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Fully suppressing edge-localized modes (ELMs), e.g., with resonant magnetic perturbations (RMPs), is essential to reach and sustain high-performance steady-state H-mode plasmas because large ELMs can significantly reduce the lifetime of divertor components in future tokamak reactors. RMP-driven ELM suppression in KSTAR has been modeled by coupling the neoclassical transport code PENTRC to the nonlinear 3D MHD code JOREK. We have found that the radial transport from the combined effects of the kink-peeling, tearing response, and neoclassical toroidal viscosity can explain the pedestal degradation observed in experiments. In addition, it has been found that the RMP response can increase the inter-ELM heat flux on the lower outer divertor by redistributing the heat transport between the divertor plates. In addition to the degraded pedestal, ELM suppression is also attributable to the RMP-induced mode interactions. While the linear stability of peeling-ballooning mode (PBMs) improves owing to the degraded pedestal, the PBM and RMP interaction increases the spectral transfer between edge harmonics, preventing catastrophic growth and the crash of unstable modes. Here, it turns out that the magnetic islands near the pedestal top can play a vital role in mediating the mode interactions.

Edge localized modes (ELMs) suppression by resonant magnetic perturbations (RMPs) was studied with the non-linear magneto-hydro-dynamic (MHD) code JOREK for the ITER H-mode scenarios at 15 MA, 12.5 MA, 10 MA/5.3 T. The main aim of this work was to demonstrate that ELMs can be suppressed by RMPs while the divertor 3D footprints of heat and particle fluxes remain within divertor material limits. The unstable peeling–ballooning modes responsible for ELMs without RMPs were modelled first for each scenario using numerically accessible parameters for ITER. Then the stabilization of ELMs by RMPs was modelled with the same parameters. RMP spectra, optimized by the linear MHD MARS-F code, with main toroidal harmonics N = 2, N = 3, N = 4 have been used as boundary conditions of the computational domain of JOREK, including realistic RMP coils, main plasma, scrape off layer (SOL) divertor and realistic first wall. The model includes all relevant plasma flows: toroidal rotation, two fluid diamagnetic effects and neoclassical poloidal friction. With RMPs, the main toroidal harmonic and the non-linearly coupled harmonics remain dominant at the plasma edge, producing saturated modes and a continuous MHD turbulent transport thereby avoiding ELM crashes in all scenarios considered here. The threshold for ELM suppression was found at a maximum RMP coils current of 45 kAt–60 kAt compared to the coils maximum capability of 90 kAt. In the high beta poloidal steady-state 10 MA/5.3 T scenario, a rotating QH-mode without ELMs was observed even without RMPs. In this scenario with RMPs N = 3, N = 4 at 20 kAt maximum current in RMP coils, similar QH-mode behaviour was observed however with dominant edge harmonic corresponding to the main toroidal number of RMPs. The present MHD modelling was limited in time by few tens of ms after RMPs were switched on until the magnetic energy of the modes saturates. As a consequence the thermal energy was still evolving on this time scale, far from the ITER confinement time scale and hence only the form of 3D footprints on the divertor targets can be indicated within this set-up. Also note, that the divertor physics was missing in this model, so realistic values of fluxes are out of reach in this modelling. However the stationary 3D divertor and particle fluxes could be simply extrapolated from these results to the stationary situation considering that a large power fraction should be radiated in the core and SOL and only about 50 MW power is going to the divertor, which is an arbitrary, but reasonable number used here. The 3D footprints with RMPs show the characteristic splitting with the main RMP toroidal symmetry. The maximum radial extension of the footprints typically was ∼20 cm in inner divertor and ∼40 cm in outer divertor with stationary heat fluxes decreasing further out from the initial strike point from ∼5 MW m−2 to ∼1 MW m−2 assuming a total power in the divertor and walls is 50 MW. The heat fluxes remain within the divertor target and baffle areas, however with rather small margin in the outer divertor which could be an issue for the first wall especially in transient regimes when part of the plasma thermal energy is released due to switching on the RMP coils. This fact should be considered when RMPs are applied with a more favorable application before or soon after the L–H transition, although optimization is required to avoid increasing the L–H power threshold with RMPs.

Both linear and quasi-linear aspects of the plasma response to the resonant magnetic perturbation (RMP) field are numerically investigated for various H-mode scenarios in ITER, covering the pre-fusion power operation and the fusion power operation phases. Linear response computations for eight ITER scenarios, with varying plasma current and toroidal magnetic field, reveal that the best coil current phasing for controlling the type-I edge localized modes (ELMs) scales roughly linearly with the edge safety factor. The coil phasing is defined as the relative toroidal phase of the coil currents between different rows, for a given toroidal harmonic. Quasi-linear initial value simulation, which is the focus of the present study, shows that application of the n = 3 (n is the toroidal mode number) RMP field has a minimum side effect on the plasma core momentum confinement but potentially a large effect on the global particle transport. Generally, the RMP field with the best (worst) coil phasing for ELM control produces the strongest (weakest) effect on the plasma edge flow and the overall density. This robustly holds for all eight ITER scenarios. Consequently, in order to minimize the RMP induced side effects while achieving ELM control (suppression) in ITER, a compromise is necessary in choosing the coil current configuration.

