Journal article

Magnetohydrodynamic stability of free-boundary quasi-axisymmetric stellarator equilibria with finite bootstrap current

The impact of the bootstrap current is investigated on the equilibrium properties of a two-period quasi-axisymmetric stellarator reactor with free boundary and on the corresponding ideal magnetohydrodynamic stability properties. Although the magnetic field strength B spectrum is dominated by a m/n = 1/0 component, the discrete filamentary coils trigger some small-amplitude symmetry-breaking components that can disturb,the quasi-symmetry of B. Finite beta causes the plasma column to shift outward in the absence of bootstrap current. With a self-consistent bootstrap current in the 1/v regime, the plasma becomes more elongated and more distorted in the horizontally elongated up-down symmetric cross section. At beta similar or equal to 3.25%, the plasma can be restored to its near-vacuum shape with the application of a vertical field with coil currents 20% of those of the modular coils, but at the expense of a significant mirror component in the B-field spectrum. The bootstrap current causes the rotational transform iota profile to increase above the critical resonant value (iota(c) = 1/2 for beta greater than or equal to 1.1%) and combines with the Pfirsch-Schluter current to destabilize a m/n = 2/1 external kink mode for beta greater than or equal to 1.8%.


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