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In this contribution we present results from the first N2 seeding experiments in JET performed after installation of the ITER-like Wall. Gas balance measurements for seeded L-mode discharges indicate very strong N2 retention as well as a potential increase in D2 retention. The possible influence of ammonia production on this apparent retention is discussed. Plasma parameters and impurity content were monitored throughout the seeded discharges as well as during subsequent clean-up discharges. These experiments give first insight into phenomena related to the use of nitrogen as seeding gas in JET with the ITER-like Wall, such as ammonia production and nitrogen legacy. © 2013 Elsevier B.V. All rights reserved.

Measurements of the plasma conditions at the low field side target plate in JET ITER-like wall ohmic and low confinement mode plasmas show minor differences in divertor plasma configurations with horizontally and vertically inclined targets. Both the reduction of the electron temperature in the vicinity of the strike points and the rollover of the ion current to the plates follow the same functional dependence on the density at the low field side midplane. Configurations with vertically inclined target plates, however, produce twice as high sub-divertor pressures for the same upstream density. Simulations with the EDGE2D-EIRENE code package predict significantly lower plasma temperatures at the low field side target in vertical than in horizontal target configurations. Including cross-field drifts and imposing a pumping by-pass leak at the low-field side plate can still not recover the experimental observations.

Recent progress in edge plasma modeling on the JET tokamak is briefly, and somewhat selectively, reviewed. This ongoing modeling activity is aimed at developing a predictive capability for ITER based on numerical models verified and validated upon JET experimental data. Topics include both steady‐state and transient particle and power exhaust, the effect of edge/SOL turbulence and edge localized modes, and first wall material migration.

Two series of JET shots, one on hot ion shots and one on hot electron shots, have been analysed in order to improve the understanding of how the temperature ratio T-e/T-i influences the performance. Comparisons are also made with ITER simulations which are in the hot electron regime. The temperature ratio has been varied between different shots, in partly interpretative simulations of one shot and locally. The effect of the variation of the temperature ratio depends strongly on how it is done but can also be different in different types of shots. In general, however, an increased electron heating deteriorates confinement rather strongly. This is the situation for increasing the alpha power in ITER. ITER simulations show, however, that the effect is not dramatic. (Some figures in this article are in colour only in the electronic version).

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.

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This contribution is focused on the magnetic signatures of type I ELM filaments. On JET a limited number of high time resolution magnetic coils were used to derive essential ELM filament parameters. The method uses forward modelling and simultaneous fitting of magnetic pickup coil signals to a simple model, motivated by observations. A new diagnostic in the form of a reciprocating probe with three magnetic pickup loops was developed for ASDEX Upgrade (AUG). Measurements during the passage of type-I ELM filaments determine the filaments to be in the scrape off layer (SOL) and to carry currents up to 1.5 kA.

Accurate and validated tools for calculating toroidal momentum sources are necessary to make reliable predictions of toroidal rotation for current and future experiments. In this work we present the first experimental validation of torque profile calculation from neutral beam injection (NBI) under toroidal field ripple. We use discharges from a dedicated experimental session on JET where neutral beam modulation technique is used together with time-dependent torque calculations from ASCOT code for making the benchmark. Good agreement between simulations and experimental results is found.

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The new multichannel Martin-Puplett interferometer for Electron Cyclotron Emission (ECE) measurements has been working at JET from the 2006 experimental campaign. After the installation, the interferometer has been extensively studied using radiation sources of different wavelengths and coherence. This paper deals with the detailed description of the installation tests performed in the millimeter wave range, and discusses the features and the limits of the different measurements performed.

In the framework of the Enhancement Projects at JET, an extension of the capabilities of the Martin-Puplett interferometer has been required for the spectral analysis of the perpendicular and oblique signals of the electron cyclotron emission (ECE) over a range of 75-400 GHz. That has been only possible with a complete redesign of the collection optics system. The new instrument is designed to analyze the incoming radiation from standard perpendicular view (two channels) and from oblique view of the ECE radiation at two different angles (four channels) at similar to 10 degrees and similar to 22 degrees from the poloidal plane. The new optical scheme needed a totally new mechanical layout of the system. This work deals with the detailed description of the mechanical realization and the first characterization of the system.

