Observation of Zero Current Density in the Core of JET Discharges with Lower Hybrid Heating and Current Drive

Hawkes, N.; Stratton, B.; Tala, T.; Challis, C.; Conway, G. D.; DeAngelis, R. J.; Giroud, C.; Hobirk, J.; Joffrin, E.; Lomas, P. J.; Lotte, P.; Mailloux, J.; Mazon, D.; Rachlew, Elisabeth; Reyes-Cortes, S.; Solano, E.R.; Zastrow, K.‐D.

doi:10.1103/physrevlett.87.115001

Cited by 143 publications

(76 citation statements)

References 16 publications

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“…On the other hand, a reversed current density with a negative density in the plasma center has not yet been observed. 3,5,6 However, if the increase in non-inductive current is formed rapidly, the toroidal electric field, transiently negative at the central region, may create a negative central current density. 6 These scenarios open the possibility of a new class of tokamak configurations, stimulating new theoretical and experimental investigations.…”

Section: Discussionmentioning

confidence: 99%

“…Plasmas are confined in tokamaks by magnetic field lines on nested axisymmetric toroidal magnetic surfaces. 1 This occurs for both conventional tokamaks, with an inductive plasma current density with a maximum at the centre, 1 and in tokamaks with a non-inductive current density with a maximum outside the centre, as presently in JET 2,3 and predicted in the forthcoming ITER. 4 However, some advanced confinement regimes with non inductive currents develop a radial hollow current profile, with a central hole where the current density is almost zero or even negative.…”

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confidence: 99%

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Analytical solutions for Tokamak equilibria with reversed toroidal current

et al. 2011

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In tokamaks, an advanced plasma confinement regime has been investigated with a central hollow electric current with negative density which gives rise to non-nested magnetic surfaces. We present analytical solutions for the magnetohydrodynamic equilibria of this regime in terms of non-orthogonal toroidal polar coordinates. These solutions are obtained for large aspect ratio tokamaks and they are valid for any kind of reversed hollow current density profiles. The zero order solution of the poloidal magnetic flux function describes nested toroidal magnetic surfaces with a magnetic axis displaced due to the toroidal geometry. The first order correction introduces a poloidal field asymmetry and, consequently, magnetic islands arise around the zero order surface with null poloidal magnetic flux gradient. An analytic expression for the magnetic island width is deduced in terms of the equilibrium parameters. We give examples of the equilibrium plasma profiles and islands obtained for a class of current density profile.

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Section: Discussionmentioning

confidence: 99%

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confidence: 99%

Analytical solutions for Tokamak equilibria with reversed toroidal current

et al. 2011

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“…1 Motional Stark effect (MSE) measurments taken 3.1 s into this shot, following about 1.6 s of off-axis lower hybrid current drive, indicate that the core toroidal current density is essentially zero out to approximately one-third of the plasma minor radius. The electron temperature profile has a high plateau across the current hole region at this instant and the density profile is flat.…”

Section: Predictions Of the 1d Modelmentioning

confidence: 99%

“…A great deal of interest has recently been generated by reports from experimental teams at the Joint European Torus (JET) 1 and JAERI Tokamak 60 (JT-60) 2 tokamaks of a newly observed phenomenon in which, following the application of large off-axis toroidal current drive sources, a region develops in the plasma around the nominal magnetic axis in which the toroidal current density is zero to within the measurement error. A noteworthy feature of this "current hole" region is that it defies 3 the prediction made by a simple one-dimensional numerical model: that the transient effect of strong off-axis current drive will be the induction of negative current in the center, which should persist until the driven current has time to diffuse inward resistively.…”

Section: Introductionmentioning

confidence: 99%

Simulation studies of the role of reconnection in the “current hole” experiments in the Joint European Torus

2003

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Injection of lower hybrid current drive into the current ramp-up phase of Joint European Torus (JET) discharges has been observed to produce an annular current distribution with a core region of essentially zero current density [Hawkes, et al., Phys. Rev. Lett. 87, 115001 (2001)]. Similar "current holes" have been observed in Japan Atomic Energy Research Institute (JAERI) Tokamak 60 Upgrade (JT-60U) discharges with off-axis current drive supplied by bootstrap current [T. Fujita, et al., Phys. Rev. Lett. 87, 245001 (2001)]. In both cases, the central current does not go negative although current diffusion calculations indicate that there is sufficient non-inductive current drive for this to occur. This is explained by Multi-level 3D code (M3D) nonlinear 2D and 3D resistive magnetohydrodynamic (MHD) simulations in toroidal geometry, which predict that these discharges undergo n = 0 reconnection events -"axisymmetric sawteeth" -that redistribute the current to hold its core density near zero. Unlike conventional sawteeth, these events retain the symmetry of the equilibrium, and thus are best viewed as a transient loss of equilibrium caused when an ι = 0 rational surface enters the plasma. If the current density profile has a central minimum, this surface will enter on axis; otherwise it will enter off-axis. In the first case, the reconnection is limited to a small region around the axis and clamps the core current at zero. In the second case, more typical of the JET experiments, the core current takes on a finite negative value before the ι = 0 surface appears, resulting in discrete periodic axisymmetric sawtooth events with a finite minor radius. Interpretation of the simulation results is given in terms of analytic equilibrium theory, and the relation to conventional sawteeth and to a recent reduced-MHD analysis of this phenomenon in cylindrical geometry [Huysmans, et al., Phys. Rev. Lett. 87, 245002 (2001)] is discussed.

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“…These configurations are closely related to the current-hole regime observed in large tokamak experiments [5,6]. Although the AC experiments in low temperature plasmas clearly show the formation of two large magnetic islands with opposite currents during the current reversal phase [7], the magnetic topology in the plasma core of current-hole experiments remains unresolved.…”

Section: Introductionmentioning

confidence: 89%

Tokamak equilibria with strong toroidal current density reversal

Ludwig¹,

Rodrigues²,

Bizarro³

2013

Nucl. Fusion

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The equilibrium of large magnetic islands in the core of a tokamak under conditions of strong toroidal current density reversal is investigated by a new method. The method uses distinct spectral representations to describe each simply connected region as well as the containing shell geometry. This ideal conducting shell may substitute for the plasma edge region or take a virtual character representing the external equilibrium field effect. The internal equilibrium of the islands is solved within the framework of the variational moment method. Equivalent surface current densities are defined on the boundaries of the islands and on the thin containing shell, giving a straightforward formulation to the interaction between regions. The equilibrium of the island–shell system is determined by matching moments of the Dirichlet boundary conditions. Finally, the macroscopic stability against a class of tilting displacements is examined by means of an energy principle. It is found out that the up–down symmetric islands are stable to this particular perturbation and geometry but the asymmetric system presents a bifurcation in the equilibrium.

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Observation of Zero Current Density in the Core of JET Discharges with Lower Hybrid Heating and Current Drive

Cited by 143 publications

References 16 publications

Analytical solutions for Tokamak equilibria with reversed toroidal current

Analytical solutions for Tokamak equilibria with reversed toroidal current

Simulation studies of the role of reconnection in the “current hole” experiments in the Joint European Torus

Tokamak equilibria with strong toroidal current density reversal

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