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Dormido-Canto S.,Culham Science Center | Vega J.,Spanish University for Distance Education (UNED) | Ramirez J.M.,Culham Science Center | Murari A.,CIEMAT | And 4 more authors.
Nuclear Fusion | Year: 2013

Prediction of disruptions from scratch is an ITER-relevant topic. The first operations with the new ITER-like wall constitute a good opportunity to test the development of new predictors from scratch and the related methodologies. These methodologies have been based on the Advanced Predictor Of DISruptions (APODIS) architecture. APODIS is a real-time disruption predictor that is in operation in the JET real-time network. Balanced and unbalanced datasets are used to develop real-time predictors from scratch. The discharges are used in chronological order. Also, different criteria to decide when to re-train a predictor are discussed. The best results are obtained by applying a hybrid method (balanced/unbalanced datasets) for training and with the criterion of re-training after every missed alarm. The predictors are tested off-line with all the discharges (disruptive/non-disruptive) corresponding to the first three JET ITER-like wall campaigns. The results give a success rate of 93.8% and a false alarm rate of 2.8%. It should be considered that these results are obtained from models trained with no more than 42 disruptive discharges. © 2013 IAEA, Vienna.

De Vries P.C.,EURATOM | De Vries P.C.,Culham Science Center | Johnson M.F.,Culham Center for Fusion Energy | Johnson M.F.,Culham Science Center | And 10 more authors.
Nuclear Fusion | Year: 2011

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. © 2011 IAEA, Vienna.

PubMed | National Institute for Laser, Plasma and Radiation Physics, Consorzio RFX, University of Lisbon and Culham Science Center
Type: Journal Article | Journal: The Review of scientific instruments | Year: 2016

The Joint European Torus (JET) neutron profile monitor ensures 2D coverage of the gamma and neutron emissive region that enables tomographic reconstruction. Due to the availability of only two projection angles and to the coarse sampling, tomographic inversion is a limited data set problem. Several techniques have been developed for tomographic reconstruction of the 2-D gamma and neutron emissivity on JET, but the problem of evaluating the errors associated with the reconstructed emissivity profile is still open. The reconstruction technique based on the maximum likelihood principle, that proved already to be a powerful tool for JET tomography, has been used to develop a method for the numerical evaluation of the statistical properties of the uncertainties in gamma and neutron emissivity reconstructions. The image covariance calculation takes into account the additional techniques introduced in the reconstruction process for tackling with the limited data set (projection resampling, smoothness regularization depending on magnetic field). The method has been validated by numerically simulations and applied to JET data. Different sources of artefacts that may significantly influence the quality of reconstructions and the accuracy of variance calculation have been identified.

Kim H.-T.,Culham Science Center | Kim H.-T.,Imperial College London | Sips A.C.C.,Culham Science Center | Sips A.C.C.,EURATOM | Sips A.C.C.,Culham Center for Fusion Energy
Nuclear Fusion | Year: 2013

This paper presents the DYON simulations of the plasma burn-through phase at Joint European Torus (JET) with the ITER-like wall. The main purpose of the study is to validate the simulations with the International Thermonuclear Experimental Reactor (ITER)-like wall, made of beryllium. Without impurities, the burn-through process of a pure deuterium plasma is described using DYON simulations, and the criterion for deuterium burn-through is derived analytically. The plasma burn-through with impurities are simulated using wall-sputtering models in the DYON code, which are modified for the ITER-like wall. The wall-sputtering models and the validation against JET data are presented. The impact of the assumed plasma parameters in DYON simulations are discussed by means of parameter scans. As a result, the operation space of prefill gas pressure and toroidal electric field for plasma burn-through in JET is compared with the Townsend avalanche criterion. © 2013 IAEA, Vienna.

Kim H.-T.,Culham Center for Fusion Energy | Kim H.-T.,Imperial College London | Kim H.-T.,EURATOM | Fundamenski W.,Culham Center for Fusion Energy | And 5 more authors.
Nuclear Fusion | Year: 2012

In this paper, new models for a plasma burn-through simulation using the DYON code are introduced in detail, and the quantitative validation of the simulation results against JET data is presented for the first time. In order to calculate the particle confinement time, a dynamic effective connection length model including an eddy current effect is used assuming ambipolar transonic transport and the Bohm diffusion model for parallel and perpendicular particle losses, respectively. Plasma-surface interaction effects are treated with an impurity sputtering yield and an exponential saturation model of the deuterium recycling coefficient. The rate and power coefficients in the Atomic Data and Analysis Structure (ADAS) package are adopted to solve energy and particle balance. The neutral screening effects are taken into account according to particle species, and the sophisticated energy and particle balances are presented. The new burn-through simulation shows good agreement against carbon-wall JET data. This indicates that the burn-through simulation can be applied to investigate the key aspect of physics in plasma burn-through and to perform a predictive simulation for ITER start-up. © 2012 IAEA, Vienna.

