS (tetramethylsilane) and HMDSO (hexamethyldisiloxane) under controlled single‐collision conditions have been analyzed. Absolute emission cross sections from threshold to 200 eV impact energy were determined for a variety of emission features. For both targets, the CH(A2Δ → X2Π) emission, the so‐called CH „4300 Å”︁ band, was found to have the largest emission cross section with values (at 100 eV) of 5.5 × 10−19 cm2 and 6.1 × 10−19 cm2 for TMS and HMDSO, respectively. Relatively high onset energies of 28.0 ± 1.5 eV (TMS) and 33.1 ± 1.5 eV (HMDSO) were measured for these emissions. Weaker emission features in both spectra were identified as CH bands corresponding to the B2∑− → X2Π transition (the CH „3900 Å”︁ system) and the C2∑ → X2Π transition, and as the atomic Si line emissions at 253 nm and 288 nm. Near‐threshold studies indicate an onset for the Si emissions of 29.0 ± 2.0 eV (TMS) and 44.6 ± 2.0 eV (HMDSO). Absolute cross sections and appearance energies were also determined for the strongest lines of the hydrogen Balmer series for both targets. The comparatively high onset energies and small emission cross sections for all observed emissions indicate that single‐step dissociative excitation processes are unlikely to play a dominant role in low‐temperature processing plasmas containing TMS and HMDSO.
Kinetic Theory of Sound Propagation in a Dissociating Gas
Authors: McLennan J. A. 
Published: July 2007

AbstractThe effects on the propagation of sound of dissociation and recombination in a diatomic gas are discussed on the basis of kinetic theory for such a gas. A perturbation theory is used in which the perturbation incorporates the collision operator for reactions and the convective term in the Boltzmann equation. Consequently the sound frequency need not be small relative to the reaction rate, so the results describe the transition between slow and fast reaction regimes.
Dielectric Theory of Collisional Absorption in Intense Laser Fields
Authors: Mulser P. , Saemann A. 
Published: July 2007

AbstractCollisional absorption of superintense laser pulses in dense plasmas can be described by the classical linear dielectric theory in a wide range of parameters. For electron oscillation velocities largely exceeding their thermal motion a linear oscillator model is presented and nearly exact solutions in terms of Anger functions are given for the time‐averaged electron‐ion collision frequency. A possible extension of the dielectric theory into higher density and/or lower temperature regions is indicated and the need for time‐dependent collision frequencies is stressed.
Wall Recombination in Glow Discharges
Authors: Behnke J. F. , Bindemann T. , Deutsch H. , Becker K. 
Published: July 2007

AbstractWe derived a model describing the radial distribution of the ion and electron densities and the radial electric field strength in the positive column of a glow discharge. The set of equations related to the plasma consists of the equations for particle and momentum conservation for the ions and electrons and the Poisson‐equation. In a novel approach, the necessary boundary conditions in our model result from a system of balance equations for the charge carriers on the insulated wall surrounding the positive column.
Analysis of Three‐Dimensional Motion of Relativistic Electrons in Ion‐Focused Electrostatic System
Authors: Liu P.‐K. 
Published: July 2007

AbstractThree‐dimensional motion of relativistic electrons in ion‐focused electrostatic system, which is the basis of investigating the ion‐channel electron cyclotron maser (ICECM) or circular polarized ion‐channel laser (CPICL), is analyzed through nonlinear numerical simulation. The results show that the projection of electron orbit in cross‐section is, in general, a precessing ellipse with a center on the axis of the ion‐channel in a cylindrical ion‐focuse electrostatic system. When injection electrons satisfy a specific condition, the electrons can have a steady circular orbit. The effect of relativistic factor on electrons' motion is discussed. Using the perturbation method, the eigenfrequency of this system is derived as Equation \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {4 - \beta _ \bot ^2 } \omega _0 $\end{document} where wo is the cyclotron frequency of the electrons. This implies that there may exist a new kind of instability in ICECM or CPICL.
Spectral and Electrical Characteristics of a Stabilized DC Arc Seeded With Potassium
Authors: Stoiljković M. M. , Holclajtner‐Antunović I. 
Published: July 2007

