T. C. Hender

Culham Centre for Fusion Energy, Abingdon-on-Thames, England, United Kingdom

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Publications (289)396.91 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The behaviour of tungsten in the core of hybrid scenario plasmas in JET with the ITER-like 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 magneto-hydrodynamical 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 cross-section. 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
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    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 ITER-like (IL) wall (a mixed beryllium/tungsten first wall). In spite of the I p quench time significantly increasing for the IL-wall compared to C-wall disruptions, the observed toroidal asymmetry time integral (∼ sideways force impulse), did not increase for IL-wall 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 IL-wall disruptions, and multi-turn rotation was sometimes observed. The I p asymmetry amplitude has no degradation with rotation frequency for either the C- or IL-wall 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.
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    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 MARS-F (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
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    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 plasma-facing components (PFCs) can have significant impact on the loads that arise during a disruption. With the ITER-like 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 closed-loop operation for currents at and above 2.5 MA in JET.
    Nuclear Fusion 08/2013; 53(9):093007. · 2.73 Impact Factor
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    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.
    Plasma Physics and Controlled Fusion 11/2012; 54(12):124032. · 2.37 Impact Factor
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    ABSTRACT: A numerical study is carried out, based on a simple toroidal tokamak equilibrium, to demonstrate the radial re-distribution 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
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    ABSTRACT: JET has been recently refurbished with an ITER-like 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 C-wall 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 C-wall 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 C-wall, 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 C-wall), but it is also correlated with enhanced radiation from the plasma core and a slow decrease of central electron temperature.
    10/2012;
  • Yueqiang Liu, R. J. Hastie, T. C. Hender
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    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
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    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
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    ABSTRACT: The non-resonant magnetic braking effect induced by a non-axisymmetric 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 bounce-averaged 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
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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
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    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 MARS-F 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 cross-sectional 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 parameter-space explored includes JET-like and ITER-like plasma shaping. While T7 can be used for rapid calculations and parameter scans, the MARS-F 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
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    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 no-wall 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 edge-localized mode (ELM)-free periods prior to the first ELM either after L–H transition or after long ELM-free 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 no-wall 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
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    ABSTRACT: Stability of high-beta plasmas is studied on discharges from a series of JET experiments on steady-state 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 mid-radius. 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 mode-trigger 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: Disruptions in tokamaks lead to high heat loads onto the plasma facing components (PFC). Two processes, of particular concern for the first wall integrity, have been studied in dedicated experiments at JET: (1) During the thermal quench, it is measured (using fast IR thermography) that 5% of the plasma stored energy is deposited onto the outer and inner poloidal limiters. More surprisingly, during the current quench, about 10% of the magnetic energy is deposited onto the outer and inner poloidal limiters via plasma wall interaction. (2) Very localised heat loads due to runaway electrons, generated in disruptions triggered by massive injection of argon and neon, are measured onto the JET upper dump plate. The temperature increase measured on the CFC tiles scales with the square of the runaway current.
    Journal of Nuclear Materials 08/2011; · 2.02 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 H-mode 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
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    ABSTRACT: The resonant magnetic perturbation (RMP) fields, including the plasma response, are computed within a linear, full toroidal, single-fluid 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 three-dimensional (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 pump-out effect in MAST experiments. Generally, the density pump-out tends to occur when the surface displacement peaks near the X-points.
    Nuclear Fusion 06/2011; 51(8):083002. · 2.73 Impact Factor
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    ABSTRACT: The high-beta capability of the spherical tokamak, coupled with a suite of world-leading diagnostics on MAST, has facilitated significant improvements in the understanding of performance-limiting 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 long-lived 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 non-axisymmetric fields to be applied for active MHD spectroscopy of instabilities near the no-wall 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 leading-edge capabilities has provided guidance to optimize operating scenarios for improved plasma performance.
    Nuclear Fusion 06/2011; 51(7):073040. · 2.73 Impact Factor

Publication Stats

2k Citations
396.91 Total Impact Points

Institutions

  • 2000–2011
    • Culham Centre for Fusion Energy
      Abingdon-on-Thames, England, United Kingdom
  • 1984–2011
    • Oak Ridge National Laboratory
      • Fusion Energy Division
      Oak Ridge, FL, United States
    • Southwestern Institute of Physics
      Hua-yang, Sichuan, China
  • 2007
    • University of Wisconsin, Madison
      • Department of Engineering Physics
      Mississippi, United States
    • United Kingdom Atomic Energy Authority
      Abingdon-on-Thames, England, United Kingdom
  • 2004
    • Jesus College, Cambridge
      Cambridge, England, United Kingdom
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Düren, North Rhine-Westphalia, 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