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.

A neoclassical tearing mode (NTM) requires a finite size seed island to become unstable. Usually the local pressure gradient is relatively large at the β-values needed for these seed islands to destabilize the NTMs. Therefore, the island has a large growth rate at mode onset and grows rapidly to its saturated island width. This width is proportional to β as long as it is well above the marginal β-limit below which the mode is stable. The marginal β-limit is independent of the seed island trigger mechanism and provides detailed information on the stabilizing terms in the modified Rutherford equation, which are not unambiguously determined theoretically. It is shown that in JET the marginal normalized β-limit for the 3/2 mode, β N,marg , is of the order of 0.5-1 for magnetic fields between 3.3 and 1 T, with q 95 ≈ 3.3, and near the H-L transition. Therefore, all H-modes with typical q-profiles (q 95 ≈ 3.3) are metastable in JET to 3/2 NTMs. In addition, the marginal island width is of the order of 2-4 cm and the stabilizing terms are such that they influence the saturated island width when it is smaller than 4-6 cm in these H-mode discharges. It is also shown that detailed analyses of the time evolution of the island width with slow β ramp-down suggest that the convective form of the stabilization term due to the 'χ ⊥ model' is more appropriate and can explain the island decay between 4 and 6 cm to the marginal island width, while the polarization current model can explain the rapid stabilization when β < β marg . The range of values of the different stabilizing terms are discussed in detail. In particular, it is shown that the mode is stabilized and has a large negative growth rate, when the

Abstract:During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimised by using hydrogen (H) plasmas where the reference ion cyclotron resonance frequency (ICRF) heating scenarios rely on minority species such as helium ( 3 He) or deuterium (D). This paper firstly describes experiments dedicated to the study of 3 He heating in H plasmas with a sequence of discharges in which 5 MW of ICRF power was reliably coupled and the 3 He concentration, controlled in real-time, was varied from below 1 % up to 10 %. The minority heating regime was observed at low concentrations (up to 2 %). Energetic tails in the 3 He ion distributions were observed with effective temperatures up to 300 keV and bulk electron temperatures up to 6 keV. At around 2 %, a sudden transition was reproducibly observed to the mode conversion regime, in which the ICRF fast wave couples to short wavelength modes, leading to efficient direct electron heating and bulk electron temperatures up to 8 keV. Secondly, experiments performed to study D minority ion heating in H plasmas are presented. This minority heating scheme proved much more difficult since modest quantities of carbon (C) impurity ions, which have the same charge to mass ratio as the D ions, led directly to the mode conversion regime.Finally, numerical simulations to interpret these two sets of experiments are under way and preliminary results are shown.

In order to simultaneously control the current and pressure profiles in high performance tokamak plasmas with internal transport barriers (ITB), a multi-variable model-based technique has been proposed. New algorithms using a truncated singular value decomposition (TSVD) of a linearised model operator and retaining the distributed nature of the system have been implemented in the JET control system. Their simplest versions have been applied to the control of the current density profile in reversed shear plasmas using three heating and current drive actuators (neutral beam injection, ion cyclotron resonant frequency heating and lower hybrid current drive). Successful control of the safety factor profile has been achieved in quasi steady state, on a time scale of the order of the current redistribution time. How the TSVD algorithm will be used in the forthcoming campaigns for the simultaneous control of the current profile and of the ITB temperature gradient is discussed in some detail, but this was not yet attempted in the present pioneering experiments.

