Edge Transport Barrier Research Articles

Simulation of H-modes discharges in ASDEX-Upgrade and MAST

A new version of the B2SOLPS5.0 transport code, which is free from numerical problems in the barrier region, has been used to simulate H-mode shots from ASDEX-Upgrade and MAST. The radial electric field inside the edge transport barrier and in the pedestal region is close to the neoclassical prediction. The shear of poloidal E → × B → drift at the inner side of the barrier is close to the value before the transition, while inside the barrier it is significantly larger. It is demonstrated that to match the experimental density and temperature radial profiles the drop in the diffusion coefficient within the barrier should be significantly larger than the drop in the electron heat conductivity.

Evidence for Strong Inversed Shear of Toroidal Rotation at the Edge-Transport Barrier in the ASDEX Upgrade

The toroidal rotation of H-mode plasmas in ASDEX Upgrade is studied in the outermost 5 cm of the confined plasma. The projection of the rotation velocity along the line of sight (approximately toroidal) is measured using charge exchange recombination spectroscopy, with a radial resolution of up to 3 mm and a temporal resolution of 1.9 ms. At about 1 cm inside the separatrix the rotation exhibits a local minimum. From there, the rotation in codirection increases towards the plasma center and towards the separatrix. The latter increase is the focus of this work. It is situated in the region of the edge transport barrier and amounts to 10-20 km/s. It is observed for D+, He2+, B5+, and C6+. The described rotation feature at the edge is not visible during an ELM crash and is probably connected to the occurrence of steep gradients in this plasma region.

Compatibility of ITER scenarios with an all-W wall

In 2008, ASDEX Upgrade has started its second experimental campaign with full tungsten coverage of the plasma facing components. In the transition from a partially W-coated device (69% tungsten coverage in 2004/2005) to a full tungsten device (since 2007), post campaign analyses show a reduction by an order of magnitude in both the carbon deposition and deuterium retention for the experimental campaigns. Spectroscopic measurements show that the outer divertor is by far the strongest tungsten source region. However, influxes from the outboard limiters are the most important source for the tungsten content in the plasma. The application of ICRH results in large W influxes due to sputtering from light impurities accelerated by electrical fields at the ICRH antennas. In H-mode discharges, ELMs reduce the inward transport of tungsten in the H-mode edge transport barrier. Central heating provided by neutral beams and the upgraded ECRH systems is used to avoid tungsten accumulation in the core of the plasma. Stationary, ITER relevant H-modes (H98 ∼ 1, βN ∼ 1.6–2), with W concentrations below 3 × 10−5, were routinely achieved. High performance H-modes have been obtained before the first boronization, achieving H98 = 1.1–1.2 and βN up to 2.6 as required for advanced scenarios in ITER. Specific ITER studies performed in 2008 include the demonstration of low voltage plasma start-up with ECRH and heating during the current rise to q95 = 3, to achieve a range of plasma inductance of 0.71–0.97. The new results reported here form the basis of further enhancing the operational space of ASDEX Upgrade with the full tungsten wall, preparing for ITER and the ITER-like wall project in JET.

A new diagnostic of the plasma density profile near the edge transport barrier from measurements of the radial profile of the Hα line intensity

An edge transport barrier is now one of the most important subjects of controlled fusion research. The edge transport barrier is located in the plasma region where hydrogen atoms readily penetrate, so the intensity of the H(alpha) (D(alpha)) line is high enough. A new diagnostic method uses the well-known property of hydrogen atoms that the ratio of the ionization rate S(i) to the excitation rate S(v) for the H(alpha) line is nearly constant over a wide range of plasma temperatures and densities. An expression has been derived that relates the radial profiles of the plasma density and H(alpha) intensity. The use of charge coupled device detectors makes it possible to measure the radial profile of H(alpha) line intensity with a resolution approximately 0.1 cm; a high intensity of the H(alpha) line ensures a high time resolution approximately 1 ms. A high resolution is thus achieved for the density profile calculated from the H(alpha) intensity profile. The method was tested when studying the plasma density profile in the region of edge transport barrier in the L-2M stellarator. It has been shown that the density gradient varies during the barrier formation and that a fine structure of the density profile correlates with a character of the plasma transport near resonance magnetic flux surfaces.

