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Magnetohydrodynamic Stability of Free-Boundary Quasi-Axisymmetric Stellarator Equilibria with Finite Bootstrap Current

Wilfred Anthony Cooper, Sergi Ferrando i Margalet, Simon J. Allfrey, Johann Kißlinger, Horst F. G. Wobig, Yoshiro Narushima, Shoichi Okamura, Chihiro Suzuki, Kiyomasa Y. Watanabe, Kozo Yamazaki, Maxim Yu. Isaev

Fusion Science and Technology / Volume 46 / Number 2 / September 2004 / Pages 365-377

Technical Papers / Stellarators /

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 causes the plasma column to shift outward in the absence of bootstrap current. With a self-consistent bootstrap current in the 1/ regime, the plasma becomes more elongated and more distorted in the horizontally elongated up-down symmetric cross section. At [approximately 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 profile to increase above the critical resonant value (c = 1/2 for 1.1%) and combines with the Pfirsch-Schlüter current to destabilize a m/n = 2/1 external kink mode for 1.8%.

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