Real-time simultaneous control of several radially distributed magnetic and kinetic plasma parameters is being investigated on JET, in view of developing integrated control of advanced tokamak scenarios. This paper describes the new model-based profile controller which has been implemented during the 2006–2007 experimental campaigns. The controller aims to use the combination of heating and current drive (H&CD) systems—and optionally the poloidal field (PF) system—in an optimal way to regulate the evolution of plasma parameter profiles such as the safety factor, q(x), and gyro-normalized temperature gradient, . In the first part of the paper, a technique for the experimental identification of a minimal dynamic plasma model is described, taking into account the physical structure and couplings of the transport equations, but making no quantitative assumptions on the transport coefficients or on their dependences. To cope with the high dimensionality of the state space and the large ratio between the time scales involved, the model identification procedure and the controller design both make use of the theory of singularly perturbed systems by means of a two-time-scale approximation. The second part of the paper provides the theoretical basis for the controller design. The profile controller is articulated around two composite feedback loops operating on the magnetic and kinetic time scales, respectively, and supplemented by a feedforward compensation of density variations. For any chosen set of target profiles, the closest self-consistent state achievable with the available actuators is uniquely defined. It is reached, with no steady state offset, through a near-optimal proportional-integral control algorithm. Conventional optimal control is recovered in the limiting case where the ratio of the plasma confinement time to the resistive diffusion time tends to zero. Closed-loop simulations of the controller response have been performed in preparation for experiments, and typical results are shown. Finally, in the last section of the paper, the first experimental results using this dynamic-model approach to control the plasma current and the safety factor profile on JET, either with the three H&CD systems or also with the PF system as an additional actuator, are presented and discussed.

Experimental evidence from the JET tokamak is presented supporting the predictions of a recent theory (Graves et al 2009 Phys. Rev. Lett. 102 065005) on sawtooth instability control by toroidally propagating ion cyclotron resonance waves. Novel experimental conditions minimized a possible alternate effect of magnetic shear modification by ion cyclotron current drive, and enabled the dependence of the new energetic ion mechanism to be tested over key variables. The results have favourable implications on sawtooth control by ion cyclotron resonance waves in a fusion reactor.

We present an ultrafast neural network (NN) model, QLKNN, which predicts core tokamak transport heat and particle fluxes. QLKNN is a surrogate model based on a database of 300 million flux calculations of the quasilinear gyrokinetic transport model QuaLiKiz. The database covers a wide range of realistic tokamak core parameters. Physical features such as the existence of a critical gradient for the onset of turbulent transport were integrated into the neural network training methodology. We have coupled QLKNN to the tokamak modelling framework JINTRAC and rapid control-oriented tokamak transport solver RAPTOR. The coupled frameworks are demonstrated and validated through application to three JET shots covering a representative spread of H-mode operating space, predicting turbulent transport of energy and particles in the plasma core. JINTRAC-QLKNN and RAPTOR-QLKNN are able to accurately reproduce JINTRAC-QuaLiKiz T i,e and n e profiles, but 3 to 5 orders of magnitude faster. Simulations which take hours are reduced down to only a few tens of seconds. The discrepancy in the final source-driven predicted profiles between QLKNN and QuaLiKiz is on the order 1%-15%. Also the dynamic behaviour was well captured by QLKNN, with differences of only 4%-10% compared to JINTRAC-QuaLiKiz observed at mid-radius, for a study of density buildup following the L-H transition. Deployment of neural network surrogate models in multi-physics integrated tokamak modelling is a promising route towards enabling accurate and fast tokamak scenario optimization, Uncertainty Quantification, and control applications.

Experiments on JET with a carbon-fibre composite wall have explored the reduction of steady-state power load in an ELMy H-mode scenario at high Greenwald fraction ∼0.8, constant power and close to the L to H transition. This paper reports a systematic study of power load reduction due to the effect of fuelling in combination with seeding over a wide range of pedestal density ((4–8) × 1019 m−3) with detailed documentation of divertor, pedestal and main plasma conditions, as well as a comparative study of two extrinsic impurity nitrogen and neon. It also reports the impact of steady-state power load reduction on the overall plasma behaviour, as well as possible control parameters to increase fuel purity. Conditions from attached to fully detached divertor were obtained during this study. These experiments provide reference plasmas for comparison with a future JET Be first wall and an all W divertor where the power load reduction is mandatory for operation.