Observation of the high-density edge transport barrier in CHS using beam emission spectroscopy

The formation of an edge transport barrier (ETB) has been observed in the high-density region (ne ∼ 1.0 × 1020 m-3) of the Compact Helical System (CHS). The high-density ETB is observed in so-called ‘reheat mode’ discharge, an improved confinement mode in which the temperature increases due to the change of density profile by controlling gas-puff fueling. In the present study, we investigated the behaviour of the density profile and the density fluctuations in the edge region accompanied by the formation of the high-density ETB using beam emission spectroscopy (BES), and we compared the behaviour with that of the normal ETB transition of CHS. BES has been implemented in CHS to simultaneously measure both local density fluctuations and gradients. It revealed that the density, density gradient, and inverse value of the density scale length increase in the edge region at both the normal ETB transition and the high-density ETB transition. The turbulence of the frequency at less than 20 kHz is suppressed at the formation of the high-density ETB.

Characteristics of high density edge transport barrier with reheat mode on CHS

Edge transport barrier (ETB) formation and a reheat mode have been simultaneously realized on the Compact Helical System (CHS). The new mode is induced by neutral particle reduction in the edge region, which is caused by shutting off fueling after a strong gas-puffing. When both the reheat mode and the ETB are simultaneously realized, the density reduction is suppressed by the ETB in the peripheral region, and the temperature continues to increase by the reheat mode. This mode provides an enhanced confinement in the high density region (n̄e ∼ 1.2 × 1020mr-3) compared to the ETB formation without the reheat mode, because of a large suppression of an anomalous transport, which is confirmed with fluctuation measurements in the edge region.

Particle transfer in edge transport barrier with stochastic magnetic field

Charged particle losses at the plasma edge affected by resonant magnetic perturbations (RMP) are considered by taking into account the electron and ion flows both parallel and perpendicular to perturbed field lines. Calculations are done for H-mode plasmas of low collisionality, i.e., under conditions where significant pump out of particles has been observed in experiments on the DIII-D tokamak [J. Luxon, Nucl. Fusion 42, 614 (2002)] with RMP from the I-coils. It is demonstrated that the perpendicular ion flux, arising by magnetic field stochastization due to the deviation of poloidal rotation from the neoclassical one, is of more importance than the parallel ion flow. With both loss contributions included, computations provide a pump out level in agreement with observations if the screening of RMP by the plasma rotation is taken into account. The impact of possible enhancement in the perpendicular electron transport due to fluctuations observed with RMP in the edge transport barrier is assessed.

Transport barriers in non-axisymmetric magnetic fields

A transport barrier which is characterized by the steeper temperature or/and density gradient has been observed both in the core region (internal transport barrier: ITB) and edge region (edge transport barrier: ETB) in the plasmas with non-axisymmetric magnetic fields compared with the gyro-Bohm type L-mode transport. A reduction of thermal conductivity and particle diffusivity in these plasmas compared with the L-mode levels is observed and the parameter dependence of thermal conductivity and particle diffusivity changes significantly from that in L-mode. A negative or at least a weak dependence of the thermal diffusivity on temperature is observed in the heat transport barrier, while a strong temperature dependence (temperature to the power of 1.5) is normally observed in the L-mode region. In this paper, transport barriers that have been observed in non-axisymmetric magnetic field configuration are discussed from the view point of the parameter dependence of transport coefficients, not the shape of temperature or density profiles. The transition between the L-mode and the improved mode with a transport barrier is also discussed based on the idea of a bifurcation of multi-state transport (discrete transport states with different relations of heat flux to temperature gradient that are self-consistent with the different turbulence states realized).

Particle confinement of pellet-fuelled H-mode plasmas in the Mega Ampere Spherical Tokamak

This paper quantifies the particle confinement of pellet-fuelled plasmas in the Mega Ampere Spherical Tokamak (MAST). The dataset is restricted mostly to neutral beam heated plasmas and to shallow pellets launched from the high field side. It is shown that the pellet deposition can be explained only by invoking the ∇B drift of the pellet ablatant. The pellet creates a zone with positive density gradient and increased temperature gradient. Simulations show that these changes could increase the level of micro-turbulence and thus enhance further the penetration of pellet-deposited particles towards the core. Post-pellet dynamics of the density profile is characterised by the pellet retention time τpel. It is shown that τpel correlates with the status of the edge transport barrier (L-mode or H-mode) and decreases rapidly for pellet deposition radius rpel approaching the plasma edge. For ELMy H-mode and ITER-like pellets, rpel ≈ 0.8a, the pellet retention time is about 20% of the energy confinement time. The fuelling requirement by the pellets for ITER is discussed.

