the ASDEX Upgrade team 4 and the EURO-fusion MST1 Team 5 .

Fast-ion redistribution and loss due to edge perturbations in the ASDEX Upgrade, DIII-D and KSTAR tokamaks M. Garcia-Munoz, S. Äkäslompolo, O. Asunta et al. Spatiotemporal response of plasma edge density and temperature to non-axisymmetric magnetic perturbations at ASDEX Upgrade

Recent experiments at ASDEX Upgrade demonstrate the compatibility of ELM mitigation by magnetic perturbations with efficient particle fuelling by inboard pellet injection. ELM mitigation persists in a high density, high collisionality regime even with the strongest applied pellet perturbations. Pellets injected into mitigation phases trigger no type-I ELM like events unlike when launched into unmitigated type-I ELMy plasmas. Furthermore, the absence of ELMs results in improved fuelling efficiency and persistent density build up. Pellet injection is helpful to access the ELM-mitigation regime by raising the edge density beyond the required threshold level, mostly eliminating the need for strong gas puff. Finally, strong pellet fuelling can be applied to access high densities beyond the density limit encountered with pure gas puffing. Core densities of up to 1.6 times the Greenwald density have been reached while maintaining ELM mitigation. No upper density limit for the ELM-mitigated regime has been encountered so far; limitations were set solely by technical restrictions of the pellet launcher. Reliable and reproducible operation at line averaged densities from 0.75 up to 1.5 times the Greenwald density is demonstrated using pellets. However, in this density range there is no indication of the positive confinement dependence on density implied by the ITERH98P(y,2) scaling.Final version, 6.1.2012 2 IntroductionThe power load deposited by type-I ELMs onto the first wall and divertor presents a severe danger for ITER. Currently, there are several options for ELM mitigation: Operating with plasma scenarios which develop no or only benign ELMs, ELM pace-making to enhance the ELM frequency by at least a factor of 15 -30 in order to reduce individual ELM losses by this same factor, or ELM mitigation or ELM suppression with non-axisymmetric magnetic perturbations. Pace-making tries to reduce the type-I ELM size by triggering the instability through an external perturbation, e.g. pellet injection with a rate higher than the natural ELM frequency. Avoidance of type-I ELMs is preferred over ELM pace-making if it can be achieved without significant deleterious impact on the plasma performance. Nonaxisymmetric magnetic perturbations have been investigated in DIII-D where full suppression or at least significant mitigation of ELMs has been achieved over a wide operational range [1]. This successful first demonstration prompted further experiments at JET [2,3], NSTX [4] and MAST [5] with different perturbation field configurations. In theses experiments, quite different results with respect to ELM mitigation were obtained, and so far full ELM suppression has not been reproduced in any of them. Recently, the ASDEX Upgrade tokamak has been enhanced with a set of in-vessel coils [6] in order to study ELM amelioration, shed light on the physics involved, and to improve the extrapolation base for ITER. In a first stage of this enhancement, n=2 magnetic perturbations are found to allow suppressing type-I ELMs and replace th...

Repetitive melting of tungsten by power transients originating from edge localized modes (ELMs) has been studied in ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the Divertor Manipulator II (DIM-II) system [1]. Designed as near replicas of the geometries used also in separate experiments on the JET tokamak [2-4], the samples featured a misaligned leading edge and a sloped ridge respectively. Both structures protrude above the default target plate surface thus receiving an increased fraction of the parallel power flux. Transient melting by ELMs was induced by moving the outer strike point to the sample location. The temporal evolution of the measured current flow from the samples to vessel potential confirmed transient melting. Current magnitude and dependency from surface temperature provided strong evidence for thermionic electron emission as main origin of the replacement current driving the melt motion. The different melt patterns observed after exposures at the two sample geometries support the thermionic electron emission model used in the MEMOS melt motion code, which assumes a strong decrease of the thermionic net current at shallow magnetic field to surface angles [5]. Post exposure ex-situ analysis of the retrieved samples show recrystallization of tungsten at the exposed surface areas to a depth of up to several mm. The melt layer transport to less exposed surface areas leads to ratcheting pile up of re-solidified debris with zonal growth extending from the already enlarged grains at the surface.

ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

Abstract. The EMC3-Eirene code package was applied for the first time to simulate the edge plasma in an ASDEX Upgrade discharge, in which the newly installed magnetic perturbation coils were used to mitigate Edge Localized Modes (ELMs). Two different points in time during this discharge were simulated, the ELM mitigated phase after turning-on of the magnetic perturbation coils and, as a reference, the ELMy H-mode phase before. The results were compared to the measurements of various edge and divertor diagnostics. Assuming the main chamber profiles to be shifted by 15mm with respect to their calibrated positions, an agreement within a factor of 2 was found between the main chamber profiles outside the separatrix and those at the outer divertor target. The most important result is the observation of several maxima and minima in the particle flux and in particular in the power deposition pattern of both the simulation and the experiment for the case with magnetic perturbations, an effect also known as strike-point splitting.

Abstract. New measurements of ion energies and currents in

The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q 95  =  5.5, ) at low density. Higher installed electron cyclotron resonance heating power   6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG. Stable high-density H-modes with MW m−1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated. Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and E r allow for inter ELM transport analysis confirming that E r is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of ‘natural’ no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback. Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle—measured for the first time—or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvénic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO.