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

Abstract

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.

Non-existence of normal tokamak equilibria with negative central current

“…Of course this singular configuration is strongly susceptible to magnetic reconnection when resistivity is included, consistent with the interpretation of rapid reconnection observed in simulations. 2,8 If finite resistivity is included, then the singularity in the field is smoothed out and loss of equilibrium can help drive reconnecting flows.…”

“…Recent non-linear toroidal resistive MHD ͑magnetohy-drodynamic͒ simulations 2,8 predict that current hole discharges undergo rapid nϭ0 reconnection events ͑axisymmetric sawteeth͒ that clamp the core current near zero. More generally, this reconnection occurs whenever the current density profile is such that the rotational transform, , goes to zero on any surface in the plasma ͑this includes the case where the current density on-axis is positive, but the current profile goes sufficiently negative somewhere off-axis that the total current enclosed by some flux surface vanishes͒.…”

“…Reduced MHD simulations in cylindrical geometry have also shown that nϭ0 resistive kink instabilities can clamp the core current density at zero when it attempts to go negative. 9 Breslau et al 8 and Stratton et al 2 stated that a second-order, large aspect ratio expansion of the MHD equations indicates that a normal toroidal equilibrium is not possible if crosses through 0 at some radius. ͑They also stated that a more general proof is needed, which we provide here.͒ A recent paper by Chu and Parks 10 used a second-order aspect ratio expansion to prove that a normal equilibrium with a peaked pressure profile is not possible with negative core current.…”

Nonlinear simulation of toroidal Alfvén eigenmode with source and sink

“…The code has been successfully applied to study a variety of MHD instabilities in tokamaks and stellarators. [6][7][8][9][10][11][12] In this work, the kinetic/magnetohydrodynamic ͑MHD͒ hybrid model of M3D is used. In the model, the thermal plasmas are treated as single fluid and the energetic particles are described using the drift-kinetic equation.…”

Tokamak equilibria with strong toroidal current density reversal

“…The role of reconnection in the formation of current holes has been studied before by means of resistive magnetohydrodynamic (MHD) simulations in toroidal geometry [21]. It was argued that reconnection events redistribute the current in the plasma core holding the current density near zero, preventing strong reversal and islands formation in the center of the discharge.…”