JOREK is a massively parallel fully implicit non-linear extended MHD code for realistic tokamak X-point plasmas. It has become a widely used versatile code for studying large-scale plasma instabilities and their control developed in an international community. This article gives a comprehensive overview of the physics models implemented, numerical methods applied for solving the equations and physics studies performed with the code. A dedicated section highlights some of the verification work done for the code. A hierarchy of different physics models is available including a free boundary and resistive wall extension and hybrid kinetic-fluid models. The code allows for flux-surface aligned iso-parametric finite element grids in single and double X-point plasmas which can be extended to the true physical walls and uses a robust fully implicit time stepping. Particular focus is laid on plasma edge and scrape-off layer (SOL) physics as well as disruption related phenomena. Among the key results obtained with JOREK regarding plasma edge and SOL, are deep insights into the dynamics of edge localized modes (ELMs), ELM cycles, and ELM control by resonant magnetic perturbations, pellet injection, as well as by vertical magnetic kicks. Also ELM free regimes, detachment physics, the generation and transport of impurities during an ELM, and electrostatic turbulence in the pedestal region are investigated. Regarding disruptions, the focus is on the dynamics of the thermal quench and current quench triggered by massive gas injection (MGI) and shattered pellet injection (SPI), runaway electron (RE) dynamics as well as the RE interaction with MHD modes, and vertical displacement events (VDEs). Also the seeding and suppression of tearing modes (TMs), the dynamics of naturally occurring thermal quenches triggered by locked modes, and radiative collapses are being studied., Comment: Comprehensive review published as special topic article in Nuclear Fusion

In both JET and MAST, the Error Field Correction Coils (EFCCs) have been used recently in order to attempt to control Type I Edge Localised Modes (ELMs), which represent a major threat to the lifetime of plasma facing components in ITER. Using vacuum magnetic modelling it is suggested that the ELM mitigation observed at JET could be related to the stochastization of the magnetic field at the edge. Indeed, the onset of ELM mitigation is found to be correlated with a certain level of the Chirikov parameter profile. Initial MAST results are presented which show an effect of EFCCs on the ELMs, again compatible with edge stochastization according to the modelling. New coils dedicated to ELM control are ready for use on MAST this year and are presented here briefly.

The suppression of ELMs with a resonant magnetic perturbation (RMP), as demonstrated in DIIID experiments [T. Evans, et al., Suppression of large edge localized modes with edge resonant magnetic fields in high confinement DIII-D plasmas, Nucl. Fusion 45 (2005) 595–607 [1] ], and widely published [K.H. Burrell, et al., Plasma Phys. Control., Fusion 47 (2005) B37–B52 [2] ; M. Becoulet, et al., Edge localized modes control by stochastic magnetic field, Nucl. Fusion 45 (2005) 1284–1292 [3] ], is an attractive solution to the problem of divertor target erosion in ITER. Ideally, the RMP should be strong at the plasma edge and weak in the core, which requires the installation of the RMP coils as close as possible to the plasma. In both ITER and JET, an installation of the RMP coils within the vacuum vessel would subject the coils to neutron irradiation, tritium contamination and all the problems of furnishing cooling and electrical services, across many interfaces. So for JET, it has been proposed to install an “ITER prototype” RMP system located ex-vessel.

A number of designs for external or in-vessel coils generating Resonant Magnetic Perturbations (RMPs) for type I ELMs control in ITER are analyzed. The RMPs generated by the I-coils in the successful DIII-D ELMs control experiments are taken as a reference. The three ITER characteristic scenarios (H-mode, hybrid and steady-state) are studied. In a second part of the paper, the first results of a self-consistent non-linear MHD modelling of the plasma response to the RMPs with the JOREK code, for a DIII-D case, are presented.

This report summarizes Type I edge localized mode (ELM) dynamics measurements from a number of tokamaks, including ASDEX-Upgrade, DIII-D, JET, JT-60U and MAST, with the goal of providing guidance and insight for the development of ELM simulation and modelling. Several transport mechanisms are conjectured to be responsible for ELM transport, including convective transport due to filamentary structures ejected from the pedestal, parallel transport due to edge ergodization or magnetic reconnection and turbulent transport driven by the high edge gradients when the radial electric field shear is suppressed. The experimental observations are assessed for their validation, or conflict, with these ELM transport conjectures.

The development of scenarios characterized by H-mode confinement and internal transport barriers (ITBs) in high triangularity, δ ~ 0.4–0.5, discharges is of particular interest for ITER advanced tokamak operation. Previous JET experiments have shown that high triangularity favours H-modes which are ELM-free or develop type I edge localized mode (ELM) activity, which inhibits long lasting ITBs. The recent experiments reported here concentrate on integrated optimization of edge and core conditions. The stability of the edge pedestal was controlled using gas injection, deuterium or light impurities, and plasma current ramps. Both methods yield more ITB-friendly edge pedestal conditions, varying from small type I to type III ELMs and, in extreme cases, resulting in L-mode. In parallel, the conditions for triggering and sustaining ITBs encompassing a large proportion of the plasma volume (outer ITBs) were optimized, as opposed to less performing ITBs located closer to the plasma centre (inner ITB). These plasmas have deeply reversed target current profiles with qmin ~ 3 and a narrow inner ITB, located typically at a small normalized radius ρ < 0.5 and close to the reversed shear region, is routinely observed. Large radius outer ITBs are only triggered at an input power in excess of 20 MW, but they do not usually survive the transition into H-mode. The best results, in terms of sustained high confinement, have been obtained with neon injection; an outer ITB is triggered during the phase with L-mode edge and survives into H-mode for about 2 s, corresponding to ~10 times the global energy confinement time τE, at q95 = 7.5, H89βN ~ 3.5–4 and ~60% of the Greenwald density limit. In summary, a high triangularity scenario has been developed, which combines the desirable characteristics of controlled edge, long lasting wide ITBs and high performance at density higher than the low triangularity JET scenarios.

