Publications (264)391.77 Total impact

Article: Tungsten transport in JET Hmode plasmas in hybrid scenario, experimental observations and modelling
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ABSTRACT: The behaviour of tungsten in the core of hybrid scenario plasmas in JET with the ITERlike wall is analysed and modelled with a combination of neoclassical and gyrokinetic codes. In these discharges, good confinement conditions can be maintained only for the first 2–3 s of the high power phase. Later W accumulation is regularly observed, often accompanied by the onset of magnetohydrodynamical activity, in particular neoclassical tearing modes (NTMs), both of which have detrimental effects on the global energy confinement. The dynamics of the accumulation process is examined, taking into consideration the concurrent evolution of the background plasma profiles, and the possible onset of NTMs. Two time slices of a representative discharge, before and during the accumulation process, are analysed with two independent methods, in order to reconstruct the W density distribution over the poloidal crosssection. The same time slices are modelled, computing both neoclassical and turbulent transport components and consistently including the impact of centrifugal effects, which can be significant in these plasmas, and strongly enhance W neoclassical transport. The modelling closely reproduces the observations and identifies inward neoclassical convection due to the density peaking of the bulk plasma in the central region as the main cause of the accumulation. The change in W neoclassical convection is directly produced by the transient behaviour of the main plasma density profile, which is hollow in the central region in the initial part of the high power phase of the discharge, but which develops a significant density peaking very close to the magnetic axis in the later phase. The analysis of a large set of discharges provides clear indications that this effect is generic in this scenario. The unfavourable impact of the onset of NTMs on the W behaviour, observed in several discharges, is suggested to be a consequence of a detrimental combination of the effects of neoclassical transport and of the appearance of an island.Nuclear Fusion 08/2014; 54(8). · 2.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A key feature of disruptions during vertical displacement events, discovered in JET in 1996, is the toroidal variation in the measured plasma current I p , i.e. the plasma current asymmetries, lasting for almost the entire current quench. The unique magnetic diagnostics at JET (full set of poloidal coils and saddle loops recorded either from two toroidally opposite or from four toroidally orthogonal locations) allow for a comprehensive analysis of asymmetrical disruptions with a large scale database. This paper presents an analysis of 4854 disruptions over an 18 year period that includes both the JET carbon (C) wall and the ITERlike (IL) wall (a mixed beryllium/tungsten first wall). In spite of the I p quench time significantly increasing for the ILwall compared to Cwall disruptions, the observed toroidal asymmetry time integral (∼ sideways force impulse), did not increase for ILwall disruptions. The I p asymmetry has a dominantly n = 1 structure. Its motion in the toroidal direction has a sporadic behaviour, in general. The distributions of the number of rotation periods are found to be very similar for both C and ILwall disruptions, and multiturn rotation was sometimes observed. The I p asymmetry amplitude has no degradation with rotation frequency for either the C or ILwall disruption. Therefore dynamic amplification remains a potentially serious issue for ITER due to possible mechanical resonance of the machine components with the rotating asymmetry.Nuclear Fusion. 01/2014; 54(7):073009.  [Show abstract] [Hide abstract]
ABSTRACT: Calculations of tearing mode stability in tokamaks split conveniently into one in an external region, where marginally stable ideal magnetohydrodynamics (MHD) is applicable, and one in a resonant layer around the rational surface where sophisticated kinetic physics is needed. These two regions are coupled by the stability parameter Δ‧. Axisymmetric pressure and current perturbations localized around the rational surface significantly alter Δ‧. Equations governing the changes in the external solution and Δ‧ are derived for arbitrary perturbations in axisymmetric toroidal geometry. These equations can be used in two ways: (i) the Δ‧ can be calculated for a physically occurring perturbation to the pressure or current; (ii) alternatively we can use these equations to calculate Δ‧ for profiles with a pressure gradient at the rational surface in terms of the value when the perturbation removes this gradient. It is the second application we focus on here since resistive magnetohydrodynamics (MHD) codes do not contain the appropriate layer physics and therefore cannot predict stability for realistic hot plasma directly. They can, however, be used to calculate Δ‧. Existing methods (Ham et al 2012 Plasma Phys. Control. Fusion 54 025009) for extracting Δ‧ from resistive codes are unsatisfactory when there is a finite pressure gradient at the rational surface and favourable average curvature because of the Glasser stabilizing effect (Glasser et al 1975 Phys. Fluids 18 875). To overcome this difficulty we introduce a specific artificial pressure flattening function that allows the earlier approach to be used. The technique is first tested numerically in cylindrical geometry with an artificial favourable curvature. Its application to toroidal geometry is then demonstrated using the toroidal tokamak tearing mode stability code T7 (Fitzpatrick et al 1993 Nucl. Fusion 33 1533) which employs an approximate analytic equilibrium. The prospects for applying this approach to resistive MHD codes such as MARSF (Liu et al 2000 Phys. Plasmas 7 3681) which utilize a fully toroidal equilibrium are discussed.Plasma Physics and Controlled Fusion 12/2013; 55(12):5015. · 2.37 Impact Factor  Nuclear Fusion 10/2013; 53(10):104008. · 2.73 Impact Factor
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ABSTRACT: Disruptions are a critical issue for ITER because of the high thermal and magnetic energies that are released on short timescales, which results in extreme forces and heat loads. The choice of material of the plasmafacing components (PFCs) can have significant impact on the loads that arise during a disruption. With the ITERlike wall (ILW) in JET made of beryllium in the main chamber and tungsten in the divertor, the main finding is a low fraction of radiation. This has dropped significantly with the ILW from 50–100% of the total energy being dissipated during disruptions in CFC wall plasmas, to less than 50% on average and down to just 10% for vertical displacement events (VDEs). All other changes in disruption properties and loads are consequences of this low radiation: long current quenches (CQs), high vessel forces caused by halo currents and toroidal current asymmetries as well as severe heat loads. Temperatures close to the melting limit have been locally observed on upper first wall structures during deliberate VDE and even at plasma currents as low as 1.5 MA and thermal energy of about 1.5 MJ only. A high radiation fraction can be regained by massive injection of a mixture of 10% Ar with 90% D2. This accelerates the CQ thus reducing the halo current and sideways impulse. The temperature of PFCs stays below 400 °C. MGI is now a mandatory tool to mitigate disruptions in closedloop operation for currents at and above 2.5 MA in JET.Nuclear Fusion 08/2013; 53(9):093007. · 2.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The new fullmetal ITERlike 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.Plasma Physics and Controlled Fusion 11/2012; 54(12):124032. · 2.37 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A numerical study is carried out, based on a simple toroidal tokamak equilibrium, to demonstrate the radial redistribution of the electromagnetic torque density, as a result of a rotating resistive plasma (linear) response to a static resonant magnetic perturbation field. The computed electromagnetic torque peaks at several radial locations even in the presence of a single rational surface, due to resonances between the rotating response, in the plasma frame, and both Alfvén and sound continuum waves. These peaks tend to merge together to form a rather global torque distribution, when the plasma resistivity is large. The continuum resonance induced net electromagnetic torque remains finite even in the limit of an ideal plasma.Physics of Plasmas 10/2012; 19(10). · 2.38 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: JET has been recently refurbished with an ITERlike Be first wall and W divertor, to study plasma wall interaction processes for ITER. In this work the new behaviour of the MHD instabilities will be characterized in the hybrid scenario, which with the Cwall in JET achieved high energy confinement, combined with good MHD stability to NTMs and ideal kinks. The same scenario developed for the ILW has produced good confinement, but interactions are observed between MHD phenomena and impurities coming from the wall. The q=1 MHD activity with the JET Cwall showed a negligible effect on plasma confinement, except NTM triggering. In some ILW hybrid pulses at the start of the heating phase a q=1 fishbone occurs, as with the Cwall, but it is often replaced by a continuous q=1 mode, with a significant reduction of confinement. ECE measurements also highlight a change from pure kink fluctuations to islands centered on q=1. NTMs have also been observed in these plasmas. Their appearance is coincident with a flattening of electron temperature profile within the island (the effect with the Cwall), but it is also correlated with enhanced radiation from the plasma core and a slow decrease of central electron temperature.10/2012;  [Show abstract] [Hide abstract]
ABSTRACT: Two possible ways of modifying the linear tearing mode index, by active magnetic feedback and by drift kinetic effects of deeply trapped particles, are analytically investigated. Magnetic feedback schemes, studied in this work, are found generally stabilizing for Δ′. The drift kinetic effects from both thermal particles and hot ions tend to reduce the power of the large solution from the outer region. This generally leads to a destabilization of Δ′ for the toroidal analytic equilibria considered here.Physics of Plasmas 09/2012; 19(9). · 2.38 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Within the single fluid theory for a toroidal, resistive plasma, the favorable average curvature effect [Glasser et al., Phys. Fluids 18, 875 (1975)], which is responsible for the strong stabilization of the classical tearing mode at finite pressure, can also introduce a strong screening effect to the externally applied resonant magnetic field. Contrary to conventional understanding, this screening, occurring at slow plasma rotation, is enhanced when decreasing the plasma flow speed. The plasma rotation frequency, below which this screening effect is observed, depends on the plasma pressure and resistivity. For the simple toroidal case considered here, the toroidal rotation frequency has to be below {approx}10{sup 5}{omega}{sub A}, with {omega}{sub A} being the Alfven frequency. In addition, the same curvature effect leads to enhanced toroidal coupling of poloidal Fourier harmonics inside the resistive layer, as well as reversing the sign of the electromagnetic torque at slow plasma flow.Physics of Plasmas 07/2012; 19(7). · 2.38 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The nonresonant magnetic braking effect induced by a nonaxisymmetric magnetic perturbation is investigated on JET and TEXTOR. The collisionality dependence of the torque induced by the n = 1, where n is the toroidal mode number, magnetic perturbation generated by the error field correction coils on JET is observed. The observed torque is located mainly in the plasma core (normalized radius ρ < 0.4) and increases with decreasing collisionality. The neoclassical toroidal plasma viscosity (NTV) torque in the collisionless regime is modelled using the numerical solution of the bounceaveraged drift kinetic equation. The calculated collisionality dependence of the NTV torque is in good agreement with the experimental observation on JET. The reason for this collisionality dependence is that the torque in the plasma core on JET mainly comes from the flux of the trapped electrons, which are still mainly in the 1/ν regime. The strongest NTV torque on JET is also located near the plasma core. The magnitude of the NTV torque strongly depends on the plasma response, which is also discussed in this paper. There is no obvious braking effect with n = 2 magnetic perturbation generated by the dynamic ergodic divertor on TEXTOR, which is consistent with the NTV modelling.Nuclear Fusion 06/2012; 52:083007. · 2.73 Impact Factor 
Article: Strong toroidal effects on tokamak tearing mode stability in the hybrid and conventional scenarios
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ABSTRACT: The hybrid scenario is thought to be an important mode of operation for the ITER tokamak. Analytic and numerical calculations demonstrate that toroidal effects at finite β have a strong influence on tearing mode stability of hybrid modes. Indeed, they persist in the large aspect ratio limit, R/a → ∞. A similar strong coupling effect is found between the m = 1, n = 1 harmonic and the m = 2, n = 1 harmonic if the minimum safety factor is less than unity. In both cases the tearing stability index, Δ' increases rapidly as β approaches ideal marginal stability, providing a potential explanation for the onset of linearly unstable tearing modes. The numerical calculations have used an improved version of the T7 code (Fitzpatrick et al 1993 Nucl. Fusion 33 1533), and complete agreement is obtained with the analytic theory for this demanding test of the code.Plasma Physics and Controlled Fusion 01/2012; 54(2):025009. · 2.37 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Two methods for calculating tearing mode stability are described in this paper. A fast method using the recently improved T7 code (Ham et al 2012 Plasma Phys. Control. Fusion54 025009) and a new method based on the MARSF MHD stability code (Liu et al 2000 Phys. Plasmas7 3681) which constructs the tearing mode solution from calculated basis functions in the full geometry of the problem. The effects of plasma toroidicity and crosssectional shaping on tearing mode stability are investigated using both of the methods; the resultant stabilizing effects are in reasonable agreement over the range of parameters investigated. The parameterspace explored includes JETlike and ITERlike plasma shaping. While T7 can be used for rapid calculations and parameter scans, the MARSF construction technique produces the more accurate value of the tearing mode stability index.Plasma Physics and Controlled Fusion 01/2012; 54(10). · 2.37 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Resonant field amplification (RFA) has been systematically measured on JET, using active MHD spectroscopy to probe plasma stability at high and low beta, and compared with theoretical predictions. RFA has been measured as a plasma response to externally applied fields. At high beta, RFA has been used to identify the ideal nowall beta limit. It was found experimentally and explained theoretically that the beta limit strongly depends on the current density and q profiles, and in particular on the qmin value, and the current density profile near the plasma edge. At low beta, RFA has been observed and analysed in detail during edgelocalized mode (ELM)free periods prior to the first ELM either after L–H transition or after long ELMfree periods during a pulse. These observations confirm that the measured increase in the RFA in some cases (e.g. at low beta) may not be connected with the nowall beta limit associated with the RWM, but may reflect a proximity to other stability thresholds. Reduction in RFA is observed during an outer mode for the first time. The first results on n = 2 probing on JET are presented.Nuclear Fusion 01/2012; 52(8). · 2.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Stability of highbeta plasmas is studied on discharges from a series of JET experiments on steadystate and hybrid advanced scenarios, with a wide range of safety factor (q) profiles and normalized beta values extending to βN = 4. Bursting and continuous forms of global n = 1 instabilities are encountered that degrade confinement or, in some cases, give rise to disruptions. Mode frequencies are well above the inverse wall time and correspond to plasma rotation at around midradius. Stability boundaries in terms of qmin and pressure peaking are examined. For relatively broad pressure profiles the stability limit decreases from βN = 4 at qmin = 1 to βN = 2 at qmin = 3, while at fixed qmin it decreases with increasing pressure peaking. Metastable and unstable regions are identified in the βN–qmin diagram by modetrigger analysis. Tearing and kink mode structures are found from phase analysis of temperature profile oscillations; for a selection of kink cases, instability conditions and mode structure are compared with ideal stability calculations.Nuclear Fusion 01/2012; · 2.73 Impact Factor  Nuclear Fusion 09/2011; 51(9):094013. · 2.73 Impact Factor
