Edge Plasma Transport Research Articles

Numerical analysis of particle recycling in the TEXTOR helical divertor

The TEXTOR helical divertor is a magnetic configuration created by the application of external resonant magnetic perturbations with the intention to control plasma edge transport and the resulting particle and heat fluxes to the divertor target. It is confirmed by 3D computer simulations that no high-recycling-like regime is established under TEXTOR relevant conditions, despite the fact that a transition to detachment (i.e. a saturation or even a roll-over of the recycling flux) is observed at high densities. The driving mechanisms are, distinct from apparently similar observations in poloidal divertors and stellarator divertors, a combination of volumetric power losses and enhanced upstream-to-downstream heat transport, but with no significant role of the momentum balance.

The effect of ion orbit loss and X-loss on the interpretation of ion energy and particle transport in the DIII-D edge plasma

Calculation models are presented for treating ion orbit loss effects in interpretive fluid transport calculations for the tokamak edge pedestal. Both standard ion orbit loss of particles following trapped or passing orbits across the separatrix and the X-loss of particles that are poloidally trapped in a narrow null-Bθ region extending inward from the X-point, where they gradB and curvature drift outward, are considered. Calculations are presented for a representative DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] shot which indicate that ion orbit loss effects are significant and should be taken into account in calculations of present and future experiments.

New approach in two-dimensional fluid modeling of edge plasma transport with high intermittency due to blobs and edge localized modes

A new approach is proposed to simulate intermittent, non-diffusive plasma transport (via blobs and filaments of edge localized modes (ELMs)) observed in the tokamak edge region within the framework of two-dimensional transport codes. This approach combines the inherently three-dimensional filamentary structures associated with an ensemble of blobs into a macro-blob in the two-dimensional poloidal cross-section and advects the macro-blob ballistically across the magnetic field, B. Intermittent transport is represented as a sequence of macro-blobs appropriately seeded in the edge plasma according to experimental statistics. In this case, the code is capable of reproducing both the long-scale temporal evolution of the background plasma and the fast spatiotemporal dynamics of blobs. We report the results from a two-dimensional edge plasma code modeling of a single macro-blob dynamics, and its interaction with initially stationary background plasma as well as with material surfaces. The mechanisms of edge plasma particle and energy losses from macro-blobs are analyzed. The effects of macro-blob sizes and advection velocity on edge plasma profiles are studied. The macro-blob impact on power loading and sputtering rates on the chamber wall and on inner and outer divertor plates is discussed. Temporal evolution of particle inventory of the edge plasma perturbed by macro-blobs is analyzed. Application of macro-blobs to ELM modeling is highlighted.

Coupling between long-range toroidal correlations and radial transport in the TJ-II boundary plasma

TJ-II is equipped with two probe systems that allow the simultaneous measurement of the radial and toroidal characteristics of the boundary plasma fluctuations with high temporal resolution. The interplay between long-range toroidal correlations, local cross-field particle transport and radial structure of the edge fluctuations has been investigated. Experimental evidence is presented that both the radial correlation and the turbulent particle transport are reduced in the region dominated by zonal flows (ZFs). Experimental findings show the key role of ZFs in the edge plasma transport regulation.

The influence of three-dimensional stochastic magnetic boundaries on plasma edge transport and the resulting plasma wall interaction

The three-dimensional (3D) features of plasma edge profiles and wall interaction patterns induced by edge resonant magnetic perturbation fields (RMP) are discussed comparing TEXTOR and DIII-D. We show that the scrape-off layer (SOL) profiles and decay lengths depend the edge safety factor, the RMP base mode as well as on the plasma rotation during RMP application indicating modification of SOL transport by the 3D perturbation fields. This is compatible with channelling of particle and heat efflux along open perturbed field lines in the very edge of the plasma boundary into a completely re-arranged, helically striated 3D divertor footprint. The distribution of the measured divertor heat and particle fluxes at DIII-D match the vacuum modelled magnetic footprint topology in L-mode while in H-mode the striation width exceeds the modelled footprint width by 15–30%. This 3D structure of the measured heat and particle fluxes results in a new situation for the material erosion properties and initial quantification of the net-erosion within the 3D footprint shows in L-mode a 50% decrease of the chemical erosion yield and evidence for a comparably small 15-20% increase in physical sputtering. Extrapolation of these findings to ITER by vacuum modelling of the magnetic footprint for the actual ELM control coils shows a similar vacuum magnetic footprint topology as found at DIII-D during RMP ELM suppression. However, the open field lines escape the CFC covered ITER divertor area potentially transferring net-erosion characteristics from the CFC domain onto the Tungsten including so far unconsidered heat and particle loads on this sensitive material.