This paper describes a new approach for modelling the pedestal energy transport in the presence of a small radial magnetic perturbation. The cases of a ballooning instability leading to Type I edge localized modes (ELMs) and a magnetic perturbation generated by external coils are treated. The model for Type I ELMs is based on the linear ideal MHD code MISHKA coupled with the non-linear energy transport code TELM in a realistic tokamak geometry. The main mechanism of the increased transport through the external transport barrier in this model of ELMs is due to the appearance of a radial velocity and a radial magnetic field perturbation due to the MHD mode. Both lead to additional transport perpendicular to the magnetic surface and hence to a relaxation of the pressure profile in the unstable zone. The typical Type I ELM time-cycle was reproduced numerically including the destabilization of the ballooning modes leading to the fast (250 μ s) collapse of the pedestal pressure followed by the edge pressure profile re-building on a diffusive time scale. A possible mechanism for the control of Type I ELMs using a stochastic plasma boundary created by external coils is modelled in this paper using data on ELM suppression by I-coils from the DIII-D experiment. In the stochastic layer the transverse transport is effectively increased by diffusion of the magnetic field lines. The modelling results demonstrate the possibility of decreasing the edge pressure gradient to a value that is just below the ideal ballooning limit, leading to a high confinement regime without Type I ELMs.

Large sub-millisecond heat pulses due to Type-I edge localized modes (ELMs) have been eliminated reproducibly in DIII-D for periods approaching nine energy confinement times (tau(E)) with small dc currents driven in a simple magnetic perturbation coil. The current required to eliminate all but a few isolated Type-l ELM impulses during a coil pulse is less than 0.4% of plasma current. Based on magnetic field line modelling, the perturbation fields resonate with plasma flux surfaces across most of the pedestal region (0.9 4-6 tau(E)) have been reproduced numerous times, on multiple experimental run days in high and intermediate triangularity plasmas, including cases matching the baseline ITER scenario 2 flux surface shape. In low triangularity, lower single null plasmas, with collisionalities near that expected in ITER, Type-l ELMs are replaced by small amplitude, high frequency Type-II-like ELMs and are often accompanied by one or more ELM-free periods approaching 1-2 tau(E). Large Type-I ELM impulses represent a severe constraint on the survivability of the divertor target plates in future burning plasma devices. Results presented in this paper demonstrate that non-axisymmetric edge magnetic perturbations provide a very attractive development path for active ELM control in future tokamaks such as ITER.

The paper reviews recent theoretical and experimental results focussing on the identification of the key factors controlling ELM energy and particle losses both in natural ELMs and in the presence of external controlling mechanisms. Present experiment and theory pointed out the benefit of the high plasma shaping, high q(95) and high pedestal density in reducing the ELM affected area and conductive energy losses in Type I ELMs. Small benign ELMs regimes in present machines (EDA, HRS, Type II, Grassy, QH, Type III in impurity seeded discharges at high delta) and their relevance for ITER are reviewed. Recent studies of active control of ELMs using stochastic boundaries, small pellets and edge current generation are presented. (c) 2004 Elsevier B.V. All rights reserved.

This paper presents the experimental characterization of pedestal parameters, edge localized mode (ELM) energy, and particle losses from the main plasma and the corresponding ELM energy fluxes on plasma facing components for a series of dedicated experiments in the Joint European Torus (JET). From these experiments, it is demonstrated that the simple hypothesis relating the peeling-ballooning linear instability to ELM energy losses is not valid. Contrary to previous observations at lower triangularities, small energy losses at low collisionality have been obtained in regimes at high plasma triangularity and q95∼4.5, indicating that the edge plasma magnetohydrodynamic stability is linked with the transport mechanisms that lead to the loss of energy by conduction during type I ELMs. Measurements of the ELM energy fluxes on the divertor target show that their time scale is linked to the ion transport along the field and the formation of a high energy sheath, in agreement with kinetic modeling of ELMs. Higher...

Mixed type I-II ELMy H-mode, a mode operation with small, frequent type II edge localized modes (ELMs) interrupted by occasional large type I ELMs, has been observed in various experimental situations. This paper combines two simple models for type I and type II ELMs, respectively (used e.g. in Lonnroth et al 2003 Plasma Phys. Control. Fusion 45 1689) into an improved scheme for modelling of mixed type I-II ELMy H-mode, which has been implemented in the 1.5D core transport code JETTO together with simple schemes for modelling of pure type I and type II ELMy H-modes based on the same ideas. In the ELM modelling, transport during the ELMs is enhanced by edge-localized radially Gaussian-shaped perturbations to the transport coefficients. Type I and type II ELMs are represented by perturbations with different widths and amplitudes and controlled by different stability limits derived from magnetohydrodynamic (MHD) stability analysis. Some justification from theory and numerical analysis is given for the representation of each ELM type. Predictive transport simulations with JETTO demonstrate that the experimental dynamics of mixed type I-II ELMy H-mode can be qualitatively reproduced using the present model. For completeness, the modelling of mixed type I-II ELMy H-mode is compared with reference simulations of pure type I and pure type II ELMy H-mode and the differences, e.g. in confinement are explained. In addition, this paper presents the results of MHD stability analysis of a number of situations experimentally found to be favourable for the occurrence of type II ELMs, namely situations with strong external neutral gas puffing, a quasi-double-null magnetic configuration, high poloidal β (ratio of the total pressure to the kinetic pressure) and combinations of high edge safety factor q95 and high triangularity δ. The results of the analysis of the given scenarios are such that the model used in this paper can explain why these situations can be favourable for mixed type I-II or pure type II ELMy H-mode.

Recent progress in experimental and theoretical studies of edge localized mode (ELM) physics is reviewed for the reactor relevant plasma regimes, namely the high confinement regimes, that is, H-modes and advanced scenarios.Theoretical approaches to ELM physics, from a linear ideal magnetohydrodynamic (MHD) stability analysis to non-linear transport models with ELMs are discussed with respect to experimental observations, in particular the fast collapse of pedestal pressure profiles, magnetic measurements and scrape-off layer transport during ELMs.High confinement regimes with different types of ELMs are addressed in this paper in the context of development of operational scenarios for ITER. The key parameters that have been identified at present to reduce the energy losses in Type I ELMs are operation at high density, high edge magnetic shear and high triangularity. However, according to the present experimental scaling for the energy losses in Type I ELMs, the extrapolation of such regimes for ITER leads to unacceptably large heat loads on the divertor target plates exceeding the material limits. High confinement H-mode scenarios at high triangularity and high density with small ELMs (Type II), mixed regimes (Type II and Type I) and combined advanced regimes at high βp are discussed for present-day tokamaks. The optimum combination of high confinement and small MHD activity at the edge in Type II ELM scenarios is of interest to ITER. However, to date, these regimes have been achieved in a rather narrow operational window and far from ITER parameters in terms of collisionality, edge safety factor and βp.The compatibility of the alternative internal transport barrier (ITB) scenario with edge pedestal formation and ELMs is also addressed. Edge physics issues related to the possible combination of small benign ELMs (Type III, Type II ELMs, quiescent double barrier) and high performance ITBs are discussed for present-day experiments (JET, JT-60U, DIII-D) in terms of their relevance for ITER. Successful plasma edge control, at high triangularity (~0.5) and high density (~0.7nGR), in ITB scenarios in JET is reported.Active control of ELMs by edge current, pellet injection, impurities and external magnetic perturbations creating an ergodic zone localized at the separatrix are discussed for present-day experiments and from the perspective of future reactors.

Analysis of Type I ELMs from ongoing experiments shows that ELM energy losses are correlated with the density and temperature of the pedestal plasma before the ELM crash. The Type I ELM plasma energy loss normalized to the pedestal energy is found to correlate across experiments with the collisionality of the pedestal plasma (ν*ped), decreasing with increasing ν*ped. Other parameters affect the ELM size, such as the edge magnetic shear, etc, which influence the plasma volume affected by the ELMs. ELM particle losses are influenced by this ELM affected volume and are weakly dependent on other pedestal plasma parameters. In JET and DIII-D, under some conditions, ELMs can be observed (`minimum' Type I ELMs with energy losses acceptable for ITER), that do not affect the plasma temperature. The duration of the divertor ELM power pulse is correlated with the typical ion transport time from the pedestal to the divertor target (τ||Front = 2πRq95/cs,ped) and not with the duration of the ELM-associated MHD activity. Similarly, the timescale of ELM particle fluxes is also determined by τ||Front. The extrapolation of the present experimental results to ITER is summarized.

Edge plasma parameters influence plasma performance in many different ways (profile stiffness is probably one of the best known examples). In the ELMy H-mode plasma, a thin region with improved transport characteristics (the edge transport barrier) links the core and the scrape-off layer. There is a strong coupling between these three areas, so that even a modest variation of plasma parameters in one region can lead to a dramatic change in the overall plasma performance. A systematic MHD stability analysis and self-consistent integrated predictive modeling of a series of JET ELMy H-mode plasmas, including scans in gas fueling and triangularity, are presented. The main conclusion is that plasma performance indeed sensitively depends on the edge plasma parameters, which should be modeled in a self-consistent way.

The 9th EFPW took place in December 2001 at Saariselka in Finland, hosted by the Technical Research Centre of Finland (VTT) and the Helsinki University, and sponsored by the European Commission. Within an overall theme of `transient events, their mitigation and their implications for plasma facing components in ITER', four topics of importance to the future development of magnetically confined fusion were discussed in detail. In addition, the key issues for the ITER design which are associated with transient events and a review of the JET scientific and technical programme under EFDA were presented. The main issues discussed and the areas identified as requiring further study are summarized here.

Analysis of Type I ELMs from present experiments shows that ELM energy losses decrease with increasing pedestal plasma collisionality ( ν ∗ ped ) and/or increasing τ Front ∥ , where ( τ ∥ Front =2π Rq 95 / c s ,ped ) is the typical ion transport time from the pedestal to the divertor target. ν ∗ ped and τ Front ∥ are not the only parameters that affect the ELMs, also the edge magnetic shear influences the plasma volume affected by the ELMs. ELM particle losses are influenced by this ELM affected volume and are weakly dependent on other pedestal plasma parameters. ‘Minimum’ Type I ELMs, with energy losses acceptable for ITER, where there is no change in the plasma temperature profile during the ELM, are observed for some conditions in JET and DIII-D. The duration of the divertor ELM power pulse is well correlated with τ Front ∥ and not with the duration of the ELM-associated MHD activity. Similarly, the time scale of ELM particle fluxes is also determined by τ Front ∥ . The extrapolation of present experimental results to ITER is summarised.

Numerical analysis of the images in visible light from the JET tangential camera show that the edge localised mode (ELM) events are characterised by impacts on the low-field side components. The increase of emission is not restricted to the components closest to the plasma. One finds also that the deposition on the low-field side components does not exhibit any poloidal or toroidal symmetry and varies from ELM to ELM. Conversely the increase of emission on the divertor baffles, or the top protection tiles, is close to axisymmetric.

This paper documents critical issues in internal transport barrier (ITB) plasmas in JET, namely the transition from type III to type I edge localized modes (ELMs), and the subsequent impact of large amplitude ELMs on the ITB. Benign type III ELMs are observed in ITB plasmas at input powers much larger (up to a factor 3) than the empirical threshold for type III/I transition derived from standard H-modes. Various measurements indirectly suggest a larger fraction of plasma current at the edge of ITB plasmas. Experimental results look consistent with a type III ELM regime controlled by a large fraction of edge current. Especially, the transition to type I ELMs does not occur for a broad current profile (li≈0.78) characterized by low edge magnetic shear (s95≈2.6), and a back transition from type I to type III has been found to well correlate with an increase of the edge current. When large ELMs occur, strong perturbations δTe on electron temperature are generated, and propagate inwards on a ballistic timescales, at vburst≈160 m s−1. It is of the order of one third (respectively one ninth) of the curvature (respectively diamagnetic) drift. This propagation looks reminiscent of non-local transport experiments. The perturbation induced by large ELMs can reach the ITB. In this case, δTe increases in the vicinity of the ITB before being strongly damped further inside. Such ELMs also lead to a transient increase of Te gradient at the ITB, which then moves inwards on a diffusive timescale (χ≈3×10−2 m2 s−1) while degrading.