The simultaneous real-time control of the current and pressure profiles could lead to the steady state sustainment of an internal transport barrier (ITB), region where heat and particle transport are strongly reduced and so to a stationary optimized plasma regime. Recent experiments in JET have demonstrated significant progress in achieving such a control: different current and temperature gradient target profiles have been reached and sustained for several seconds using a controller based on a static linear model identification. Nevertheless, these experiments have shown that the controller was sensitive to rapid plasma events such as transient ITBs during the safety factor profile evolution or Magneto-Hydrodynamics (MHD) instabilities which modify the pressure profiles on the confinement time scale. The control technique has been improved by using a multiple-time-scale approximation in order to better respond to these rapid plasma events. The paper describes the theoretical analysis and closed-loop simulations using the controller that will be tested experimentally in JET during the forthcoming campaign.

A series of hot-electron plasmas were produced at JET with the aim of clarifying the dependence of the energy confinement time on the electron to ion temperature ratio, T-e/T-i. Although these plasmas showed a clear increase in the ion heat diffusivity with higher T-e/T-i, their energy confinement time remained constant. In this paper we propose that these observations are consistent with the predictions of the Weiland model of energy transport. A feedback loop amplifying the ion diffusivity simultaneously flattens the ion temperature profile, and keeps the ion heat flux constant. The electron transport shows no dependence on T-e/T-i, and hence there is no scaling of the confinement time with the temperature ratio. To support these claims we show a series of numerical simulations using the Weiland model. For scaling law purposes it is also interesting to note that the energy confinement time does not show any dependence on the effective input power ratio, P-e/P-i, either.

Real time control of many plasma parameters will be an essential aspect in the development of reliable high performance operation of Next Step Tokamaks. The main prerequisites for any feedback scheme are the precise real-time determination of the quantities to be controlled, requiring top quality and highly reliable diagnostics, and the availability of robust control algorithms. A new set of real time diagnostics was recently implemented on JET to prove the feasibility of determining, with high accuracy and time resolution, the most important plasma quantities. With regard to feedback algorithms, new model–based controllers were developed to allow a more robust control of several plasma parameters. Both diagnostics and algorithms were successfully used in several experiments, ranging from H-mode plasmas to configuration with ITBs. Since elaboration of computationally heavy measurements is often required, significant attention was devoted to non-algorithmic methods like Digital or Cellular Neural/Nonlinear Networks. The real time hardware and software adopted architectures are also described with particular attention to their relevance to ITER., Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France)

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ELMy H-mode experiments at JET in 2000/mid-2002 have focused on discharges with normalized parameters for plasma density, energy confinement and beta similar to those of the ITER Q(DT) = 10 reference regime (n/n(GW) similar to 0.85, H-98(y,H-2) similar to 1, beta(N) similar to 1.8). ELMy H-mode plasmas have been realized reaching or even exceeding those parameters in steady-state conditions (up to similar to5 s or 12tau(epsilon)) in a reproducible way and only limited by the duration of the additional heating phase. These results have been obtained (a) in highly triangular plasmas, by increasing the average triangularity delta towards the ITER reference value (delta similar to 0.5), and (b) in plasmas at low triangularity (delta similar to 0.2) by seeding of Ar and placing the X-point of the plasma on the top of the septum. Pellet injection from the high field side is a third method yielding high density and high confinement, albeit not yet under steady-state conditions. In highly triangular plasmas the influence of input power, plasma triangularity and impurity seeding with noble gases has been studied. Density profile peaking at high densities has been obtained in (a) impurity seeded low triangularity discharges, (b) ELMy H-modes with low levels of input power and (c) discharges fuelled with pellet injection from the high field side. New ELM behaviour has been observed in high triangularity discharges at high density, opening a possible route to ELM heat load mitigation, which can be further amplified by Ar impurity seeding. Current extrapolations of the ELM heat load to ITER show possibly a window for Type I ELM operation. Confinement scaling studies indicate an increase in confinement with triangularity and density peaking, and a decrease in confinement with the Greenwald number. In addition, experiments in H isotope and He indicate tau(E) proportional to M(0.19)Z(-0.59). The threshold power for the L-H transition in He plasmas shows the same parametric dependence as in D plasmas, but with a 50% higher absolute value.