Comparative Study of the SN, DDN and DN Diverter Plasma Simulations for HCSB-DEMO

A comparative study of SN (single null), DDN (disconnected double-null) and DN (double null) diverters for HCSB-DEMO (helium-cooled solid breeder, HCSB) is reported in this paper by using the 2D edge plasma transport code SOLPS5.0. There is a heat power of 400 MW crossing CIB (core interface boundary). The peak heat flux at targets with different upstream densities is investigated. It is indicated that the peak heat flux at the outer target with a SN diverter is lower than that at the outer-down target with a DDN diverter under the same upstream density, but is higher than that at the outer target with a DN diverter. The diverter should operate at a proper upstream density to avoid strong high heat flux at the targets. The peak heat flux at the targets and first wall are sensitive to the SOL (scrape-off-layer) grid thickness. The simulated results will provide data for the design of diverter in HCSB-DEMO.

Validating the 3D edge code EMC3-EIRENE against 2D simulations with EDGE2D-EIRENE for JET single null configurations

The 3D edge plasma transport code EMC3-EIRENE [1] was successfully adapted to the single null configuration of JET. A test bed with the well established 2D edge plasma code EDGE2D-EIRENE [2,3] was prepared to check the adaptation of the EMC3-EIRENE code and isolate any discrepancies when compared. While both codes solve essentially the same set of reduced Braginskii’s fluid equations, they use very different numerical techniques: a 3D Lagrangian Monte Carlo method in EMC3 and a 2D Eulerian finite differences scheme in EDGE2D. The results should be very similar as both codes are iteratively coupled to the same kinetic neutral particle Monte Carlo code EIRENE [4]. We show that both codes are in a fairly good quantitative agreement regarding both midplane and target plasma profiles as well as transition from low to high recycling regime with increasing core plasma density.

Modeling of dust impact on tokamak edge plasmas

The first self-consistent modeling of impact of dust on edge plasmas in tokamaks with the coupled dust–plasma transport code DUSTT/UEDGE is presented. The code validation for the modeling of lithium dust with radius ∼20μm in the plasmas is performed using 3D reconstructed dust trajectories measured during lithium dust injection experiments on NSTX. The modeling demonstrates that the dust injection with rates of several of tens mg/s can have profound effect on the edge plasma profiles, transport, and stability. The differences between the dust injection and the injection of equivalent amounts of gaseous impurities in the plasmas are discussed.

Pfirsch–Schlüter impurity transport in stellarator edge plasmas with large radial gradients

Large radial gradients of the bulk ion temperature and density in edge plasmas cause heavy impurity ions to redistribute themselves within flux surfaces, thereby making the impurity density vary poloidally (and, in a stellarator, toroidally). This redistribution reduces the radial impurity flux. In this paper, the effects of stellarator geometry on this mechanism are investigated, which lead to qualitatively different results compared with axisymmetric systems, in which the plasma can rotate freely. We derive analytic expressions for the radial impurity flux and the impurity density. The resulting differential equations are solved analytically in some limiting cases and numerically for W7-X and W7-AS.

Edge Plasma Transport in the Helical Divertor Configuration in LHD

AbstractTo understand the edge plasma transport in the complex magnetic field lines structure in the LHD heliotoron, the heat and particle fluxes profiles on divertor plates were investigated experimentally and theoretically by using EMC3‐EIRENE code. The heat flux profiles on a divertor plate in different magnetic configuration were measured and the main channel of the heat transport was revealed to be the field lines with long connection lengths which connect the divertor plate and the stochastic field lines layer. A comparison of the heat and the ion fluxes profiles on the divertor plates between the experimental data and the results of the calculation using EMC3‐EIRENE code was conducted. It was revealed that the change of the cross‐field transport in the edge plasma region causes not only the change of the heat and particle flux profiles on divertor plates, but also the changes of the intrinsically non‐uniform global heat and particle deposition profiles on the helical diveror. The cross‐field transport seems to be larger in the higher edge electron temperature discharge in the experimental conditions in this paper (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modelling of passive spectroscopy in the ITER divertor: the first hydrogen Balmer lines

