V. Riccardo

JET ITER-like wall—overview and experimental programme*

This paper reports the successful installation of the JET ITER-like wall and the realization of its technical objectives. It also presents an overview of the planned experimental programme which has been optimized to exploit the new wall and other JET enhancements in 2011/12.

Overview of the ITER-like wall project

Work is in progress to completely replace, in 2008/9, the existing JET CFC tiles with a configuration of plasma facing materials consistent with the ITER design. The ITER-like wall (ILW) will be cr ...

Impact and mitigation of disruptions with the ITER-like wall in JET

Disruptions are a critical issue for ITER because of the high thermal and magnetic energies that are released on short timescales, which results in extreme forces and heat loads. The choice of material of the plasma-facing components (PFCs) can have significant impact on the loads that arise during a disruption. With the ITER-like wall (ILW) in JET made of beryllium in the main chamber and tungsten in the divertor, the main finding is a low fraction of radiation. This has dropped significantly with the ILW from 50?100% of the total energy being dissipated during disruptions in CFC wall plasmas, to less than 50% on average and down to just 10% for vertical displacement events (VDEs). All other changes in disruption properties and loads are consequences of this low radiation: long current quenches (CQs), high vessel forces caused by halo currents and toroidal current asymmetries as well as severe heat loads. Temperatures close to the melting limit have been locally observed on upper first wall structures during deliberate VDE and even at plasma currents as low as 1.5 MA and thermal energy of about 1.5?MJ only. A high radiation fraction can be regained by massive injection of a mixture of 10% Ar with 90% D2. This accelerates the CQ thus reducing the halo current and sideways impulse. The temperature of PFCs stays below 400??C. MGI is now a mandatory tool to mitigate disruptions in closed-loop operation for currents at and above 2.5?MA in JET.

JET disruption studies in support of ITER

Plasma disruptions affect plasma-facing and structural components of tokamaks due to electromechanical forces, thermal loads and generation of high energy runaway electrons (REs). Asymmetries in poloidal halo and toroidal plasma current can now be routinely measured in four positions 90° apart. Their assessment is used to validate the design of the ITER vessel support system and its in-vessel components. The challenge of disruption thermal loads comes from both the short duration over which a large energy has to be lost and the potential for asymmetries. The focus of this paper will be on localized heat loads. Resonant magnetic perturbations failed to reduce the generation of REs in JET. An explanation of the limitations applying to these attempts is offered together with a minimum guideline. The REs generated by a moderate, but fast, Ar injection in limiter plasmas show evidence of milder and more efficient losses due to the high Ar background density.

Forces between plasma, vessel and TF coils during AVDEs at JET

In JET, during some vertical displacement events (VDEs), the plasma current and position are toroidally asymmetric. These events are referred to as asymmetric VDEs (AVDEs). Analysis of the interactions among the current carrying systems reveals that the repelling force between the plasma and the vessel is of little significance in asymmetric events, even if it is the main symmetric repelling force. The sideways force acting on the vessel is shown to be due mainly to the interaction with the toroidal field of the asymmetric circulation of currents in the wall. The asymmetric current path in the vessel is then calculated analytically for a simplified geometry and used together with experimental data to estimate the sideways force on the vessel.

Plasma current asymmetries during disruptions in JET

A key feature of disruptions during vertical displacement events, discovered in JET in 1996, is the toroidal variation in the measured plasma current Ip, i.e. the plasma current asymmetries, lasting for almost the entire current quench. The unique magnetic diagnostics at JET (full set of poloidal coils and saddle loops recorded either from two toroidally opposite or from four toroidally orthogonal locations) allow for a comprehensive analysis of asymmetrical disruptions with a large scale database. This paper presents an analysis of 4854 disruptions over an 18 year period that includes both the JET carbon (C) wall and the ITER-like (IL) wall (a mixed beryllium/tungsten first wall). In spite of the Ip quench time significantly increasing for the IL-wall compared to C-wall disruptions, the observed toroidal asymmetry time integral (~ sideways force impulse), did not increase for IL-wall disruptions. The Ip asymmetry has a dominantly n = 1 structure. Its motion in the toroidal direction has a sporadic behaviour, in general. The distributions of the number of rotation periods are found to be very similar for both C- and IL-wall disruptions, and multi-turn rotation was sometimes observed. The Ip asymmetry amplitude has no degradation with rotation frequency for either the C- or IL-wall disruption. Therefore dynamic amplification remains a potentially serious issue for ITER due to possible mechanical resonance of the machine components with the rotating asymmetry.

Progress in Understanding Halo Current at JET

The poloidal distribution of the halo current density on the top dump plate in JET can now be measured thanks to a new set of Rogowskii coils. These are the first measurements in JET able to offer ...

Parametric analysis of asymmetric vertical displacement events at JET

A small fraction of plasma disruptions at JET lead to a significant horizontal displacement in the vacuum vessel. This can threaten the integrity of the vessel and, in particular, of various vessel attachments such as the main vertical ports and the neutral injector boxes. These events are referred to as asymmetric vertical disruption events (AVDEs). In this paper we attempt to identify associations between plasma parameters and large asymmetric vessel displacements by analysis of data from some hundreds of disruptive JET pulses. As far as the pre-disruption parameters are concerned, the amplitude of sideways vessel displacements shows a trend with the boundary safety factor and the poloidal beta. No link has been identified with the plasma triangularity or elongation. With regard to plasma parameters during the disruption, the data show a trend with the amplitude of the instability mode n = 1, and a trend with the plasma current quench rate and the minimum value of the boundary safety factor. A reliable short cut to select disruptions with high asymmetric loads and to estimate the effects on the vessel using disruption data has been proposed. Hardly any practical operational space has been found to be wholly safe from triggering such sideways vessel displacements.

Modelling magnetic forces during asymmetric vertical displacement events in JET

Asymmetric vertical displacement events (AVDEs) are fortunately rare, but can induce large lateral forces which can cause significant mechanical damage to tokamaks. In this paper we present a simple model which allows the lateral forces generated during such a disruption to be estimated as a function of relatively easily obtained electromagnetic parameters: the asymmetries in the vertical current moment. This model is validated by using it to predict the displacement history of the JET tokamak caused by a number of major AVDEs. It is shown that the predicted forces and displacements agree well with quantities measured during these disruptions. One conclusion from the model is that the maximum sideways displacement scales with the product of the plasma current and the toroidal field, and this recipe is now used at JET to assess a priori the hazards of performing high current and high field pulses when they are known to be likely to disrupt.

Plasma facing materials for the jet iter-like wall

Abstract The chosen materials for plasma facing components for the deuterium/tritium phase of ITER are beryllium and tungsten. These materials have already been widely investigated in various devices like ion beam or electron beam tests. However, the operation of this material combination in a large tokamak including plasma wall interaction, material degradation, erosion and material mixing has not been proven yet. The ITER-like Wall, which has been recently installed in JET, consists of a combination of bulk tungsten and tungsten coated CFC divertor tiles as well as bulk beryllium and beryllium coated INCONEL in the main chamber. The experiments in JET will provide the first fully representative test of the ITER material choice under relevant conditions. This paper concentrates on material research and developments for the materials of the JET ITER-like Wall with respect to mechanical and thermal properties. The impact of these materials and components on the JET operating limits with the ITER-like Wall and implications for the ongoing scientific program will be summarised.