Particle confinement of pellet-fuelled tokamak plasma

This paper quantifies the particle confinement of pellet-fuelled plasmas as measured in the Mega Ampere Spherical Tokamak. The dataset is restricted mostly to neutral beam heated plasmas in H-mode and to shallow pellets launched from the high-field side. It is shown that the pellet deposition can be explained only by invoking the ∇B drift of the pellet ablatant. The pellet creates a zone with positive density gradient and increased temperature gradient. Simulations show that these changes could increase the level of micro-turbulence and thus enhance further the penetration of pellet-deposited particles towards the core. Post-pellet dynamics of the density profile is characterized by the pellet retention time τpel. It is shown that τpel correlates with the status of the edge transport barrier (L-mode or H-mode) and decreases rapidly for pellet deposition radius rpel approaching the plasma edge. For ELMy H-mode and pellet deposition radius of rpel ≈ 0.8a, the pellet retention time is about 20% of the energy confinement time. The fuelling requirement by the pellets for ITER and the Component Test Facility based on the spherical tokamak is discussed.

Paleoclassical transport explains electron transport barriers in RTP and TEXTOR

The recently developed paleoclassical transport model sets the minimum level of electron thermal transport in a tokamak. This transport level has proven to be in good agreement with experimental observations in many cases when fluctuation-induced anomalous transport is small, i.e. in (near-)ohmic plasmas in small to medium size tokamaks, inside internal transport barriers (ITBs) or edge transport barriers (H-mode pedestal). In this paper predictions of the paleoclassical transport model are compared in detail with data from such kinds of discharges: ohmic discharges from the RTP tokamak, EC heated RTP discharges featuring both dynamic and shot-to-shot scans of the ECH power deposition radius and off-axis EC heated discharges from the TEXTOR tokamak.For ohmically heated RTP discharges the Te profiles predicted by the paleoclassical model are in reasonable agreement with the experimental observations, and various parametric dependences are captured satisfactorily.The electron thermal ITBs observed in steady state EC heated RTP discharges and transiently after switch-off of off-axis ECH in TEXTOR are predicted very well by the paleoclassical model.

Toroidally non-uniform formation of the edge-transport-barrier by low-to-high confinement transition on the Compact Helical System

In the Compact Helical System, the radial structure of the edge-transport-barrier (ETB) and the characteristics of electrostatic fluctuations were measured for the first time at three toroidally separated locations (the upper side of the vertically elongated section and the outboard and inboard sides in the horizontally elongated section) by using triple Langmuir probes. Electron density (ne) near the plasma edge increased noticeably across the low-to-high confinement (L–H) transition without a clear electron temperature rise. A rapid vertical expansion and outward shift of the formed particle ETB were observed just after the transition. Radial profiles of ne and radial electric field at the inboard of the torus evolved to a peculiar shape across the transition, suggesting the presence of a sizable magnetic island at the rational surface of ι/2π = 1 (ι/2π: rotational transform). Turbulent particle flux derived experimentally was clearly reduced at the upper and outboard locations just after transition, but enhanced at the inboard location. Total turbulent flux passing through a whole LCFS is inferred to be reduced.

Modelling of pedestal transport during ELM suppression by external magnetic field perturbations

