Scrape-off Layer Transport Research Articles

Far scrape-off layer particle and heat fluxes in high density – High power scenarios

The far scrape-off layer transport is studied in ASDEX Upgrade H-mode discharges with high divertor neutral density N0,div, high power across the separatrix Psep and nitrogen seeding to control the divertor temperature. Such conditions are expected for ITER but usually not investigated in terms of turbulent SOL transport. At high N0,div and Psep the H-mode discharges enter a regime of high cross-field particle and power transport in the SOL which is accompanied by a significant change of the turbulence characteristic analogous to the transition from conductive to convective transport in L-mode. Parallel particle and power flux densities of several 1023m−2s−1 and 10MWm−2 have been detected about ∼40 to 45mm outside the separatrix mapped to the outer mid-plane. The particle flux fall-off length reached up to 45mm. This paper presents for the first time an empirical condition to enter the high transport regime in H-mode and the relation of this regime to changes in the filamentary transport.

Effects of heating power on divertor in-out asymmetry and scrape-off layer flow in reversed field on Experimental Advanced Superconducting Tokamak

The dependence of divertor asymmetry and scrape-off layer (SOL) flow on heating power has been investigated in the Experimental Advanced Superconducting Tokamak (EAST). Divertor plasma exhibits an outboard-enhanced in-out asymmetry in heat flux in lower single null configuration for in reversed (ion ∇B drift direction toward the upper X-point) field directions. Upper single null exhibits an inboard-favored asymmetry in low heating power condition, while exhibits an outboard-favored asymmetry when increasing the heating power. Double null has the strongest in-out asymmetry in heat flux, favoring the outer divertor. The in-out asymmetry ratios of qt,out/qt,in and Pout/Ptotal increase with the power across the separatrix Ploss, which is probably induced by the enhanced radial particle transport due to a large pressure gradient. The characteristics of the measured SOL parallel flow under various discharge conditions are consistent with the Pfirsch-Schlüter (PS) flow with the parallel Mach number M∥ decreasing with the line averaged density but increasing with Ploss, in the same direction as the PS flow. The contributions of both poloidal E×B drift and parallel flow on poloidal particle transport in SOL on EAST are also assessed.

An experimental investigation of the high density transition of the scrape-off layer transport in ASDEX Upgrade

A multidiagnostic approach, utilizing Langmuir probes in the midplane, X-point and divertor walls, along with lithium beam and infrared measurements is employed to evaluate the evolution of the scrape-off layer (SOL) of ASDEX Upgrade across the L-mode density transition leading to the formation of a density shoulder. The flattening of the SOL density profiles is linked to a regime change of filaments, which become faster and larger, and to a similar flattening of the q∥ profile. This transition is related to the beginning of outer divertor detachment and leads to the onset of a velocity shear layer in the SOL. Experimental measurements are in good agreement with several filament models which describe the process as a transition from conduction to convection-dominated SOL perpendicular transport caused by an increase of parallel collisionality. These results could be of great relevance since both ITER and DEMO will feature detached divertors and densities largely over the transition values, and might therefore exhibit convective transport levels different to those observed typically in present-day devices.

Implications of high density operation on SOL transport: A multimachine investigation

The transition reported in L-mode plasmas leading to the formation of a density “shoulder” in the Scrape-off Layer (SOL) of tokamaks is investigated in the three ITER stepladder machines: COMPASS, ASDEX Upgrade (AUG) and JET. In AUG and JET, the broadening of the profiles can be observed at similar relative densities (ne/nGW∼0.4–0.5) and coincides with a clear increase in the size of SOL filaments and the onset of the outer divertor detachment. In COMPASS, no transition is observed in the same density range: the divertor does not detach and no clear effect is observed neither in the profiles nor in the turbulence. This is explained as the result of an increase of intermittent transport triggered by the increased resistivity along filaments associated to divertor detachment. The transition conditions in AUG and JET are compared to foreseen ITER SOL parameters, and the implications are briefly discussed.

Effect of chaos on plasma filament dynamics and turbulence in the scrape-off layer

Naturally occurring error fields as well as resonant magnetic perturbations applied for stability control are known to cause magnetic field-line chaos in the scrape-off layer (SOL) region of tokamaks. Here, 2D simulations with the BOUT++ simulation framework are used to investigate the effect of the field-line chaos on the SOL and in particular on its width and peak particle flux. The chaos enters the SOL dynamics only through the connection length, which is evaluated using a Poincaré map. The variation of experimentally relevant quantities, such as the SOL gradient length scale and the intermittency of the particle flux in the SOL, is described as a function of the strength of the magnetic perturbation. It is found that the effect of the chaos is to broaden the profile of the sheath-loss coefficient, which is proportional to the inverse connection length. That is, the SOL transport in a chaotic field is equivalent to that in a model where the sheath-loss coefficient is replaced by its average over the unperturbed flux surfaces. The model does not include the effects of chaotic features other than the parallel connection length.

Characterization of scrape-off layer transport in JET limiter plasmas

The JET scrape-off layer (SOL) has been characterized with a reciprocating probe in inner wall (IW), and outer wall (OW), limited plasmas. Experiments revealed that SOL profiles are substantially broader (by a factor of ∼5–7.5 in the power e-folding length) for IW limited than in OW limited plasmas. Results are consistent with the larger radial turbulent transport found for IW limited plasmas. Major differences are observed between IW and OW limited plasmas on the density and electron temperature e-folding lengths, parallel flow, radial turbulent transport as well as on the temporal and spatial characteristics of the fluctuations. Experimental findings on JET suggest that the differences in the SOL characteristics for both configurations are due to a combination of a poloidal asymmetry in radial transport with a reduced cross-field transport across the last closed flux surface associated with the confinement improvement observed for OW limited plasmas.The dependence of the SOL power e-folding length on the main plasma parameters was also investigated for IW limited plasmas and a modest negative dependence on both the plasma current and the line-averaged density found. Finally, it is shown that the SOL radial transport and the amplitude of the fluctuations increase with plasma current and decrease with line-averaged density for IW limited plasmas.

Study of the L–I–H transition with a new dual gas puff imaging system in the EAST superconducting tokamak

The intermediate oscillatory phase during the L–H transition, termed the I-phase, is studied in the EAST superconducting tokamak using a newly developed dual gas puff imaging (GPI) system near the L–H transition power threshold. The experimental observations suggest that the oscillatory behaviour appearing at the L–H transition could be induced by the synergistic effect of the two components of the sheared m, n = 0 E × B flow, i.e. the turbulence-driven zonal flow (ZF) and the equilibrium flow. The latter arises from the equilibrium, and is, to leading order, balanced by the ion diamagnetic term in the radial force balance equation. A slow increase in the poloidal flow and its shear at the plasma edge are observed tens of milliseconds prior to the I-phase. During the I-phase, the turbulence recovery appears to originate from the vicinity of the separatrix with clear wave fronts propagating both outwards into the far scrape-off layer (SOL) and inwards into the core plasma. The turbulence Reynolds stress is directly measured using the GPI system during the I-phase, providing direct evidence of kinetic energy transfer from turbulence to ZFs at the plasma edge. The GPI observations strongly suggest that the SOL transport physics and the evolution of pressure gradient near the separatrix play an important role in the L–I–H transition dynamics. To highlight these new physics, the previous predator–prey model is extended to include a new equation for the SOL physics. The model successfully reproduces the L–I–H transition process with several features comparing favourably with GPI observations.

Operation and coupling of LH waves with the ITER-like wall at JET

In this paper important aspects of the lower hybrid (LH) operation with the ITER-like wall (ILW) [] at JET are reported. Impurity release during LH operation was investigated and it was found that there is no significant Be increase with LH power. The concentration of W was analysed in more detail and it was concluded that LH negligibly contributes to its increase. No cases of W accumulation in LH-only heating experiments were observed so far. LH wave coupling was studied and optimised to achieve the level of system performance similar to before ILW installation. Measurements by Li-beam were used to study systematic dependencies of the scrape-off layer (SOL) density on the gas injection rate from a dedicated gas introduction module and the LH power and launcher position. Experimental results are supported by SOL transport modelling. Observations of arcs in front of the LH launcher and hotspots on magnetically connected sections of the vessel are reported. Overall, a relatively trouble-free operation of the LH system up to 2.5 MW of coupled radio frequency power in L-mode plasma was achieved with no indication that the power cannot be increased further.

Benchmarking of a 1D scrape-off layer code SOLF1D with SOLPS and its use in modelling long-legged divertors

A 1D code modelling scrape-off layer (SOL) transport parallel to the magnetic field (SOLF1D) is benchmarked with 2D simulations of MAST-U SOL performed via the SOLPS code for two different collisionalities. Based on this comparison, SOLF1D is then used to model the effects of divertor leg stretching in 1D, in support of the planned Super-X divertor on MAST. The aim is to separate magnetic flux expansion from volumetric power losses due to recycling neutrals by stretching the divertor leg either vertically (∇B∥ = 0 in the divertor) or radially (B ∝ 1/R).