PubMed | CAS Hefei Institutes of Physical Science, Hefei University of Technology, Culham Science Center and Dalian University of Technology
Type: Journal Article | Journal: Physical review letters | Year: 2016

Evidence of a nonlinear transition from mitigation to suppression of the edge localized mode (ELM) by using resonant magnetic perturbations (RMPs) in the EAST tokamak is presented. This is the first demonstration of ELM suppression with RMPs in slowly rotating plasmas with dominant radio-frequency wave heating. Changes of edge magnetic topology after the transition are indicated by a gradual phase shift in the plasma response field from a linear magneto hydro dynamics modeling result to a vacuum one and a sudden increase of three-dimensional particle flux to the divertor. The transition threshold depends on the spectrum of RMPs and plasma rotation as well as perturbation amplitude. This means that edge topological changes resulting from nonlinear plasma response plays a key role in the suppression of ELM with RMPs.

Kazakov Ye.O.,Culham Science Center | Kazakov Ye.O.,Chalmers University of Technology | Kiptily V.G.,Culham Science Center | Kiptily V.G.,EURATOM | And 4 more authors.
Nuclear Fusion | Year: 2012

Various ion cyclotron resonance heating (ICRH) scenarios relevant for the D-T phase of the JET tokamak are studied. Recent ICRH experiments in JET ( 3He)-D and ( 3He)-H plasmas confirmed the possibility of electron heating enhancement in the mode conversion (MC) regime due to the constructive interference of the reflected fast waves. Such a heating enhancement in D-T plasma is investigated first for JET-like conditions for both dipole and +π/2 ICRH antenna phasing, and for T concentration varied from 0% to 100%. It is shown that most of the MC scenarios at comparable concentrations of D and T species suffer from a parasitic absorption by fusion-born alpha-particles and NBI-produced fast ions whereas the impact of such fast ions in the minority heating (MH) ICRH schemes is substantially smaller. A possibility of ion heating enhancement due to the interference effect is shown for the MH scenarios. It is found that thermal ion heating becomes dominant in tritium-rich plasmas with T concentration ∼80%. The efficiency of ion heating in such a scenario is compared with the alternative 3He minority ICRH scenario in D:T=50:50 plasmas. © 2012 IAEA, Vienna.

Meakins A.J.,Culham Science Center | Meakins A.J.,EURATOM | McDonald D.C.,Culham Science Center | McDonald D.C.,EURATOM
Plasma Physics and Controlled Fusion | Year: 2010

Understanding the processes which establish the H-mode edge transport barrier (ETB) and the scaling of those processes with the plasma properties local to the plasma edge is of critical importance for optimizing the performance of power-station scale fusion plasmas. In this paper, data from 67 JET pulses were assembled and classified by confinement mode. A neural network classification technique was applied to identify the nature of the dependence of the L-H boundary on plasma parameters local to the plasma edge. Strong dependences on Te, ne, 〈B〉 and a weak dependence on q80 were found. In applying the neural network model as a confinement mode identification tool, the correct operating mode was identified for 98.86% of the time slices in a test data set. Using an extended data set the boundary predictions from five theoretical ETB models were evaluated. These models consider a range of different physical processes for the ETB. One model was clearly rejected. While other models proved more favourable, none of the tested models robustly described the JET L-H boundary. © 2010 IOP Publishing Ltd.

Bokshi A.,University of York | Dickinson D.,University of York | Roach C.M.,Culham Science Center | Wilson H.R.,University of York
Plasma Physics and Controlled Fusion | Year: 2016

We consider a time-dependent linear global electrostatic toroidal fluid ion-temperature gradient (ITG) model to study the evolution of toroidal drift modes in tokamak plasmas as the equilibrium flow-shear varies with time. While we consider the ITG mode as a specific example, the results are expected to be valid for most other toroidal microinstabilities. A key result is that when there is a position in the plasma with a maximum in the instability drive (e.g. ITG), there is a transient burst of stronger growth as the flow-shear evolves through a critical value. This transient burst is expected to drive a filamentary plasma eruption, reminiscent of small-ELMs. The amplitude of the dominant linear mode is initially peaked above or below the outboard midplane, and rotates through it poloidally as the flow-shear passes through the critical value. This theoretical prediction could provide an experimental test of whether this mechanism underlies some classes of small-ELMs. © 2016 IOP Publishing Ltd.

Fitzgerald M.,Culham Science Center | Hole M.J.,Australian National University | Qu Z.S.,Australian National University
Plasma Physics and Controlled Fusion | Year: 2015

In this work, we generalise linear magnetohydrodynamic (MHD) stability theory to include equilibrium pressure anisotropy in the fluid part of the analysis. A novel 'single-adiabatic' (SA) fluid closure is presented which is complementary to the usual 'double-adiabatic' (CGL) model and has the advantage of naturally reproducing exactly the MHD spectrum in the isotropic limit. As with MHD and CGL, the SA model neglects the anisotropic perturbed pressure and thus loses non-local fast-particle stabilisation present in the kinetic approach. Another interesting aspect of this new approach is that the stabilising terms appear naturally as separate viscous corrections leaving the isotropic SA closure unchanged. After verifying the self-consistency of the SA model, we re-derive the projected linear MHD set of equationsrequired for stability analysis of tokamaks in the MISHKA code. The cylindrical wave equationis derived analytically as done previously in the spectral theory of MHD and clear predictions are made for the modification to fast-magnetosonic and slow ion sound speeds due to equilibrium anisotropy. © 2015 EURATOM.

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