Recent experiments on the Type I ELMy H-mode regime performed at JET with improved diagnostics have expanded the range of parameters for the study of Type I ELM energy and particle losses. Deviations from the standard behaviour of such losses in some areas of the Type I ELMy H-mode operating space have revealed that the ELM losses are correlated with the parameters (density and temperature) of the pedestal plasma before the ELM crash, while other global ELM characteristics (such as ELM frequency) are a consequence of the ELM-driven energy and particle flux and of the in-between ELM energy and particle confinement. The relative Type I ELM plasma energy loss (to the pedestal energy) is found to correlate well with the collisionality of the pedestal plasma, showing a weak dependence on the method used to achieve those pedestal plasma parameters: plasma shaping, heating, pellet injection and impurity seeding. Effects of edge plasma collisionality and transport along the magnetic field on the Type I ELM particle and energy fluxes onto the divertor target have also been observed. Two possible physical mechanisms that may give rise to the observed collisionality dependence of ELM energy losses are proposed and their consistency with the experimental measurements investigated: collisionality dependence of the edge bootstrap current with its associated influence on the ELM MHD origin and the limitation of the ELM energy loss by the impedance of the divertor target sheath to energy flow during the ELM event.

This paper discusses how the proximity to the L-H threshold affects the confinement of ELMy H-modes at high density. The largest reduction in confinement at high density is observed at the transition from the Type I to the Type III ELMy regime. At medium plasma triangularity, δ0.3 (where δ is the average triangularity at the separatrix), JET experiments show that by increasing the margin above the L-H threshold power and maintaining the edge temperature above the critical temperature for the transition to Type III ELMs, it is possible to avoid the degradation of the pedestal pressure with density, normally observed at lower power. As a result, the range of achievable densities (both in the core and in the pedestal) is increased. At high power above the L-H threshold power the core density was equal to the Greenwald limit with H970.9. There is evidence that a mixed regime of Type I and Type II ELMs has been obtained at this intermediate triangularity, possibly as a result of this increase in density. At higher triangularity, δ0.5, the power required to achieve similar results is lower.

A spectroscopic study of VUV emission from injected and intrinsic low-Z impurities has been carried out for Tore Supra ergodic divertor (ED) plasmas. Analysis of plasmas in which a nearby limiter effectively provides a spatially localized source of recycled impurities provides information illuminating the dynamical processes of impurity penetration in ED plasmas. The profile evolution behaviour is found to be consistent with an interpretation which identifies both ED-influenced edge confinement and scrape-off layer and edge phenomena provided by the localized source. Preferentially increased edge impurity transport is a beneficial aspect of the ED.

The two-dimensional (2D) edge plasma density distribution in the scrape off layer magnetically connected to the ion cyclotron resonance heating (ICRH) antennae is studied experimentally and numerically in the tokamak Tore Supra. A local density decrease in front of the loaded ICRH antenna (“pump-out” effect) is demonstrated by Langmuir probe measurements in a low recycling regime. An up–down asymmetry in the heat-flux and in the antenna erosion is also observed, and is associated with poloidal variations of the local density. These density redistributions are ascribed to an E×B convection process linked with rf sheaths. To assess this interpretation, the 2D transport code CELLS for modeling the density distribution near an ICRH antenna was developed and described in this paper. The code takes into account perpendicular diffusion, parallel transport and convection in rf-sheath-driven potentials. The strong density differences obtained in simulations reproduce up–down asymmetries of the heat fluxes.

Recent experimental results and plasma edge stability studies of ELMs are presented for both H-mode and optimized shear (OS) plasmas in JET. High triangularity (δ~0.5) ELMy \mbox{H-mode} experiments in JET achieved simultaneously high confinement (HIPB(y,2) = 1) and high density (~0.9 nGR). At high pedestal density (~1.1nGR), an anomalous decrease of type I ELMs frequency and energy losses per ELM was observed. A specific for high triangularity and high density (δ~0.5 and ne~1.1nGR) MHD activity with broad-band frequency spectrum (

The role of high-energy charge exchange (CX) particles during radio-frequency (RF) heating is examined in an attempt to develop a predictive model for wall saturation and desaturation processes. Tore Supra discharges in which an apparent saturated wall, previously obtained through a succession of ohmic discharges, was transformed into a pumping wall with the application of RF heating are analyzed. Direct measurements show the increased mean energy of the CX spectrum upon application of RF heating. This leads to deeper CX neutrals penetration reaching the remaining a:C–D unsaturated layer and restoring the wall pumping efficiency. The conversion from a saturated to a pumping state with the application of RF heating, and the time scales for saturation during the ohmic and the heating phases, are reasonably well explained by several existing models. The deeper particle wall penetration observed during RF plasma experiments points to a potential mechanism which can increase tritium trapping in a fusion reactor.

Experimental evidence of the location of the ergodic divertor separatrix is shown to agree with the predicted value given by codes. Variation of this position modifies the divertor tightness, defined as the ratio of the divertor to core density. This effect is governed by laminar transport, i.e., transport proportional to the magnitude of the perturbation. Operation with feedback control of the divertor temperature allows one to optimise the choice of injected impurity species. At 10 eV divertor temperature, nitrogen is shown to lead to the largest decrease in energy flux to the divertor at lowest contribution to Zeff. Parallel energy fluxes as low as 2 MW m −2 are thus achieved on the target plates. For this impurity, radiation is localised in the divertor volume thus leading to radiation compression close to 10. The ergodic divertor appears as a powerful tool to control plasma–wall interaction with no loss of core confinement or plasma current.