Plasma confinement modification and convective transport suppression in the scrape-off layer using additional gas puffing in the STOR-M tokamak

The influence of short gas puffing (GP) pulses on the scrape-off layer (SOL) transport is studied. Similar responses of ion saturation current and floating potential measured near the GP injection valve and in the 90° toroidally separated cross-section suggest that the GP influence on the SOL region should be global. A drop in plasma temperature and a decrease in the rotational velocity of the plasma are observed in the SOL region immediately after the GP pulse; however, an unexpected increase in electron and ion temperatures is observed in the second stage of the plasma response. The decrease in floating potential fluctuations indicates that the turbulent transport is dumped immediately after the GP pulse. The GP-induced modification of turbulence properties in the SOL points to a convective transport suppression in the STOR-M tokamak. A substantial decrease in the skewness and kurtosis of ion saturation current fluctuations is observed in the SOL region resulting in the probability distribution function (PDF) getting closer to the Gaussian distribution. The plasma potential reduction, the change in plasma rotation and the suppression of turbulent transport in the SOL region indicate that the plasma confinement is modified after the GP injection. Some features of the H-mode-like confinement in the plasma bulk also accompany the SOL observations after application of the additional sharp GP pulse.

First analysis of tungsten transport in the edge of Tore Supra plasmas

A series of Tungsten (W) injections was performed in the Tore Supra (TS) tokamak during the 2011 experimental campaign. The first goal of this experiment was to establish Vacuum Ultra Violet W spectroscopy in view of the WEST project that consists in equipping TS with a full W divertor and the second one to study the W screening properties of the TS Scrape-Off Layer (SOL). The injections are performed by Laser Blow Off injections and by a new injection system developed at IPP Garching. The latter was mounted on the TS fast reciprocating manipulator system allowing quantitatively controlled injections of W molecules at different radial positions. The W quasi-continuum observed at 5nm is the best candidate for W concentration assessment in WEST. Regarding SOL transport, the experimental results suggest a screening effect of the SOL in qualitative agreement with modelling of the W transport by EMC3 EIRENE.

Comparison of scrape-off layer transport in inner and outer wall limited JET plasmas

The JET scrape-off layer has been characterized with a reciprocating probe in inner wall, IW, and outer wall, OW, limited plasmas. Broad SOL profiles are observed for IW limited plasmas with power e-folding length substantially larger (by a factor of ∼5–7.5) than in OW limited plasmas. The properties of the fluctuations in the SOL parameters indicate larger turbulent transport for IW limited plasmas. The striking differences observed between IW and OW limited plasmas on the power e-folding length, parallel flow, turbulent transport as well as the characteristics of the fluctuations support the existence of a poloidally localized region of enhanced radial transport near the outboard midplane. The dependence of the SOL power e-folding length on the main plasma parameters was also investigated for IW limited plasmas and a modest negative dependence on both the plasma current and the line-averaged density found.

3-D simulations of plasma transport in the ring limiter scrape-off layer of tokamak Aditya

Aditya tokamak has a circular poloidal ring limiter at one toroidal location and therefore its contact with plasma produces a complex 3D flow pattern in the SOL plasma region [1]. Presently plasma flow in the SOL region is being measured using Mach probes at a few discrete locations and can be properly understood only with the help of a full 3D simulation taking the toroidal asymmetry introduced by the ring limiter into account. We have therefore carried out plasma transport studies for the Aditya configuration using the 3D Monte-Carlo plasma–neutral code EMC3-Eirene [1,2]. The toroidal asymmetry of the plasma parameters and flows indicate the importance of limiter as a localized sink of plasma. The plasma and neutral particle density show a corresponding variation in the toroidal direction as well as in its radial penetration depth. The plasma configurations are characterized by sharp gradients in the edge and show the presence of extended zones of large parallel flow shear in the SOL and edge regions. The plasma flow patterns show poloidal modulations which correspond to effects of a rational surface interacting with the limiter.

Implementation of a diffusion convection surface evolution model in WallDYN

In thermonuclear fusion experiments with multiple plasma facing materials the formation of mixed materials is inevitable. The formation of these mixed material layers is a dynamic process driven the tight interaction between transport in the plasma scrape off layer and erosion/(re-) deposition at the surface. To track this global material erosion/deposition balance and the resulting formation of mixed material layers the WallDYN code has been developed which couples surface processes and plasma transport. The current surface model in WallDYN cannot fully handle the growth of layers nor does it include diffusion. However at elevated temperatures diffusion is a key process in the formation of mixed materials. To remedy this shortcoming a new surface model has been developed which, for the first time, describes both layer growth/recession and diffusion in a single continuous diffusion/convection equation. The paper will detail the derivation of the new surface model and compare it to TRIDYN calculations.

