Simulation Studies of the Role of Reconnection in the ''Current Hole'' Experiments in the Joint European Torus
Injection of lower-hybrid current drive into the current ramp-up phase of the Joint European Torus (JET) plasma 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 (2001) 115001]. Similar ''current holes'' have been observed in the Japan Atomic Energy Research Institute (JAERI) Tokamak 60 Upgrade (JT-60U) plasma discharges with off-axis current drive supplied by the bootstrap current [T. Fujita, et al., Phys. Rev. Lett. 87 (2001) 245001]. In both cases, the central current does not go negative although current diffusion calculations indicate that there is sufficient noninductive current drive for this to occur. This is explained by the Multi-level 3-D code (M3D) nonlinear 2-D and 3-D resistive magnetohydrodynamic (MHD) simulations in toroidal geometry, which predict that these plasma discharges undergo n = 0 reconnection events--''axisymmetric sawteeth''--that redistribute th e 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 iota = 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 iota = 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 (2001) 245002] is discussed.
- Research Organization:
- Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
- Sponsoring Organization:
- USDOE Office of Science (US)
- DOE Contract Number:
- AC02-76CH03073
- OSTI ID:
- 809965
- Report Number(s):
- PPPL-3770; TRN: US0302663
- Resource Relation:
- Other Information: PBD: 21 Jan 2003
- Country of Publication:
- United States
- Language:
- English
Similar Records
The poloidal distribution of turbulent fluctuations in the Mega-Ampere Spherical Tokamak
Density perturbation mode structure of high frequency compressional and global Alfvén eigenmodes in the National Spherical Torus Experiment using a novel reflectometer analysis technique
Related Subjects
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
BOOTSTRAP CURRENT
CURRENT DENSITY
DIFFUSION
DISTRIBUTION
GEOMETRY
JAERI
MAGNETOHYDRODYNAMICS
MODE RATIONAL SURFACES
PLASMA
SIMULATION
SYMMETRY
TRANSIENTS
CURRENT DRIVE
NUMERICAL STIMULATION
RECONNECTION