The control of particle and heat fluxes at the plasma edge is a critical issue on the path to achieving fusion in a magnetic confinement device. Over the past 10 years, the ergodic divertor was successfully operated on the Tore Supra tokamak to fulfil this task. In ergodic divertor discharges, a magnetic perturbation, superimposed upon the equilibrium magnetic field, modifies transport properties by creating open field lines at the plasma edge. The characteristics of the resulting edge ergodic layer (hence, the induced effects on transport, MHD stability and other physical properties) depend on the features of both the existing equilibrium and the applied perturbation. Until recently, the description of the ergodic layer was solely based upon results from numerical simulations which had been poorly validated against experimental data. We report here on the results of a series of experiments dedicated to this issue, and particularly to locating the separatrix position (i.e., the boundary between closed and open field lines).

The 3-D geometry of radiation losses is investigated in the Tore Supra ergodic divertor. Measurements from passive bolometers located on the divertor coils show evidence of toroidal and poloidal radiation modulations. They were interpreted using a 3-D code solving heat transport equation that gives the whole geometry of plasma radiation in a divertor configuration close to Tore Supra. The results of the code are in qualitative agreement with the measurements and they show that the total radiated power is underestimated when inferred from standard bolometers located between divertor modules. Maximum of radiation in front of the modules is explained by the multiplication of radiative zones at this place due to the intersection of field lines with the vessel wall. This effect leads to non-monotonic temperature profiles along field lines in the boundary plasma.

Poloidal impurity emission profiles measured with the Tore Supra grazing incidence duochromator exhibit a complex spatial structure during ergodic divertor operation with an outboard poloidal guard limiter. As previous measurements with inboard-wall limited plasmas have shown that these profiles give important information about the ergodic field structure, so the contribution of local neon recycling from the limiter-induced plume has been modeled. This permits a discrimination of edge and core transport effects. The BBQ 3D scrape-off layer code calculates the asymmetric contribution to the emission amd MIST 1D simulation gives the symmetric part. A systematic increase is observed in the decay rate of neon emission after injection as the ergodic divertor strength is increased. The calculations permit identification of the limiter plume contribution to the profile structure, and, with this identification, the effect of the divertor to enhance impurity efflux can be seen from the decay data.

The present paper addresses the issue of particle recirculation in discharges where low-energy flux to ergodic divertor target plates is achieved in highly-radiating detached ohmic plasmas. Plasma temperature and particle flux are measured by flush-mounted probes in the divertor plates and by an upstream fast scanning Mach probe. The scalings with core density of the ion flux and electron temperature are well described by the simple two-point model used in axisymmetric poloidal divertors. The detachment signature is a pressure drop that occurs when the edge temperature falls below 10 eV. The parallel ion flux gradient is always positive, indicating that recombination is unlikely to play an important role in detachment. Visible spectroscopy of a neutralizer plate shows that attainment of cold detached plasmas near the density limit coincides with an abrupt increase of fuelling efficiency for both deuterium and impurities. A feedback algorithm based on real-time Langmuir probe measurements has been developed to monitor detachment and avoid disruptions.

The type I ELMy H-mode is the baseline operating scenario for ITER. While it is known that the type I edge-localized mode (ELM) ultimately results from the peeling–ballooning instability, there is growing experimental evidence that a mode grows up before the ELM crash that may modify the edge plasma, which then leads to the ELM event due to the peeling–ballooning mode. The triggered mode results in the release of a large number of particles and energy from the core plasma but the precise mechanism by which these losses occur is still not fully understood and hence makes predictions for future devices uncertain. Recent progress in understanding the processes that trigger type I ELMs and the size of the resultant energy loss are reviewed and compared to experimental data and ideas for further development are discussed.

The three dimensional plasma boundary displacements induced by either longlasting core MHD instabilities or by applied non-axisymmetric magnetic perturbations have been measured in ASDEX Upgrade, DIII-D, JET, MAST and NSTX. For the core instabilities, the displacements are usually small, although in extreme cases in MAST when the rotation braking is strong and global a significant displacement can be observed. The most concerning instability for ITER is the saturated internal kink, or helical core, which can be found in plasmas with a wide region of low magnetic shear such as the hybrid scenario, which can lead to non-negligible boundary displacements. Nonetheless, the boundary displacement resultant from core MHD instabilities in ITER is predicted to be less than ±1% of the minor radius. The displacements arising from applied resonant magnetic perturbations (RMPs) are measured up to ±5% of the minor radius in present day machines. Good agreement has been found between these experimental measurements and a range of models ­ from vacuum field line tracing, to ideal three dimensional MHD equilibrium reconstruction, to nonlinear plasma amplification. The measured displacement across a range of machines is found to correlate linearly with the applied resonant field predicted by vacuum modelling. The RMP-induced displacements foreseen in ITER are expected to lie within the range of those predicted by the different models, meaning less than ±1.75% (±3.5cm) of the minor radius in the H-mode baseline and less than ±2.5% (±5cm) in a 9MA plasma. Whilst a displacement of 7cm peak-to-peak in the baseline scenario is marginally acceptable from both a plasma control and heat loading perspective, it is important that ITER adopts a plasma control system which can account for a three dimensional boundary corrugation to avoid an n = 0 correction which would otherwise exacerbate the displacement caused by the applied fields.

