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Nonlinear and Non-ideal Effects on FRC Stability

Description: New computational results are presented which advance the understanding of the stability properties of the Field-Reversed Configuration (FRC). We present results of hybrid and two-fluid (Hall-MHD) simulations of prolate FRCs in strongly kinetic and small-gyroradius, MHD-like regimes. The n = 1 tilt instability mechanism and stabilizing factors are investigated in detail including nonlinear and resonant particle effects, particle losses along the open field lines, and Hall stabilization. It is shown that the Hall effect determines the mode rotation and change in the linear mode structure in the kinetic regime; however, the reduction in the growth rate is mostly due to the finite Larmor radius effects. Resonant particle effects are important in the large gyroradius regime regardless of the separatrix shape, and even in cases when a large fraction of the particle orbits are stochastic. Particle loss along the open field lines has a destabilizing effect on the tilt mode and contributes to the ion spin up in toroidal direction. The nonlinear evolution of unstable modes in both kinetic and small-gyroradius FRCs is shown to be considerably slower than that in MHD simulations. Our simulation results demonstrate that a combination of kinetic and nonlinear effects is a key for understanding the experimentally observed FRC stability properties.
Date: October 21, 2002
Creator: Belova, E.V.; Davidson, R.C.; Ji, H. & Yamada, M.
Partner: UNT Libraries Government Documents Department

Simulation Studies of the Role of Reconnection in the ''Current Hole'' Experiments in the Joint European Torus

Description: 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., ...
Date: January 21, 2003
Creator: Breslau, J.A.; Jardin, S.C. & Park, W.
Partner: UNT Libraries Government Documents Department