A quasi-isodynamic stellarator with poloidally closed contours of the magnetic field strength B (Mikhailov 2002 Nucl. Fusion 42 L23) has been obtained by an integrated physics optimization comprising MHD and neoclassical theory. For a configuration with six periods and aspect ratio approximately 12, a main result is the attainability of an essentially MHD-stable high-β (β ≈ 0.085) plasma with low neoclassical transport, approximately vanishing bootstrap current in the long-mean-free-path regime and excellent α-particle confinement.

Using analytical methods, the authors investigate the axial region of the magnetic configurations of ℓ = 3 traps (stellarator and torsatron) when a uniform transverse field is present. They determine the conditions necessary for the existence of a rosette-type structure of the magnetic surfaces in the axial region as a function of various magnetic trap parameters such as toroidality, the angle of inclination of the helical conductors and the strength of the transverse field. Expressions are found for the critical transverse magnetic field value at which the rosette-type structure disappears.

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Using analytical methods, the authors study the reasons for the formation of a magnetic-surface island structure in the central region of ℓ = 3 stellarator and torsatron magnetic configurations. Possible methods of substantially reducing the dimensions of this structure at minimum magnetic-configuration distortions are discussed. Magnetic configurations are considered both in vacuum and in the presence of a plasma.

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