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ABSTRACT: The equilibrium equation for a rotating plasma is constructed supposing the thermal Mach number is much less than unity. The canonical profile of angular rotation velocity is defined as the profile which minimizes the total plasma energy while conserving toroidal current and obeying the equilibrium condition. The transport model based on this canonical profile, with stiffness calibrated by JET ELMy Hmode and hybrid mode data, reasonably describes the velocity of the forced toroidal rotation. The RMS deviations of the calculated rotation profiles from the experimental ones do not exceed 10–15%. The developed model is also applied to the modeling of MAST rotation.Plasma Physics and Controlled Fusion 06/2011; 53(8):085025. · 2.37 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The resonant magnetic perturbation (RMP) fields, including the plasma response, are computed within a linear, full toroidal, singlefluid resistive magnetohydrodynamic (MHD) model, and under realistic plasma conditions for MAST and ITER. The response field is found to be considerably reduced, compared with the vacuum field produced by the magnetic perturbation coils. This field reduction relies strongly on the screening effect from the toroidal plasma rotation. Computations also quantify threedimensional (3D) distortions of the plasma surface, caused by RMP fields. A correlation is found between the computed mode structures, the plasma surface displacement and the observed density pumpout effect in MAST experiments. Generally, the density pumpout tends to occur when the surface displacement peaks near the Xpoints.Nuclear Fusion 06/2011; 51(8):083002. · 2.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The highbeta capability of the spherical tokamak, coupled with a suite of worldleading diagnostics on MAST, has facilitated significant improvements in the understanding of performancelimiting core instabilities in high performance plasmas. For instance, the newly installed motional Stark effect diagnostic, with radial resolution <25 mm, has enabled detailed study of saturated longlived modes in hybrid scenarios. Similarly, the upgraded Thomson scattering system, with radial resolution <10 mm and the possibility of temporal resolution of 1 µs, has allowed detailed analysis of the density and temperature profiles during transient activity in the plasma, such as at a sawtooth crash. High resolution charge exchange recombination spectroscopy provided measurement of rotation braking induced by both applied magnetic fields and by magnetohydrodynamic (MHD) instabilities, allowing tests of neoclassical toroidal viscosity theory predictions. Finally, MAST is also equipped with internal and external coils that allow nonaxisymmetric fields to be applied for active MHD spectroscopy of instabilities near the nowall beta limit. MAST has been able to operate above the pressure at which the resonant field amplification is observed to strongly increase. In order to access such high pressures, the resistive wall mode must be damped, and so numerical modelling has focused on assessing the kinetic damping of the mode and its nonlinear interaction with other instabilities. The enhanced understanding of the physical mechanisms driving deleterious MHD activity given by these leadingedge capabilities has provided guidance to optimize operating scenarios for improved plasma performance.Nuclear Fusion 06/2011; 51(7):073040. · 2.73 Impact Factor 
Article: Survey of disruption causes at JET
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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, nonphysics, 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 neoclassical 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.Nuclear Fusion 04/2011; 51(5):053018. · 2.73 Impact Factor
Publication Stats
2k  Citations  
391.77  Total Impact Points  
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Institutions

2000–2011

Culham Centre for Fusion Energy
AbingdononThames, England, United Kingdom


1984–2011

Oak Ridge National Laboratory
 Fusion Energy Division
Oak Ridge, FL, United States 
Southwestern Institute of Physics
Huayang, Sichuan, China


2007

University of Wisconsin, Madison
 Department of Engineering Physics
Mississippi, United States 
United Kingdom Atomic Energy Authority
AbingdononThames, England, United Kingdom


2004

Jesus College, Cambridge
Cambridge, England, United Kingdom 
Forschungszentrum Jülich
 Zentralabteilung für Chemische Analysen (ZCH)
Düren, North RhineWestphalia, Germany


2003

Chalmers University of Technology
Goeteborg, Västra Götaland, Sweden 
Princeton University
 Princeton Plasma Physics Laboratory
Princeton, New Jersey, United States


1981

Royal Holloway, University of London
Эгхем, England, United Kingdom
