The influence of an ITER-like wall on disruptions at JET
TL;DR: De Vries et al. as discussed by the authors conducted a survey to identify the root causes, the chain-of-events and classifying each disruption, similar to a previous analysis for carbon-wall operations.
Abstract: In order to preserve the integrity of large tokamaks such as ITER, the number of disruptions has to be limited. JET has operated previously with a low frequency of disruptions (i.e., disruption rate) of 3.4% [P. C. de Vries et al., Nucl. Fusion 51, 053018 (2011)]. The start of operations with the new full-metal ITER-like wall at JET showed a marked rise in the disruption rate to 10%. A full survey was carried out to identify the root causes, the chain-of-events and classifying each disruption, similar to a previous analysis for carbon-wall operations. It showed the improvements made to avoid various disruption classes, but also indicated those disruption types responsible for the enhanced disruption rate. The latter can be mainly attributed to disruptions due to too high core radiation but also due to density control issues and error field locked modes. Detailed technical and physics understanding of disruption causes is essential for devising optimized strategies to avoid or mitigate these events.
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TL;DR: In this article, the impact of ICRF heating on core impurity content in a variety of 2.5 MA JET-ILW H-mode plasmas was presented.
Abstract: Ion cyclotron resonance frequency (ICRF) heating has been an essential component in the development of high power H-mode scenarios in the Jet European Torus ITER-like wall (JET-ILW). The ICRF performance was improved by enhancing the antenna-plasma coupling with dedicated main chamber gas injection, including the preliminary minimization of RF-induced plasma-wall interactions, while the RF heating scenarios where optimized for core impurity screening in terms of the ion cyclotron resonance position and the minority hydrogen concentration. The impact of ICRF heating on core impurity content in a variety of 2.5 MA JET-ILW H-mode plasmas will be presented, and the steps that were taken for optimizing ICRF heating in these experiments will be reviewed.
70 citations
TL;DR: The European roadmap to the realization of fusion electricity breaks the quest into eight missions, and for each mission, it reviews the current status of research, identifies open issues, and proposes a research and development programme.
Abstract: The European roadmap to the realization of fusion electricity breaks the quest into eight missions. For each mission, it reviews the current status of research, identifies open issues, and proposes...
51 citations
General Atomics1, Columbia University2, ITER3, Massachusetts Institute of Technology4, Lawrence Livermore National Laboratory5, École Polytechnique Fédérale de Lausanne6, Princeton Plasma Physics Laboratory7, Max Planck Society8, University of Cagliari9, Lehigh University10, University of Milan11, Chinese Academy of Sciences12
TL;DR: In this article, the authors present a survey of the authors of this article: J.C. Buttery, P.J. Baruzzo, J.M.Baruzzo et al.
Abstract: E.J. Strait, J.L. Barr, M. Baruzzo, J.W. Berkery, R.J. Buttery, P.C. de Vries, N.W. Eidietis, R.S. Granetz, J.M. Hanson, C.T. Holcomb, D.A. Humphreys, J.H. Kim, E. Kolemen, M. Kong, M.J. Lanctot, M. Lehnen, E. Lerche, N.C. Logan, M. Maraschek, M. Okabayashi, J.K. Park, A. Pau, G. Pautasso, F.M. Poli, C. Rea, S.A. Sabbagh, O. Sauter, E. Schuster, U.A. Sheikh, C. Sozzi, F. Turco, A.D. Turnbull, Z.R. Wang, W.P. Wehner, L. Zeng
51 citations
TL;DR: In this paper, a hierarchy of actions consisting of (I) recovery of the discharge to full performance or at least continuation with a less disruption-prone backup scenario, (II) complete avoidance of the disruption to sustain the discharge, (III) only as last resort, a disruption mitigation.
Abstract: Routine reaction to approaching disruptions in tokamaks is currently largely limited to machine protection by mitigating an ongoing disruption, which remains a basic requirement for ITER and DEMO [1]. Nevertheless, a mitigated disruption still generates stress to device. Additionally, in future fusion devices, high-performance discharge time itself will be very valuable. Instead of reacting only on generic features, occurring shortly before the disruption, the ultimate goal is to actively avoid approaching disruptions at an early stage, sustain the discharges whenever possible and restrict mitigated disruptions to major failures. Knowledge of the most relevant root causes and the corresponding chain of events leading to disruption, the disruption path, is a prerequisite. For each disruption path, physics-based sensors and adequate actuators must be defined and their limitations considered. Early reaction facilitates the efficiency of the actuators and enhances the probability of a full recovery. Thus, sensors that detect potential disruptions in time are to be identified. Once the entrance into a disruption path is detected, we propose a hierarchy of actions consisting of (I) recovery of the discharge to full performance or at least continuation with a less disruption-prone backup scenario, (II) complete avoidance of the disruption to sustain the discharge or at least delay it for a controlled termination and, (III), only as last resort, a disruption mitigation. Based on the understanding of disruption paths, a hierarchical and path-specific handling strategy must be developed. Such schemes, testable in present devices, could serve as guideline for ITER and DEMO operation. For some disruption paths, experiments have been performed at ASDEX Upgrade and TCV. Disruptions were provoked in TCV by impurity injection into ELMy H-mode discharges and in ASDEX Upgrade by forcing a density limit in H-mode discharges. The new approach proposed in this paper is discussed for these cases. For the H-mode density limit sensors used so far react too late. Thus a plasma-state boundary is proposed, that can serve as an adequate early sensor for avoiding density limit disruptions in H-modes and for recovery to full performance.
