Numerical Simulation of Non-Inductive Startup of the Pegasus Toroidal Experiment.

摘要

The dynamics and relaxation of magnetic flux ropes produced during non-inductive startup of the Pegasus Toroidal Experiment are simulated with nonlinear magnetohydrodynamic and two-fluid plasma models. A current filament is produced by a single injector and directed along multiple passes by toroidal and vertical vacuum magnetic field components. Adjacent passes of the current filament merge and reconnect, releasing an axisymmetric current ring from the driven channel. Squashing degree analysis indicates the presence of a quasi-separatrix layer (QSL) during ring formation, but the QSL does not solely correspond to magnetic reconnection. Chaotic scattering is also apparent from the distribution of magnetic field-line lengths. The merging of adjacent passes constitutes coherent dynamo action that affects the toroidally-averaged magnetic-field distribution. The MHD dynamo--primarily from the vertical displacement of the current channel--concentrates symmetric poloidal flux and transfers significant energy to the forming flux-rope ring. Accumulation of poloidal flux over many reconnection events contributes to the development of a poloidal magnetic field null near the central column that redirects the driven current filament, such that its path traces a toroidal surface. After cessation of the simulated current drive, temperature and current profiles broaden and closed flux surfaces form rapidly and encompass a large plasma volume. High toroidal-mode number harmonics of the magnetic energy decay preferentially, leaving a tokamak-like plasma suitable for transition to other forms of current drive. Computations with the two-fluid terms in Ohm's Law produce qualitatively similar plasma evolution to the MHD computations. However, for the computations with the two-fluid plasma model, the ion fluid significantly decouples from the electron fluid, weakening the dynamics during magnetic reconnection. This effect is quantified by comparing global and local plasma parameters in both sets of calculations. Synthetic Mirnov coils at the locations used in the experiment observe magnetic fluctuation amplitudes of order 5 % and significant activity in the 10-20 kHz range in agreement with experimental measurements. Based on the cross-power spectrograms, the two-fluid calculation more accurately reproduces the frequency response of the experimental dynamics.