AbstractA systematic investigation of spectral and voltage‐current (U‐I) characteristics of a DC arc plasma was performed. The vertical, argon and wall stabilized DC arc plasma was seeded by an easily ionizable element (EIE), potassium, in a wide range of concentrations. It was found that different arc currents and potassium contents in the plasma have considerable influence on the plasma properties. A lower potassium content (< 10 g/l of KCl in solution) introduced into the arc at lower currents (<6A) causes an increase in arc voltage and a change in the arc spectrum (intensity of lines), while at higher arc currents (>6A) it causes no change in the electrical characteristics, but a further change in the intensity of the arc spectrum. Higher potassium concentrations, >20 g/l of KCl, increase the arc voltage at higher currents (>6A). There is a critical quantity of potassium in the plasma which suppresses high energy processes (ionization, excitation…) for a fixed arc current, while spectral composition and lateral distribution of the residual emitted spectra become essentially changed. Amounts of potassium above critical do not cause further changes in the plasma spectral composition nor in the I‐U characteristic, except in the spectral line intensities.
Spatially Homogeneous Modelling of a Stabilized XeCl* Laser Discharge
Authors: Gortchakov S. , Loffhagen D. , Winkler R. 
Published: July 2007

AbstractRecently an improved technique to create a stable and homogeneous discharge for pumping excimer laser plasmas has been proposed in [1]. This technique is based on the additional use of a stabilizing low‐current preliminary discharge.For the study of such laser discharges a self‐consistent spatially homogeneous model with the inclusion of the time‐dependent Boltzmann equation for the electrons, of the rate equation system arising from the heavy particle and photon kinetics and of the electrical circuit equations has been employed. A brief presentation of important features of the comprehensive model is given. An iterative procedure to solve self‐consistently the system of kinetic, rate and circuit equations has been generalized and applied to discharge conditions where a dominant electron‐electron interaction occurs.Model calculations have been performed for typical XeCl* laser plasmas which operate according to the improved discharge technique. On this basis a detailed analysis of main features of this laser discharge is given and the impact of various parameters on significant characteristics of the discharge is calculated and discussed.
Ergodic Divertor : Present Status of Understanding
Authors: Grosman A. , Ghendrih Ph. , Capes H. , Féron S. 
Published: July 2007

AbstractThe ergodic divertor (ED) experiments on Tore Supra have demonstrated the capability of open divertors to reach significant results in terms of plasma‐wall interaction control. Standard statistical analysis of stochastic systems allows one to derive the diffusion of free particles when the threshold to stochasticity is reached. However, transport analysis modifies this picture owing to boundary effects due to sources and sinks. The energy deposition onto the target plate is shown to be governed by parallel transport over the parallel coherence scale of the temperature field. This transport referred to as laminar transport is very similar to that found in axisymmetric divertors. Using field line tracing (MASTOC code), one can locate precisely the location of the peak in energy deposition. Qualitative agreement with experimental evaluation of the peaking factor is obtained using the radial penetration of field lines given by Mastoc and a WKB approximation.
Multi‐Species Developments in the EDGE2D Code
Authors: Fichtmüller M. , Corrigan G , Radford G , Simonini R , Spence J , Taroni A 
Published: July 2007

AbstractThe scrape‐off layer code EDGE2D has been extended to accommodate more than one impurity species [1]. The parallel transport coefficients are calculated from a 21 moment description using the method of reduced charge states (RCS). Simulations with carbon and helium as impurities have been carried out to validate the multi‐species version of EDGE2D. It is shown that further use of RCS allows the reduction of the number of momentum equations. The computing time is thereby substantially reduced. At the same time the code results show little difference between using the reduced set of momentum equations and using the full set.
Modelling of Impurity Generation and Transport with a Monte Carlo Method based on Single Particle Motions
Authors: Kawamura T. , Ono T. , Kenmotsu T. , Ohyabu N. 
Published: July 2007

AbstractThe generation and transport of impurity particles in a boundary plasma are analysed on the basis of a single particle motion together with a Monte Carlo method, where the Langevin‐type equation of an impurity particle is used. The impurity generation and transport in a scrape‐off layer are modelled after the Large Helical Device (LHD). The screening efficiency of the SOL for the impurity penetration into a core plasma, impurity redeposition on the first wall and the other effects are analysed from the statistics of traces of many impurites.