The impact of the ITER-like wall at JET on disruptions
TL;DR: The full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions, yielding higher plasma temperatures after the thermal quench and thus longer current quench times.
Abstract: The new full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions. The main difference is a significantly lower fraction (by up to a factor of 5) of energy radiated during the disruption process, yielding higher plasma temperatures after the thermal quench and thus longer current quench times. Thus, a larger fraction of the total energy was conducted to the wall resulting in larger heat loads. Active mitigation by means of massive gas injection became a necessity to avoid beryllium melting already at moderate levels of thermal and magnetic energy (i.e. already at plasma currents of 2 MA). A slower current quench, however, reduced the risk of runaway generation. Another beneficial effect of the ILW is that disruptions have a negligible impact on the formation and performance of the subsequent discharge.
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TL;DR: In this paper, a brief overview on the disruption loads and mitigation strategies for ITER and the physics basis which is continuously refined through the current disruption R&D programs is discussed.
294 citations
TL;DR: An overview of the present status of research toward the final design of the ITER disruption mitigation system (DMS) is given in this paper, where the authors discuss the physics and engineering constraints of the design of this system due to limitations on port access and the amount and species of injected impurities.
Abstract: An overview of the present status of research toward the final design of the ITER disruption mitigation system (DMS) is given. The ITER DMS is based on massive injection of impurities, in order to radiate the plasma stored energy and mitigate the potentially damaging effects of disruptions. The design of this system will be extremely challenging due to many physics and engineering constraints such as limitations on port access and the amount and species of injected impurities. Additionally, many physics questions relevant to the design of the ITER disruption mitigation system remain unsolved such as the mechanisms for mixing and assimilation of injected impurities during the rapid shutdown and the mechanisms for the subsequent formation and dissipation of runaway electron current.
220 citations
TL;DR: In this paper, the ITER-like wall (ILW) was installed in the JET to enable a direct comparison of operation with all carbon plasma facing components (PFCs) to an all metal beryllium/tungsten first-wall under identical conditions.
146 citations
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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
"The impact of the ITER-like wall at..." refers background in this paper
...P.C. de Vries, G. Arnoux, A. Huber, J. Flanagan, M. Lehnen, V. Riccardo, C. Reux, S. Jachmich, C Lowry, G. Calabro, D. Frigione, M Tsalas, N. Baumgarten, S. Brezinsek, M. Clever, D. Douai, M. Groth, T.C. Hender, E. Hodille, E. Joffrin, U. Kruezi, G.F. Matthews, J.A. Morris, R. Neu, V. Philipps, G. Sergienko, M. Sertoli and JET EFDA contributors EFDA–JET–CP(12)04/12 The Impact of the ITER-Like Wall at JET on Disruptions “This document is intended for publication in the open literature....
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...T.C. Hender et al 2007 Nuclear Fusion 47 S128 [2]....
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TL;DR: In addition to the operational limits imposed by MHD stability on plasma current and pressure, an independent limit on plasma density is observed in confined toroidal plasmas as mentioned in this paper, where all toroidal confinement devices considered operate in similar ranges of (suitably normalized) densities.
Abstract: In addition to the operational limits imposed by MHD stability on plasma current and pressure, an independent limit on plasma density is observed in confined toroidal plasmas. This review attempts to summarize recent work on the phenomenology and physics of the density limit. Perhaps the most surprising result is that all of the toroidal confinement devices considered operate in similar ranges of (suitably normalized) densities. The empirical scalings derived independently for tokamaks and reversed-field pinches are essentially identical, while stellarators appear to operate at somewhat higher densities with a different scaling. Dedicated density limit experiments have not been carried out for spheromaks and field-reversed configurations, however, `optimized' discharges in these devices are also well characterized by the same empirical law. In tokamaks, where the most extensive studies have been conducted, there is strong evidence linking the limit to physics near the plasma boundary: thus, it is possible to extend the operational range for line-averaged density by operating with peaked density profiles. Additional particles in the plasma core apparently have no effect on density limit physics. While there is no widely accepted, first principles model for the density limit, research in this area has focussed on mechanisms which lead to strong edge cooling. Theoretical work has concentrated on the consequences of increased impurity radiation which may dominate power balance at high densities and low temperatures. These theories are not entirely satisfactory as they require assumptions about edge transport and make predictions for power and impurity scaling that may not be consistent with experimental results. A separate thread of research looks for the cause in collisionality enhanced turbulent transport. While there is experimental and theoretical support for this approach, understanding of the underlying mechanisms is only at a rudimentary stage and no predictive capability is yet available.
469 citations
"The impact of the ITER-like wall at..." refers background in this paper
...M. Greenwald 2002 Plasma Physics and Controlled Fusion 44 R27 [22]....
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TL;DR: In this paper, an analysis of runaway electron formation and its evolution during disruptions in large tokamaks, where avalanche phenomena play a crucial role, is presented, but sufficiently accurate, analytical model suitable for one dimensional (1-D) transport codes is proposed.
Abstract: An analysis is presented of runaway electron formation and its evolution during disruptions in large tokamaks, where avalanche phenomena play a crucial role A simplified, but sufficiently accurate, analytical model suitable for one dimensional (1-D) transport codes is proposed Validation of the model was done by comparison with Monte Carlo calculations
429 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
"The impact of the ITER-like wall at..." refers background in this paper
...F.C. Schüller 1995 Plasma Physics and Controlled Fusion 37 A135 [4]....
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TL;DR: In this article, a survey of the causes of all 2309 JET disruptions over the last decade of JET operations was carried out to obtain a complete picture of all possible disruption causes, in order to devise better strategies to prevent or mitigate their impact.
Abstract: A survey has been carried out into the causes of all 2309 disruptions over the last decade of JET operations. The aim of this survey was to obtain a complete picture of all possible disruption causes, in order to devise better strategies to prevent or mitigate their impact. The analysis allows the effort to avoid or prevent JET disruptions to be more efficient and effective. As expected, a highly complex pattern of chain of events that led to disruptions emerged. It was found that the majority of disruptions had a technical root cause, for example due to control errors, or operator mistakes. These bring a random, non-physics, factor into the occurrence of disruptions and the disruption rate or disruptivity of a scenario may depend more on technical performance than on physics stability issues. The main root cause of JET disruptions was nevertheless due to neo-classical tearing modes that locked, closely followed in second place by disruptions due to human error. The development of more robust operational scenarios has reduced the JET disruption rate over the last decade from about 15% to below 4%. A fraction of all disruptions was caused by very fast, precursorless unpredictable events. The occurrence of these disruptions may set a lower limit of 0.4% to the disruption rate of JET. If one considers on top of that human error and all unforeseen failures of heating or control systems this lower limit may rise to 1.0% or 1.6%, respectively.
202 citations
"The impact of the ITER-like wall at..." refers background in this paper
...P.C. de Vries et al 2011 Nuclear Fusion 51 053018 [5]....
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