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 δ calculated at the separatrix ∼0.45-0.5 and elongation κ ∼ 1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the thermal energy confinement time and the maximum density achievable in steady state for a given confinement enhancement factor increase with δ. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density n e ∼ 1.1n GR for H 98 ∼ 0.96. In this plasma configuration, at 2.5 MA/2.7 T (q 95 ∼ 2.8), a line average density ∼95% n GR with H 98 = 1 and β N ∼ 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 ∼ n GR (and n e ∼ 1.1n GR). A change in the Type I ELMs behaviour is observed for pedestal densities n ped 70% n GR , with their frequency decreasing with density (at constant P sep), enhanced divertor D α 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

The dynamic ergodic divertor (DED) on the TEXTOR tokamak allows for the creation of static and rotating helical magnetic perturbation fields. In the 3/1 configuration the strong m/n = 2/1 sideband excites a locked 2/1 tearing mode above a critical perturbation field strength. The mode onset threshold depends strongly on the plasma fluid rotation with respect to the mode. Rotation in plasma current direction destabilizes the mode in a certain range of rotation frequencies, whereas counter-rotation has a stabilizing influence. The threshold shows a minimum when the frequency of the external perturbation equals the MHD frequency of the mode.

The main objective of this paper is investigation of methods for reduction of divertor heat loads in order to increase the lifetime of divertor tiles in future fusion reactors. Special emphasis is given to studies of reduction of transient heat loads due to edge localized modes (ELMs). Two methods are compared: argon seeded type-I ELMy H-modes and nitrogen seeded type-III ELMy H-modes. In both scenarios, the impurity seeding leads to a reduction in the pedestal energy and hence a reduction in the energy released by the ELM. This consequentially reduces the power load to the divertor targets. At high radiative power fractions in type-III ELMy H-modes, part of that released ELM energy (small ELMs, below 20 kJ) is dissipated by radiation in the scrape off layer (SOL). Modelling of the ELM mitigation supports the experimental findings. This ELM mitigation by radiative dissipation is not effective for larger ELMs. In between ELMs, the plasma is detached and radiates strongly from the X-point region. During an ELM, the nitrogen in the X-point and divertor region becomes ionized into more weakly radiating higher charge states and the plasma re-attaches for large ELMs. At JET, argon radiates predominantly in the main plasma and not so much in the cold divertor region. Hence, the effect of radiative dissipation of ELM heat fluxes by argon is very low due to the limited argon density in the divertor region. Nevertheless, both scenarios might be compatible with an integrated ITER scenario, with respect to acceptable divertor lifetime and acceptable confinement.

Impurity injection in the JET ELMy H-mode regime has produced high-confinement, quasi-steady-state plasmas with densities close to the Greenwald density. However, at large Ar densities, a sudden loss of confinement is observed. A possible correlation between loss of confinement and the observed MHD phenomena, both in the core and in the edge of the plasma, was considered. The degradation in confinement coincided with impurity profile peaking following the disappearance of sawtooth activity. In addition, impurity density profile analysis confirmed that central MHD modes prevented impurity peaking. Experiments were designed to understand the role of sawtooth crashes in redistributing impurities. Ion-cyclotron radio frequency heating was used to control the central q-profile and maintain sawtooth activity. This resulted in quasi-steady-state, high-performance plasmas with high Ar densities. At H 98y * f GWD ∼ 0.8 and high Ar injection rates, quasi-steady-states, which previously only lasted <1τ E , were now maintained for the duration of the heating (t ∼ 9τ E). The increased central heating may have an additional beneficial effect in opposing impurity accumulation by changing the core power balance and modifying the impurity transport as predicted by neo-classical theory.

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