Particle and energy transport in the edge transport barrier is analysed in the presence of magnetic field perturbations from external resonant coils successfully used recently for the mitigation of type I edge localized modes (ELMs). The modification of transport due to charged particle and heat flows along perturbed field lines in a small region near the separatrix, spanning from 2% to 4% of the total poloidal flux, where complete stochastization is provided by the overlap of the main magnetic islands, is taken into account. The observed reduction of the density in plasmas of low collisionality is explained by the generation of charged particle flows along perturbed field lines, the increase in the electron and ion temperatures in the barrier—by the reduction of the perpendicular neoclassical transport with decreasing density and non-locality of parallel heat transport. On the basis of the heat flux limit concept in a deeply collisionless regime, the parallel thermal conductivities are taken to be 17 times smaller for electrons and 7 times smaller for ions than from a standard free-streaming estimate. The model elaborated before is developed further by taking into account the radial variation of the inclination angle of stochastic field lines and convective energy losses including the acceleration of ions by the pressure gradient and ambipolar electric field. It is demonstrated that convection of parallel kinetic energy of ions gives greater losses than parallel thermal conduction in the outer 50% of the stochastic layer and its inclusion improves the agreement with experimental results. This modelling is performed by assuming in agreement with observations that the influx of recycling neutrals through the separatrix is not reduced with I-coils compared with its level between ELMs before the mitigation stage. By trying to match experimental profiles with this influx decrease, some enhanced thermal losses of another nature than that considered here are needed in order to mitigate the drop in the perpendicular thermal conductivities for the assumed density scaling. The impact of the neutral particle influx increase by gas puffing applied in order to restore the plasma density is investigated.

Effects of an externally produced static magnetic island on edge MHD modes in the Large Helical Device

In high beta plasmas and plasmas with an edge transport barrier on the Large Helical Device (LHD), pressure-driven edge MHD modes such as m/n = 1/1, 3/4, 2/3, 3/5 and 1/2 (m, n: poloidal and toroidal mode numbers) are excited in the magnetic hill region of the plasma edge. When a sizable static magnetic island with the mode number of m/n = 1/1 was externally generated at the ι/2π = 1 surface near the plasma edge using the perturbation field coils called the local island divertor coil, the edge pressure gradient was modified in the toroidal and poloidal directions by the formation of an X-point and an O-point of the island and the achieved volume-averaged beta was appreciably reduced. This modification changed the excited mode numbers of these edge MHD modes and their radial mode structures at the inboard and outboard sides of LHD. In particular, these edge MHD fluctuations observed by soft x-ray detectors exhibited obvious enhancement on the outboard side (larger major radius) and reduction on the inboard side, regardless of the O-point/X-point position.

Ideal MHD stability of double transport barrier plasmas in DIII-D

The ideal MHD stability for double transport barrier (DTB or DB) plasmas with varying edge and internal barrier width and height was investigated, using the ideal MHD stability code GATO. A moderate ratio of edge transport barriers (ETB) height to internal transport barriers (ITBs) height is found to be beneficial to MHD stability and the βN is limited by global low n instabilities. For moderate ITB width DB plasmas, if the ETB is weak, the stability is limited by n = 1 (n is the toroidal mode number) global mode; whereas if the ETB is strong it is limited by intermediate-n edge peeling–ballooning modes. Broadening the ITB can improve stability if the ITB half width wi ≲ 0.3. For very broad ITB width plasmas the stability is limited by stability to a low n (n > 1) global mode.

Active control of the H-mode transition on MAST

In the presence of strong α-particle heating, active control of the formation or the properties of the edge transport barrier (ETB) might be necessary to meet the confinement and exhaust needs of burning fusion plasmas. In particular, the sensitive dependences on the magnetic configuration and divertor geometry are suitable candidates for controlling the ETB. Using both methods on the Mega Ampére Spherical Tokamak (MAST) short L-mode periods of a few milliseconds duration have been introduced into strongly heated H-modes.In double null configuration a vertical plasma shift by ΔZ < 2.5 cm opposite to the ion ∇B-drift direction led to L-mode periods of 15 ms < ΔtL < 20 ms length. The peak power load to the targets at the H–L transition was a factor of 2 below that measured during edge localized modes (ELMs). At the H–L transition a rapid change in toroidal He II velocity indicating a fast collapse of the radial electric field within was observed spanning a wider radial region than the collapse during ELMs. In single null configuration the change in the connection length of the outer divertor leg by 2 m < ΔLc < 4 m led to L-mode periods of 2 ms < ΔtL < 5 ms duration. In both cases fluid modelling indicates a more negative Er and consequently an increase in the E × B shear in the L-mode just inside the last closed flux surface due to the changes to the magnetic geometry, which lead to improved H-mode access.The active control techniques demonstrated here may help to develop suitable scenarios for future devices and improve the understanding of the physics determining the ETB.

Edge profile stiffness and insensitivity of the density pedestal to neutral fuelling in Alcator C-Mod edge transport barriers

Mechanisms determining the structure of edge temperature and density pedestals, which are associated with edge transport barrier (ETB) formation in tokamaks, are investigated on Alcator C-Mod. Experiments suggest a strong role for critical gradient behaviour in setting profile characteristics of edge plasma. The maximum pressure gradient scales as the square of plasma current in both H-modes without edge-localized modes, and in the near scrape-off layer in Ohmic discharges. In either case, the pressure gradient obtained, normalized to the square of plasma current, is a function of local collisionality, hinting that common physics may contribute to setting profile gradients in both confinement regimes. Varying the neutral fuelling source has little effect on density gradient scale lengths in the ETB and a relatively weak impact on the height of the density pedestal, even during aggressive deuterium puffing. Strong screening of neutrals in the ETB are observed, creating a challenge for fuelling H-modes in C-Mod beyond their ‘natural’ density. A simple pedestal fuelling model does not reproduce the typically clamped density gradients seen in experiment during H-mode puffing. These results suggest that a simple diffusive model for plasma transport is deficient, and that a critical gradient assumption for transport may be essential for pedestal modelling.

Characteristics of the H-mode pedestal in improved confinement scenarios in ASDEX Upgrade, DIII-D, JET and JT-60U

Pedestal and global plasma parameters are compared in conventional ELMy H-modes and improved confinement discharges from ASDEX Upgrade (AUG), DIII-D, JET and JT-60U with varying net input power. Both electron and ion pedestal pressures are studied. The pedestal top pressure pPED increases moderately with power in all tokamaks, in broad agreement with the power dependence of the IPB98(y, 2) scaling. Higher pedestal pressures are observed in AUG improved H-modes and in JT-60U high βpol discharges at q95 ∼ 6.5 and high triangularity. For all machines and all scenarios a robust correlation between the total and the pedestal thermal stored energy is observed, with the ratio of the two varying between ∼0.3 and 0.5. However the relative importance of pedestal and core confinement varies from regime to regime. In AUG the confinement improvement with respect to the IPB98(y, 2) scaling is due to improved pedestal confinement in improved H-modes with early heating and to both improved pedestal and core confinement in improved H-modes with late heating. In DIII-D hybrid discharges the increase in confinement factor compared with conventional H-modes is due to improved confinement in the plasma core. JT-60U reversed shear H-modes have strong internal transport barriers and thus improved core performance. In all four tokamaks improved edge stability is correlated with increasing total βpol and H98(y,2) increases with pedestal βpol. The analysed multimachine data set supports a scaling expression for the pedestal stored energy derived under the assumption that the dominant loss term for the pedestal is by thermal conduction in the edge transport barrier region.

H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod

This paper reports on studies of the edge transport barrier and transition threshold of the high confinement (H) mode of operation on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)], over a wide range of toroidal field (2.6–7.86T) and plasma current (0.4–1.7MA). The H-mode power threshold and edge temperature at the transition increase with field. Barrier widths, pressure limits, and confinement are nearly independent of field at constant current, but the operational space at high B shifts toward higher temperature and lower density and collisionality. Experiments with reversed field and current show that scrape-off-layer flows in the high-field side depend primarily on configuration. In configurations with the B×∇B drift away from the active X-point, these flows lead to more countercurrent core rotation, which apparently contributes to higher H-mode thresholds. In the unfavorable case, edge temperature thresholds are higher, and slow evolution of profiles indicates a reduction in thermal transport prior to the transition in particle confinement. Pedestal temperatures in this case are also higher than in the favorable configuration. Both high-field and reversed-field results suggest that parameters at the L-H transition are influencing the evolution and parameters of the H-mode pedestal.

H-Mode Pedestal and L-H Transition Studies on Alcator C-Mod

H-mode research on Alcator C-Mod is described, with a focus on the edge transport barrier (ETB). ETB pedestals are characterized using several diagnostics, leading to a thorough description of profile structure in H-mode. L-H transition criteria are discussed, along with the fast evolution of the pedestal following the L-H transition. H-mode regimes are described in terms of their edge transport characteristics and the local edge parameters favoring each. Empirical scalings of the pedestal with operational parameters are found, helping to illuminate physics governing the pedestal structure, and the relationship between edge transport and global confinement is discussed. Dimensionless comparisons between discharges on different tokamaks are discussed. Finally, ongoing work and directions for the future are described.