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The thermal and electromagnetic loads related to disruptions in ITER are substantial and require careful design of tokamak components to ensure they reach the projected lifetime and to ensure that safety relevant components fulfil their function for the worst foreseen scenarios. The disruption load specifications are the basis for the design process of components like the full-W divertor, the blanket modules and the vacuum vessel and will set the boundary conditions for ITER operations. This paper will give a brief overview on the disruption loads and mitigation strategies for ITER and will discuss the physics basis which is continuously refined through the current disruption R&D programs.

The first priority for the 2013 JET campaigns will be to provide relevant data for the ITER decision on the day one divertor material selection. In particular, a tungsten melt experiment using a specially modified divertor module will study the characteristics and impact of edge localised mode (ELM) induced melting on plasma operation—only JET has large enough ELMs to perform such a test. The second priority is to better understand why the change to an all metal wall has restricted access to scenarios with high normalized energy confinement ( H -factor). This unexpected result may be partly due to the impact of the higher fuelling needed to control tungsten levels but in some scenarios there is also an effect related to the loss of a beneficial effect of carbon as a plasma impurity. In parallel with pushing JET's core plasma parameters as close as possible to those in ITER, we also intend to develop ITER demonstration pulses including as many of ITER's requirements as possible but not focusing on core performance. Another high priority item is the study of the generation and mitigation of runaway electrons which are so far not created in JET disruptions but are potentially very challenging for JET and ITER. Finally, there is still important work to be continued on key plasma–wall interaction issues ranging from material migration to fuel removal studies.

Experiments have been performed on the JET tokamak with 2nd harmonic ion cyclotron resonance heating (ICRH) of hydrogen in deuterium plasmas to assess the role of finite Larmor radius (FLR) effects on the resonant ion distribution function. More specifically, the clamping of high-energy resonant particle distribution due to weak wave-particle interaction at high energy is studied. The distributions of ICRH heated hydrogen ions have been measured with a high-energy neutral particle analyser in the range of 0.29-1.1 MeV. By changing the electron density the energy E*, around which the wave-particle interaction becomes weak, is varied. The dependence of the ion distribution on E* is experimentally observed for a number of discharges and FLR effects are clearly seen to affect the high energy tail shape. Experiments have been analysed with the combination of ICRH modelling codes PION and FIDO, including FLR effects, and good agreement with measurements have been found.

Injection of Lower Hybrid Heating and Current Drive (LHCD) into the current ramp-up phase of JET discharges has produced extremely reversed q-profiles characterized by a core region of near zero current density (within Motional Stark Effect diagnostic measurement errors). Non-inductive, off-axis co-current drive induces a back electromotive force inside the non-inductive current radius that drives a negative current in the plasma core. The core current density does not go negative, although current diffusion calculations indicate that there is sufficient LHCD to cause this. A possible explanation of the core current density clamping near zero is that MHD instabilities redistribute the core current soon after it goes negative. This is seen in reduced MHD simulations in cylindrical geometry and nonlinear resistive MHD simulations in toroidal geometry which predict that these discharges undergo n=0 reconnection events which clamp the core current near zero. Understanding the physics of the current hole in present devices is important for enabling predictions of current profile evolution in next step facilities with strong non-inductive current drive.