Plasma Physics Seminar ( Phys 769)

Stefan Gerhardt, PPPL, Princeton University

Studies of Free Boundary Field-Reversed Configurations with Improved Stability in the Magnetic Reconnection Experiment

A recent campaign of field reversed configuration (FRC) studies in the Magnetic Reconnection Experiment (MRX) has produced MHD-regime oblate FRCs with improved stability to low-n MHD modes, with an associated extension of the plasma performance. These FRCs are produced by the merging of two spheromaks with anti-parallel toroidal fields. The plasma shape in controlled through three independently energized pairs of equilibrium field (EF) coils; we parameterize the shape of the equilibrium field by its mirror ratio. A conducting center column has been used in some cases to provide important stabilization of the spheromaks during formation and translation before merging, and provides some stabilization of the final FRC state. The FRC without a conducting center column is observed to be unstable to n=1 tilt/shift motions. The dangerous n=1 tilt is suppressed by the center column except at very low mirror ratio, but n=2 & 3 modes often remain. These modes can be reduced by forming plasmas with very large EF mirror ratio. These very oblate plasmas have a minimal amplitude of low-n perturbations, and the longest lifetime. Note that both shape control and passive stabilization are required: plasmas whose n=1 activity is reduced by the central conductor may still be quickly terminated by n=2 activity if the mirror ratio is not sufficiently large. Free-boundary equilibria for these FRC plasmas are computed with a new Grad-Shafranov solver, which constrains the equilibrium solution to the available data. The calculations indicate that as the EF mirror ratio varies from 2 to 4.5, the elongation (kappa) varies from 1 to 0.6 and the triangularity (delta) varies from 0.5 to -0.4. The equilibria are used in a model for rigid-body tilt/shift motions, which indicates that these oblate FRCs are in the MHD tilt-stable regime. Initial 3D MHD calculations with the HYM code indicate improved stability to both radially and axially polarized low-n instabilities.

Movies from some of these simulations are available here