The first lines of the hydrogen Balmer series are investigated in ITER divertor conditions using a line shape code and a plasma edge transport code. It is shown that most of the emissivity originates from a localized, cold and dense region close to the divertor target plates, where the plasma is in the recombining regime. We simulate the signal obtained by pointing a spectrometer at this zone. The physical processes which contribute to the spectral line formation are examined, with a special emphasis on the Stark effect, photon absorption and stimulated emission. It is shown that, even though the Stark effect is significant, local information on the Doppler atomic temperature can be obtained from a fitting analysis of the Dα spectral line shape.

Multi‐Fluid Modeling of Low‐Recycling Divertor Regimes

AbstractThe low‐recycling regimes of divertor operation in a single‐null NSTX magnetic configuration are studied using computer simulations with the edge plasma transport code UEDGE. The edge plasma transport properties pertinent to the low‐recycling regimes are demonstrated. These include the flux‐limited character of the parallel heat transport and the high plasma temperatures with the flattened profiles in the scrape‐off‐layer. It is shown that to maintain the balance of particle fluxes at the core interface the deuterium gas puffing rate should increase as the divertor recycling coefficient decreases. The radial profiles of the heat load to the outer divertor plate, the upstream radial plasma profiles, and the effects of the cross‐field plasma transport in the low‐recycling regimes are discussed. It is also shown that recycling of lithium impurities evaporating from the divertor plate at high surface temperatures can reverse the low‐recycling divertor operational regime to the high‐recycling one and may cause thermal instability of the divertor plate (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulations of NSTX with a Liquid Lithium Divertor Module

AbstractA strategy to develop self‐consistent simulations of the behavior of lithium in the Liquid Lithium Divertor (LLD) module to be installed in NSTX is described. In this initial stage of the plan, the UEDGE edge plasma transport code is used to simulate an existing NSTX shot, with UEDGE's transport coefficients set using midplane and divertor diagnostic data. The LLD is incorporated into the simulations as a reduction in the recycling coefficient over the outer divertor. Heat transfer calculations performed using the resulting heat flux profiles provide preliminary estimates on operating limits for the LLD as well as input data for subsequent steps in the LLD modeling effort (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of Boundary Plasma for HCSB-DEMO by Using 2D Edge Plasma Transport Code SOLPS5.0

A simulation on HCSB-DEMO (helium-cooled solid breeder, HCSB) edge plasma, by using 2D edge plasma transport code SOLPS5.0, is presented. There is 400 MW heat power crossing CIB (core interface boundary). The heat flux profiles and peak flux at the divertor targets with different boundary densities are investigated. It is indicated that the HCSB-DEMO divertor should operate at a proper upstream density in order to avoid a high heat flux at the divertor targets. When the upstream density is 0.63×1020 m−3, the peak heat flux at the divertor targets will be above 17 MW/m2. The cross-field transport, the power crossing CIB and the power fraction taken by electrons and ions and SOL (scrape-off-layer) thickness are analyzed as unknown parameters. It is shown that the peak flux at the divertor target is very sensitive to these parameters. The simulation results will be used in the divertor design for HCSB-DEMO.

Three-dimensional edge transport simulations for DIII-D plasmas with resonant magnetic perturbations

The impact of resonant magnetic perturbations on plasma edge transport at DIII-D is investigated with the EMC3-EIRENE code. The code includes a self-consistent fluid treatment of particle, parallel momentum and energy transport for the plasma as well as recycling and kinetic neutral particle transport. It is shown that the magnetic field structure is reflected in the plasma structure as well, both in the patterns of particle and heat loads on the divertor target and in a 3D spatial modulation of plasma parameter in the X-point region and near the midplane on the high field side. In particular, a very strong, toroidally and poloidally localized increase in electron density and temperature outside the separatrix is found. The presented findings of target loads are consistent with earlier 3D heat transport modelling results at DIII-D, but not with heat flux measurements, while reasonable agreement is found for the particle flux. The poloidal modulations introduced by short magnetic flux tubes are consistent with observations and 3D modelling results at TEXTOR and we demonstrate that even for this rather simple flux tube geometry it is necessary to include the full 3D plasma and neutral transport.

Block-structured grids in Lagrangian 3D edge plasma transport simulations

Distinct from conventional Eulerian 2D fluid solvers, applied routinely to magnetic fusion edge plasma studies, complex 3D magnetic topologies are currently treated by the geometrically more flexible Lagrangian schemes, supplemented by Monte Carlo procedures for higher order derivatives (dissipative terms due to diffusion processes) and sources. These particle based algorithms are combined with a field line reconstruction technique for dealing with partially ergodic magnetic fields, involving field aligned regular grids. A generalization to block-structured grids is carried out, which greatly enhances the applicability range of present 3D fusion plasma edge codes, in particular also to poloidally magnetic diverted configurations, as currently envisaged for the largest magnetic fusion device under construction: ITER.

Simulation of hydrocarbon reflection from carbon and tungsten surfaces and its impact on codeposition patterns on plasma facing components

In order to investigate the redeposition characteristics of hydrocarbons released by chemical sputtering, a Monte Carlo simulation of the transport in edge plasmas is performed, where the reflection and dissociation efficiencies at tungsten (W) and carbon (C) surfaces are calculated using molecular dynamics simulation. The redeposition probability for W and W–C mixed material is strongly suppressed due to strong break-up of hydrocarbons on the surface. Sticking coefficient for ion species increases with increasing plasma temperature, whereas for low plasma temperature (<3 eV) it approaches to the value for neutral species which is independent of the temperature. Our calculations reproduce the redeposition distribution and the difference between C and W roof-like limiters in 13CH 4 injection experiments at TEXTOR. The redeposition distribution is more localized for physical sputtering at a W–C mixed layer formed on the W limiter than for chemical sputtering of hydrogenated and amorphized carbon on the C limiter.

Comparison of kinetic and fluid neutral models for attached and detached state

Neutral behavior has an important role in the transport simulations of the edge plasma. Most of the edge plasma transport codes treat neutral particles by a simple fluid model or a kinetic model. The fluid model allows faster calculations. However, the applicability of the fluid model is limited. In this study, simulation results of JT-60U from kinetic neutral model and fluid neutral model are compared under the attached and detached state, using the 2D edge plasma code package, SOLPS5.0. In the SOL region, no significant differences are observed in the upstream plasma profiles between kinetic and fluid neutral models. However, in the divertor region, large differences are observed in plasma and neutral profiles. Therefore, further optimization of the fluid neutral model should be performed. Otherwise kinetic neutral model should be used to analyze the divertor region.

Edge turbulence and SOL transport in tokamaks

Some key topics in tokamak edge plasma transport and turbulence are reviewed. Multi-device results reveal a new paradigm of scrape-off layer (SOL) transport. Radial transport is driven by intermittency throughout the SOL, in between edge localized modes (ELMs) in H-mode, and comprised of plasma filaments that are generated near the last closed flux surface likely by interchange instability. The filaments travel radially at speeds of ∼1 km/s into the SOL and have a poloidal size of 1–3 cm in most devices. The radial transport in the SOL is poloidally asymmetric, by factors of 2–5, causing a pressure peak in the low field side. This asymmetry and other neo-classical terms, such as Pfirsch–Schlüter currents, are found to drive strong SOL flows. The intermittent particle flux, is 20% of the total, including ELMs, at low collisionality, becoming 70% of total at high collisionality. Numerical and analytical models can reproduce the scaling of intermittency with collisionality as well as many details of the filament dynamics in the SOL.

Edge plasma modelling for transport analysis on JT-60U tokamak

At the edge plasma the cross-field transport is thought to play a major role in determining the shape of density and temperature profiles. The edge plasma transport code B2.5I is used for modelling experimental measurements of the edge plasmas in L-mode and H-mode shots on JT-60U tokamak and the radial transport coefficients at the edge have been obtained by matching the results of the transport code directly to experimental measurements, by which the radial transport coefficients are determined by the combination of the experiment and the transport modelling.