Type-I Edge Localised Modes (ELMs) have been mitigated in MAST through the application of n = 3, 4 and 6 resonant magnetic perturbations (RMPs). For each toroidal mode number of the non-axisymmetric applied fields, the frequency of the ELMs has been increased significantly, and the peak heat flux on the divertor plates reduced commensurately. This increase in ELM frequency occurs despite a significant drop in the edge pressure gradient, which would be expected to stabilise the peeling-ballooning modes thought to be responsible for type-I ELMs. Various mechanisms which could cause a destabilisation of the peeling-ballooning modes are presented, including pedestal widening, plasma rotation braking, three dimensional corrugation of the plasma boundary and the existence of radially extended lobe structures near to the X-point. This leads to a model aimed at resolving the apparent dichotomy of ELM control, that is to say ELM suppression occurring due to the pedestal pressure reduction below the peeling-ballooning stability boundary, whilst the reduction in pressure can also lead to ELM mitigation, which is ostensibly a destabilisation of peeling-ballooning modes. In the case of ELM mitigation, the pedestal broadening, 3d corrugation or lobes near the X-point degrade ballooning stability so much that the pedestal recovers rapidly to cross the new stability boundary at lower pressure more frequently, whilst in the case of suppression, the plasma parameters are such that the particle transport reduces the edge pressure below the stability boundary which is only mildly affected by negligible rotation braking, small edge corrugation or short, broad lobe structures., 23 pages, 12 figures. Copyright (2013) United Kingdom Atomic Energy Authority. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

The interaction of static Resonant Magnetic Perturbations (RMPs) with the plasma flows is modeled in toroidal geometry, using the non-linear resistive MHD code JOREK, which includes the X-point and the scrape-off-layer. Two-fluid diamagnetic effects, the neoclassical poloidal friction and a source of toroidal rotation are introduced in the model to describe realistic plasma flows. RMP penetration is studied taking self-consistently into account the effects of these flows and the radial electric field evolution. JET-like, MAST, and ITER parameters are used in modeling. For JET-like parameters, three regimes of plasma response are found depending on the plasma resistivity and the diamagnetic rotation: at high resistivity and slow rotation, the islands generated by the RMPs at the edge resonant surfaces rotate in the ion diamagnetic direction and their size oscillates. At faster rotation, the generated islands are static and are more screened by the plasma. An intermediate regime with static islands which sl...

As part of the ITER Design Review and in response to the issues identified by the Science and Technology Advisory Committee, the ITER physics requirements were reviewed and as appropriate updated. The focus of this paper will be on recent work affecting the ITER design with special emphasis on topics affecting near-term procurement arrangements. This paper will describe results on: design sensitivity studies, poloidal field coil requirements, vertical stability, effect of toroidal field ripple on thermal confinement, material choice and heat load requirements for plasma-facing components, edge localized modes control, resistive wall mode control, disruptions and disruption mitigation. © 2009 IAEA, Vienna.

Large Type-I edge localized modes (ELMs) are completely eliminated with small n = 3 resonant magnetic perturbations (RMP) in low average triangularity, , plasmas and in ITER similar shaped (ISS) plasmas, , with ITER relevant collisionalities . Significant differences in the RMP requirements and in the properties of the ELM suppressed plasmas are found when comparing the two triangularities. In ISS plasmas, the current required to suppress ELMs is approximately 25% higher than in low average triangularity plasmas. It is also found that the width of the resonant q95 window required for ELM suppression is smaller in ISS plasmas than in low average triangularity plasmas. An analysis of the positions and widths of resonant magnetic islands across the pedestal region, in the absence of resonant field screening or a self-consistent plasma response, indicates that differences in the shape of the q profile may explain the need for higher RMP coil currents during ELM suppression in ISS plasmas. Changes in the pedestal profiles are compared for each plasma shape as well as with changes in the injected neutral beam power and the RMP amplitude. Implications of these results are discussed in terms of requirements for optimal ELM control coil designs and for establishing the physics basis needed in order to scale this approach to future burning plasma devices such as ITER.

Type-I edge-localized modes (ELMs) have been mitigated at the JET tokamak using a static external n=1 perturbation field generated by four error field correction coils located far from the plasma. During the application of the n=1 field the ELM frequency increased by a factor of 4 and the amplitude of the D(alpha) signal decreased. The energy loss per ELM normalized to the total stored energy, DeltaW/W, dropped to values below 2%. Transport analyses shows no or only a moderate (up to 20%) degradation of energy confinement time during the ELM mitigation phase.

ELMy H-modes in helium-4 plasmas provide valuable information on ELMy H-mode physics as well as a possible early low activation operational phase for next-step tokamaks, such as ITER. With this in mind, a series of helium-4 H-mode experiments were performed on JET with pure helium-4 NBI auxiliary heating (up to 12 MW). A set of ELMy H-mode plasmas were produced, in both the Type I ELM regime and a second regime, which showed characteristics similar to the deuterium Type III regime, but with a reverse ELM frequency dependence on power. Sawteeth were also observed, and had similar behaviour to those seen in deuterium. Compared with deuterium plasmas, Type I ELMy H-mode confinement is seen to be 28 +/- 6% poorer in helium-4 plasmas and the L-H power threshold 42 +/- 10% larger. This is the opposite of the behaviour predicted by experimental isotope mass scalings from hydrogenic plasmas.Comparison with a wider hydrogenic database, enables the effects of isotopic charge and mass to be studied independently.

ELMy H-mode experiments at JET in 2000/mid-2002 have focused on discharges with normalized parameters for plasma density, energy confinement and beta similar to those of the ITER Q(DT) = 10 reference regime (n/n(GW) similar to 0.85, H-98(y,H-2) similar to 1, beta(N) similar to 1.8). ELMy H-mode plasmas have been realized reaching or even exceeding those parameters in steady-state conditions (up to similar to5 s or 12tau(epsilon)) in a reproducible way and only limited by the duration of the additional heating phase. These results have been obtained (a) in highly triangular plasmas, by increasing the average triangularity delta towards the ITER reference value (delta similar to 0.5), and (b) in plasmas at low triangularity (delta similar to 0.2) by seeding of Ar and placing the X-point of the plasma on the top of the septum. Pellet injection from the high field side is a third method yielding high density and high confinement, albeit not yet under steady-state conditions. In highly triangular plasmas the influence of input power, plasma triangularity and impurity seeding with noble gases has been studied. Density profile peaking at high densities has been obtained in (a) impurity seeded low triangularity discharges, (b) ELMy H-modes with low levels of input power and (c) discharges fuelled with pellet injection from the high field side. New ELM behaviour has been observed in high triangularity discharges at high density, opening a possible route to ELM heat load mitigation, which can be further amplified by Ar impurity seeding. Current extrapolations of the ELM heat load to ITER show possibly a window for Type I ELM operation. Confinement scaling studies indicate an increase in confinement with triangularity and density peaking, and a decrease in confinement with the Greenwald number. In addition, experiments in H isotope and He indicate tau(E) proportional to M(0.19)Z(-0.59). The threshold power for the L-H transition in He plasmas shows the same parametric dependence as in D plasmas, but with a 50% higher absolute value.

Washboard (WB) modes (Smeulders P et al 1999 Plasma Phys. Control. Fusion 41 1303) are a very common edge instability regularly observed in the H-mode regime in JET. They are detected as (normally several) bands of continuously fluctuating magnetic activity rotating in the direction of the electron diamagnetic drift with typical frequencies in the range of 10–90 kHz. The time evolution of the WB mode frequency is found to follow qualitatively the evolution of the electron temperature measured near the pedestal top, probably due to the strong diamagnetic drift associated with the large pedestal gradients. Evidence for their involvement in the pedestal and ELM dynamics will be presented. Increasing WB mode amplitude is correlated with an increase in the time between consecutive type-I ELMs. In situations in which a sudden increase (decrease) of WB mode activity is observed, the build-up of the pedestal temperature (and, linked to this, also of the pedestal pressure) of the electrons is seen to become slower (faster). This is a strong indication that the WB mode activity has a regulating effect on the pedestal and that it is responsible for an enhanced transport of energy across the separatrix. The occurrence of a class of type-I ELM precursor modes commonly observed in JET in discharges with low to moderate collisionality ( , roughly) (Perez C P et al EFDA-JET Preprint EFD-P(02)11) is found to be associated with a weakening of the WB modes. The underlying mechanism for this interaction has not been yet identified. In contrast to low triangularity discharges, WB activity is found to increase with gas puffing at high triangularity. This can provide an explanation for the regime recently identified on JET that has been called the mixed type-I/type-II ELM regime (Saibene G et al 2002 Plasma Phys. Control. Fusion 44 1769). A modified version of the peeling–ballooning cycle for type-I ELMs on JET that takes into account the WB mode phenomenon and is consistent with the experimental observations is proposed.This article was due to be published in issue 9 of Plasma Phys. Control. Fusion. To access this special issue, please follow this link: http://www.iop.org/EJ/toc/0741-3335/45/9.

This paper presents the results of JET experiments aimed at studying the operational space of plasmas with a Type III ELMy edge, in terms of both local and global plasma parameters. In JET, the Type III ELMy regime has a wide operational space in the pedestal n(e)-T-e diagram, and Type III ELMs are observed in standard ELMy H-modes as well as in plasmas with an internal transport barrier (ITB). The transition from an H-mode with Type III ELMs to a steady state Type I ELMy H-mode requires a minimum loss power, P-TypeI-P-TypeI decreases with increasing plasma triangularity. In the pedestal n(e)-T-e diagram, the critical pedestal temperature for the transition to Type I ELMs is found to be inversely proportional to the pedestal density (T-crit proportional to 1/n) at a low density. In contrast, at a high density, T-crit, does not depend strongly on density. In-the density range where T-crit proportional to 1/n, the critical power required for the transition to Type I ELMs decreases with increasing density. Experimental results are presented suggesting a common mechanism for Type III ELMs at low and high collisionality. A single model for the critical temperature for the transition from Type III to Type I ELMs, based on the resistive interchange instability with magnetic flutter, fits well the density and toroidal field dependence of the JET experimental data. On the other hand, this model fails to describe the variation of the Type III n(e)-T-e operational space with isotopic mass and q(95). Other results are instead suggestive of a different physics for Type III ELMs. At low collisionality, plasma current ramp experiments indicate a role of the edge current in determining the transition from Type III to Type I ELMs, while at high collisionality, a model based on resistive ballooning instability well reproduces, in term of a critical density, the experimentally observed q(95) dependence of the transition from Type I to Type III ELMs. Experimental evidence common to Type III ELMs in standard ELMy H-modes and in plasmas with ITBs indicates that they are driven by the same instability.

We present the results of experiments in JET to study the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged,for the standard,Q = 10 operation in ITER. The experiments described are single lower null plasmas,with standard q profile, neutral beam heating and. gas fuelling, with average plasma triangularity delta calculated at the separatrix similar to0.45-0.5 and elongation kappa similar to 1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the and the maximum density achievable in thermal energy confinement time steady state for a given confinement enhancement factor increase with delta. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density n(e) similar to 1.1 n(GR) for H-98 similar to 0.96. In this plasma configuration, at 2.5 MA/2.7 T (q(95) similar to 2.8), a line average density similar to95% n(GR) with H-98 = 1 and beta(N) similar to 2 are obtained, with plasma thermal stored energy content W-th being approximately constant with increasing density, as long as the discharge maintains Type I ELMs, up to n(ped) similar to n(GR) (and n(e) similar to 1.1n(GR)).A change in the Type I ELMs behaviour is observed for pedestal densities n(ped) greater than or similar to 70% n(GR), with their frequency decreasing with density (at constant P-sep), enhanced divertor D-alpha emission and increased inter-ELM losses. We show that this change in the ELM character at high pedestal density is due to a change in transport and/or stability in the pedestal region, with the ELMS changing from Type I to mixed Type I and Type II. The similarity of these observations with those in the Type II ELM regime in ASDEX Upgrade and with other small ELM regimes in DIII-D, JT-60U and Alcator C-MOD is discussed.Finally, we present the first results of experiments by studying in more detail the effects of the plasma boundary geometry, in particular by investigating separately the effect of the upper and lower triangularity, at high average delta. We show that the changes to the lower delta (or of the radial position of the x-point) affect the pedestal parameters, the size of ELM energy losses as well as the global energy confinement of the plasma.