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Hybrid simulations of rotational instabilities in FRCs

Author: Elena V. Belova
Requested Type: Consider for Invited
Submitted: 2006-12-18 19:19:55

Co-authors: R. C. Davidson, H. Ji, M. Yamada, S. P. Gerhardt

Contact Info:
Princeton Plasma Physics Laboratory
MS 18, PO Box 451
Princeton, NJ   08543

Abstract Text:
Three-dimensional nonlinear hybrid simulations have been performed to study the ion toroidal spin-up and rotational instabilities in FRC experiments, including the effects of finite toroidal magnetic field. The initial configuration is assumed to have zero ion rotation and dimensionless parameters corresponding to the FRX-L plasma. The ion spin-up in the simulations is related to the particle loss caused by the resistive poloidal flux decay. In about 20-30 Alfven times, the ion rotational velocity becomes comparable to the ion diamagnetic velocity. The n=1 tilt instability is the most unstable linear mode for the initial (non-rotating) configuration. However, at t~30t_A, growth of the n=2 and n=3 rotational modes can be seen depending on the initial conditions. The rotational mode has the largest amplitude in the nonlinear regime. Another set of nonlinear simulations was performed including finite initial toroidal field with peak value of up to 20% of external field. It is found that the ion toroidal spin-up is not modified significantly by the finite initial toroidal magnetic field. A separate set of linearized simulations for a non-rotating FRC configuration shows that a small equilibrium toroidal field has a weak destabilizing effect on the MHD modes due to the effective reduction of the average ion Larmor radius, and correspondingly weaker finite-Larmor-radius stabilization. In contrast to the linear behavior, the finite toroidal field is found to have a significant stabilizing effect in the nonlinear phase of the simulations, reducing the growth rates and nonlinear amplitudes of the rotational modes. The physical mechanism for the toroidal field stabilization of the rotational modes is being investigated.

Characterization: A2,E5


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