Simulation Studies of the Role of Reconnection in the "Current Hole" Experiments in the Joint European Torus
Authors: J.A. Breslau, S.C. Jardin, and W. Park
Date of PPPL Report: January 2003
Published in: Physics of Plasmas 10, No. 5 (May 2003) 1665-1695.
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 3-D code (M3D) nonlinear 2-D and 3-D 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 i = 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 i = 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.