43 citations
References
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European Atomic Energy Community1, General Atomics2, Columbia University3, Chalmers University of Technology4, École Polytechnique Fédérale de Lausanne5, Massachusetts Institute of Technology6, ITER7, Max Planck Society8, Japan Atomic Energy Agency9, University of Wisconsin-Madison10, Eindhoven University of Technology11, Princeton Plasma Physics Laboratory12, Forschungszentrum Jülich13, Kurchatov Institute14, Keldysh Institute of Applied Mathematics15, Polytechnic University of Turin16
TL;DR: A review of recent advances in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document (1999 Nucl. Fusion 39 2137-2664), is reviewed in this paper.
Abstract: Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document (1999 Nucl. Fusion 39 2137-2664), is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER baseline ELMy H-mode scenario, but the tools exist to avoid or control them through localized current drive or fast ion generation. Active control of other MHD instabilities will most likely be also required in ITER. Extrapolation from existing experiments indicates that stabilization of neoclassical tearing modes by highly localized feedback-controlled current drive should be possible in ITER. Resistive wall modes are a key issue for advanced scenarios, but again, existing experiments indicate that these modes can be stabilized by a combination of plasma rotation and direct feedback control with non-axisymmetric coils. Reduction of error fields is a requirement for avoiding non-rotating magnetic island formation and for maintaining plasma rotation to help stabilize resistive wall modes. Recent experiments have shown the feasibility of reducing error fields to an acceptable level by means of non-axisymmetric coils, possibly controlled by feedback. The MHD stability limits associated with advanced scenarios are becoming well understood theoretically, and can be extended by tailoring of the pressure and current density profiles as well as by other techniques mentioned here. There have been significant advances also in the control of disruptions, most notably by injection of massive quantities of gas, leading to reduced halo current fractions and a larger fraction of the total thermal and magnetic energy dissipated by radiation. These advances in disruption control are supported by the development of means to predict impending disruption, most notably using neural networks. In addition to these advances in means to control or ameliorate the consequences of MHD instabilities, there has been significant progress in improving physics understanding and modelling. This progress has been in areas including the mechanisms governing NTM growth and seeding, in understanding the damping controlling RWM stability and in modelling RWM feedback schemes. For disruptions there has been continued progress on the instability mechanisms that underlie various classes of disruption, on the detailed modelling of halo currents and forces and in refining predictions of quench rates and disruption power loads. Overall the studies reviewed in this chapter demonstrate that MHD instabilities can be controlled, avoided or ameliorated to the extent that they should not compromise ITER operation, though they will necessarily impose a range of constraints.
1,051 citations
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TL;DR: In this paper, the authors describe a two-stage MHD instability, where the first stage is caused by a loss of energy from the central region and the second stage is due to an increase in impurity radiation.
Abstract: In JET, both high density and low-q operation are limited by disruptions. The density limit disruptions are caused initially by impurity radiation. This causes a contraction of the plasma temperature profile and leads to an MHD unstable configuration. There is evidence of magnetic island formation resulting in minor disruptions. After several minor disruptions, a major disruption with a rapid energy quench occurs. This event takes place in two stages. In the first stage there is a loss of energy from the central region. In the second stage there is a more rapid drop to a very low temperature, apparently due to a dramatic increase in impurity radiation. The final current decay takes place in the resulting cold plasma. During the growth of the MHD instability the initially rotating mode is brought to rest. This mode locking is believed to be due to an electromagnetic interaction with the vacuum vessel and external magnetic field asymmetries. The low-q disruptions are remarkable for the precision with which they occur at qψ = 2. These disruptions do not have extended precursors or minor disruptions. The instability grows and locks rapidly. The energy quench and current decay are generally similar to those of the density limit.
410 citations
TL;DR: In this article, the successful installation of the JET ITER-like wall and the realization of its technical objectives is reported, and an overview of the planned experimental program which has been optimized to exploit the new wall and other JET enhancements in 2011/12.
Abstract: 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.
281 citations
TL;DR: The CRONOS suite as mentioned in this paper is a suite of numerical codes for the predictive/interpretative simulation of a full tokamak discharge, which integrates, in a modular structure, a 1D transport solver with general 2D magnetic equilibria, several heat, particle and impurities transport models, as well as heat and particle and momentum sources.
Abstract: CRONOS is a suite of numerical codes for the predictive/interpretative simulation of a full tokamak discharge. It integrates, in a modular structure, a 1D transport solver with general 2D magnetic equilibria, several heat, particle and impurities transport models, as well as heat, particle and momentum sources. This paper gives a first comprehensive description of the CRONOS suite: overall structure of the code, main available models, details on the simulation workflow and numerical implementation. Some examples of applications to the analysis of experimental discharges and the predictions of ITER scenarios are also given.
252 citations
TL;DR: The cause and dynamics of disruptions will be described for the many different scenarios that these violent events can follow andibilities will be discussed to avoid or at least to ameliorate the damaging effects of disruptions.
Abstract: Disruptions and related vertical displacement events pose a major problem to the design and operation of future tokamak reactors. The cause and dynamics of disruptions will be described for the many different scenarios that these violent events can follow. Possibilities will be discussed to avoid or at least to ameliorate the damaging effects of disruptions.
209 citations