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Progress in the definition of the requirements for edge localized mode (ELM) control and the application of ELM control methods both for high fusion performance DT operation and non-active low-current operation in ITER is described. Evaluation of the power fluxes for low plasma current H-modes in ITER shows that uncontrolled ELMs will not lead to damage to the tungsten (W) divertor target, unlike for high-current H-modes in which divertor damage by uncontrolled ELMs is expected. Despite the lack of divertor damage at lower currents, ELM control is found to be required in ITER under these conditions to prevent an excessive contamination of the plasma by W, which could eventually lead to an increased disruptivity. Modelling with the non-linear MHD code JOREK of the physics processes determining the flow of energy from the confined plasma onto the plasma-facing components during ELMs at the ITER scale shows that the relative contribution of conductive and convective losses is intrinsically linked to the magnitude of the ELM energy loss. Modelling of the triggering of ELMs by pellet injection for DIII-D and ITER has identified the minimum pellet size required to trigger ELMs and, from this, the required fuel throughput for the application of this technique to ITER is evaluated and shown to be compatible with the installed fuelling and tritium re-processing capabilities in ITER. The evaluation of the capabilities of the ELM control coil system in ITER for ELM suppression is carried out (in the vacuum approximation) and found to have a factor of ∼2 margin in terms of coil current to achieve its design criterion, although such a margin could be substantially reduced when plasma shielding effects are taken into account. The consequences for the spatial distribution of the power fluxes at the divertor of ELM control by three-dimensional (3D) fields are evaluated and found to lead to substantial toroidal asymmetries in zones of the divertor target away from the separatrix. Therefore, specifications for the rotation of the 3D perturbation applied for ELM control in order to avoid excessive localized erosion of the ITER divertor target are derived. It is shown that a rotation frequency in excess of 1 Hz for the whole toroidally asymmetric divertor power flux pattern is required (corresponding to n Hz frequency in the variation of currents in the coils, where n is the toroidal symmetry of the perturbation applied) in order to avoid unacceptable thermal cycling of the divertor target for the highest power fluxes and worst toroidal power flux asymmetries expected. The possible use of the in-vessel vertical stability coils for ELM control as a back-up to the main ELM control systems in ITER is described and the feasibility of its application to control ELMs in low plasma current H-modes, foreseen for initial ITER operation, is evaluated and found to be viable for plasma currents up to 5-10 MA depending on modelling assumptions.

No abstract

Probe measurements of electrostatic plasma fluctuations in the scrape-off layer (SOL) of the TCV tokamak are compared with the results from twodimensional interchange turbulence simulations. Excellent agreement is found for both the radial variation of statistical moments and temporal correlations, clearly indicating that turbulent transport in the tokamak SOL is due to radial advection of blob-like filamentary structures. This offers an explanation both for the basic mechanism driving the anomalous SOL particle transport and the now commonly observed broad particle density profiles, extending deep into the SOL and thought to be the cause of high levels of main chamber plasma-wall interactions.

Plasma fluctuations in the scrape-off layer (SOL) of the TCV tokamak exhibit statistical properties which are universal across a broad range of discharge conditions. Electron density fluctuations, from just inside the magnetic separatrix to the plasma-wall interface, are described well by a gamma distributed random variable. The density fluctuations exhibit clear evidence of self-similarity in the far SOL, such that the corresponding probability density functions collapse upon renormalization solely by the mean particle density. This constitutes a demonstration that the amplitude of the density fluctuations is simply proportional to the mean density and is consistent with the further observation that the radial particle flux fluctuations scale solely with the mean density over two orders of magnitude. Such findings indicate that it may be possible to improve the prediction of transport in the critical plasma-wall interaction region of future large scale tokamaks.

In this paper, two simplified models of edge localized mode (ELM) power exhaust are developed, one based on the kinetic and the other on the fluid treatment of parallel losses. These models are found to capture many (though not all) of the salient features of kinetic simulations at substantial savings in both cost and complexity (CPU time in seconds versus days), making them ideal as real time interpretive tools or as modules in non-linear MHD, transport and/or turbulence codes. The kinetic model offers analytic expressions for the ion and electron powers deposited on the divertor, parametrized in terms of transient sheath energy transmission coefficients γ i and γ e , in good agreement with particle-in-cell simulations. The fluid model successfully reproduces ELM filament densities and electron energies measured at the outer poloidal limiter on JET, as well as recent measurements of ELM filament ion energies in the JET far-scrape-off layer (SOL). Taking confidence from this favourable comparison, the same model is then used to predict ion impact energies due to the incidence of Type-I ELM filaments on the ITER limiter. Although the models are applied here exclusively to ELMs, they have a potential application to other tokamak transients, such as intermittent SOL bursts and the disruption thermal quench.

The heating of tungsten monoblocks at the ITER divertor vertical targets is calculated using the heat flux predicted by three-dimensional ion orbit modelling. The monoblocks are beveled to a depth of 0.5 mm in the toroidal direction to provide magnetic shadowing of the poloidal leading edges within the range of specified assembly tolerances, but this increases the magnetic field incidence angle resulting in a reduction of toroidal wetted fraction and concentration of the local heat flux to the unshadowed surfaces. This shaping solution successfully protects the leading edges from inter-ELM heat loads, but at the expense of (1) temperatures on the main loaded surface that could exceed the tungsten recrystallization temperature in the nominal partially detached regime, and (2) melting and loss of margin against critical heat flux during transient loss of detachment control. During ELMs, the risk of monoblock edge melting is found to be greater than the risk of full surface melting on the plasma-wetted zone. Full surface and edge melting will be triggered by uncontrolled ELMs in the burning plasma phase of ITER operation if current models of the likely ELM ion impact energies at the divertor targets are correct. During uncontrolled ELMs in pre-nuclear deuterium or helium plasmas at half the nominal plasma current and magnetic field, full surface melting should be avoided, but edge melting is predicted.

Two reciprocating probe systems, in the same poloidal position at the top of the JET torus but toroidally separated by 180 o , have been used to measure parallel flow in the Scrape-Off Layer (SOL). One system uses the entrance slit plates of a Retarding Field Analyser (RFA) to record upstream and downstream flux densities, and the second system uses two pins of a nine pin Turbulent Transport Probe, (TTP). Measurements have been made in both forward and reverse field directions.Results from both systems are similar.In the forward field direction, that is with B r ∇ drifts downward towards the divertor, there is a strong parallel flow in the direction outer to inner divertor. The flow generally has a low value, Mach number M~0.2, close to the separatrix and rises in the region of high magnetic shear close to the separatrix to a maximum of M~0.5 some 20mm outside of the separatrix. The flow in the reverse field direction is small, close to zero, and generally is again in the same direction as that for forward field close to the separatrix, with M~0.2. Code results using EDGE2D with drifts suggested an almost symmetrical flow about zero when the field direction was changed in an earlier work, [1]. However, this was for particularly low density, high temperature edge conditions, and the predicted symmetry is not evident for more usual edge conditions, reported here. Experimentally, the flow is found to be quite asymmetric about zero, particularly at high density. There is some symmetry in flow, but about an offset value of M~0.2. The form of M(r) is similar to experiment but the major code result is the low value of M generally < 0.1. The effect of gas (deuterium) and impurity (carbon) puffing in the code has been investigated. We are unable to say why the magnitude of M(r) from experimental data and codes do not agree. However, results presented in this paper suggest that the probe itself may be exhibiting an influence on the magnitude of the flow as measured in the SOL.