New experiments in 2013-2014 have investigated the physics responsible for the decrease in H-mode pedestal confinement observed in the initial phase of JET-ILW operation (2012 Experimental Campaigns). The effects of plasma triangularity, global beta and neutrals-both D and low-Z impurities-on pedestal confinement and stability have been investigated systematically. The stability of JET-ILW pedestals is analysed in the framework of the Peeling-Ballooning model and the pedestal predictive code EPED. Low D neutrals content in the plasma, achieved either by low D 2 gas injection rates or by divertor configurations with optimum pumping, and high beta are necessary conditions for good pedestal (and core) performance. In such conditions the pedestal stability is consistent with the Peeling-Ballooning paradigm. Moderate to high D 2 gas rates, required for W control and stable H-mode operation with the ILW, lead to increased D neutrals content in the plasma and additional physics in the pedestal models may be required to explain the onset of the ELM instability. The physics mechanism leading to the beneficial increase in pedestal temperature with N 2 seeding in high triangularity JET-ILW H-modes is not yet understood. The changes in H-mode performance associated with the change in JET wall composition from C to Be/W point to D neutrals and low-Z impurities playing a role in pedestal stability, elements which are not currently included in pedestal models. These aspects need to be addressed in order to progress towards full predictive capability of the pedestal height.

Abstract:JET underwent a transformation from a full carbon-dominated tokamak to a full metallic device with the ITER-like wall combination for the activated phase with Beryllium main chamber and Tungsten divertor. The ITER-Like Wall (ILW) experiment at JET provides an ideal test bed for ITER and shall demonstrate as primary goals the plasma compatibility with metallic walls and the reduction in fuel retention. We report on a set of experiments ( = 2.0 , = 2.0 − 2.4 , = 0.2 − 0.4) in different confinement and plasma conditions with global gas balance analysis demonstrating a strong reduction of the long term retention rate by a factor ten with respect to carbon references. All experiments have been executed in a series of identical plasma discharges in order to achieve maximum plasma duration until the analysis limit of the active gas handling system has been reached. The composition analysis shows high purity of the recovered gas, typically 99% D. For typical L-mode discharges ( = 0.5 ), type III ( = 5.0 ), and type I ELMy H-mode plasmas ( = 12.0 ) a drop of the retention rate normalised to the operational time in divertor configuration has been measured from 1.27 × 10 has been obtained with the ILW. The observed reduction by one order of magnitude confirms the expected predictions concerning the plasma-facing material change in ITER and widens the operation without active cleaning in the DT phase in comparison to a full carbon device.

In ASDEX Upgrade, experimental efforts aim to establish pace making and mitigation of type-I edge localized modes (ELMs) in high confinement mode (H-mode) discharges. Injection of small size cryogenic deuterium pellets (∼(1.4 mm)2 × 0.2 mm ≈ 2.5 × 1019 D) at rates up to 83 Hz imposed persisting ELM control without significant fuelling, enabling for investigations well inside the type-I ELM regime. The approach turned out to meet all required operational features. ELM pace making was realized with the driving frequency ranging from 1 to 2.8 times the intrinsic ELM frequency, the upper boundary set by hardware limits. ELM frequency enhancement by pellet pace making causes much less confinement reduction than by engineering means like heating, gas bleeding or plasma shaping. Confinement reduction is observed in contrast to the typical for engineering parameters. Matched discharges showed triggered ELMs ameliorated with respect to intrinsic counterparts while their frequency was increased. No significant differences were found in the ELM dynamics with the available spatial and temporal resolution. By breaking the close correlation of ELM frequency and plasma parameters, pace making allows the establishment of fELM as a free parameter giving enhanced operational headroom for tailoring H-mode scenarios with acceptable ELMs. Use was made of the pellet pace making tool in several successful applications in different scenarios. It seems that further reduction of the pellet mass could be possible, eventually resulting in less confinement reduction as well.

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A comparison of the L–H power threshold (Pthr) in JET with all carbon, JET-C, and beryllium/tungsten wall (the ITER-like choice), JET-ILW, has been carried out in experiments with slow input power ramps and matched plasma shapes, divertor configuration and IP/BT pairs. The low density dependence of the L–H power threshold, namely an increase below a minimum density ne,min, which was first observed in JET with the MkII-GB divertor and C wall and subsequently not observed with the current MkII-HD geometry, is observed again with JET-ILW. At plasma densities above ne,min, Pthr is reduced by ∼30%, and by ∼40% when the radiation from the bulk plasma is subtracted (Psep), with JET-ILW compared to JET-C. At the L–H transition the electron temperature at the edge, where the pedestal later develops, is also lower with JET-ILW, for a given edge density. With JET-ILW the minimum density is found to increase roughly linearly with magnetic field, , while the power threshold at the minimum density scales as . The H-mode power threshold in JET-ILW is found to be sensitive both to variations in main plasma shape (Psep decreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of Psep and Zeff or subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Zeff and/or subdivertor neutral pressure in the L–H transition physics. Depending on the chosen divertor configuration, Pthr can be up to a factor of two lower than the ITPA scaling law for densities above ne,min. A shallow edge radial electric field well is observed at the L–H transition. The edge impurity ion poloidal velocity remains low, close to its L-mode values, ⩽5 km s−1 ± 2–3 km s−1, at the L–H transition and throughout the H-mode phase, with no measureable increase within the experimental uncertainties. The edge toroidal rotation profile does not contribute to the depth of the negative Er well and thus may not be correlated with the formation of the edge transport barrier in JET.