Modification of scrape-off layer transport and turbulence by non-axisymmetric magnetic perturbations in ASDEX Upgrade

A brief survey of the status of ELM mitigation by non-axisymmetric magnetic perturbations in ASDEX Upgrade is presented. ELM mitigation occurs for a wide range of parameters when the peripheral plasma density is above a density threshold. The focus of this paper is on the influence of magnetic perturbations on scrape-off layer transport and turbulence in L- and H-mode discharges. In low density L-mode strike-line splitting in the divertor as footprint of the 3D magnetic topology with magnetic perturbations is well visible in the heat flux density profiles. At enhanced densities strike-line splitting cannot be observed, transport masks the 3D substructure and the flattening of the density profile around the separatrix causing an enhanced SOL density vanishes. In H-mode strike-line splitting occurs before the ELMs are mitigated. With magnetic perturbation the SOL cross-field transport rises, with ELM mitigation onset intermittency increases.

A new model of the L–H transition in tokamaks**This letter is an intended contribution to the upcoming Nuclear Fusion special issue: H-MODE.

A new model of the L–H transition in tokamaks is proposed, based on the criterion that the transition occurs when plasma turbulence and shear Alfvén waves compete in the vicinity of the last-closed flux surface. The model is used to predict the scaling of the H-mode access power, PL–H, with magnetic and plasma variables. The predictions are in good agreement with the experimental scaling of PL–H with plasma size, density, magnetic field and edge safety factor (plasma current). They are also qualitatively consistent with the dependence of PL–H on ion mass and charge, limiter versus divertor plasmas, single versus double null configurations, the divertor leg length and H–L versus L–H hysteresis. Most notably, the model explains the appearance of the minimum in PL–H with plasma density (in terms of the transition between sheath limited and conduction limited scrape-off layer (SOL) transport) and correctly predicts the scaling of the density minimum with magnetic field and Greenwald number. Finally, the effect of toroidal field reversal is included by making the normalized correlation length of the eddy and maximum safety factor in the edge-SOL layer functions of B × ∇B direction.

Bohm-like Dependence of Transport in Scrape-off Layer Plasmas

Nondiffusive radial transport in scrape-off layer (SoL) is investigated using the two-dimensional SoL interchange turbulence simulation code, which reproduces plasma blobs. From nonlinear simulations, it is found that a mesoscale convective cell emerges from interchange turbulence and propagates in the radial direction. Blob structures are formed autonomously and their ballistic propagation dominates SoL radial transport. A high-density tail in the radial profile is reproduced. This indicates the existence of a 2nd SoL. The effective transport coefficient is analyzed and a Bohm-like dependence is found. It is concluded that the flux-driven turbulence in SoL induces Bohm-like “nondiffusive” transport. The PDF of the turbulence-driven flux is also investigated. It is found that the PDF distribution follows a stretched Gaussian distribution.

Diffusive–convective transition for scrape-off layer transport and the heat-flux width

Transport of plasma from the edge pedestal gradient region into the scrape-off layer (SOL) forms the heat exhaust channel. The properties of this channel are critical for future tokamak devices. The SOL heat-flux width is believed to be set by a competition between classical parallel transport and turbulent cross-field transport. In previous work, (Myra et al 2011 J. Nucl. Mat. 415 S605) focusing on modeling of the heat-flux width in the National Spherical Torus Experiment (NSTX), the possibility of a transition from quasi-diffusive to convective transport in the SOL was noticed. This transition, and the scaling of the heat-flux width, is explored here through additional SOL turbulence simulations using the SOLT code (Russell et al 2009 Phys. Plasmas 16 122304). At the transition, the transport becomes intermittent, and the SOL width is broadened due to blob emission. Critical parameters for the transition are investigated, including the power flux into the SOL, the field line pitch, the connection length and the plasma collisionality. An inverse dependence of the heat-flux width on the poloidal field, also seen routinely in experiments, is noted and explained qualitatively.

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.

Comparison between 2D simulation and the standard 2-point model for scrape-off layer transport including effects of spatially varying transport coefficients

The effect of 2D transport in the tokamak scrape-off layer (SOL) on relating measured divertor heat-flux profiles to midplane plasma profiles is explored with the UEDGE code for a range of transport assumptions. The relationship between the divertor heat-flux profile and the midplane plasma temperature profile as determined from UEDGE is compared to commonly used predictions from 1D and “2-point” analytic relation. Results show that the parametric variation in λq,div with midplane separatrix values follows the conduction-limited 2-point model, though the actual value of q// on a given flux surface is much lower than implied by the midplane Te,sep, and λq,div is ∼ twice as wide as expected from these models. Poloidal variations in χ result in minimal change to radial profiles and probably can’t be distinguished experimentally from cases with uniform χ having the same flux-surface average value.