Maxim Yu. Isaev

Improved alpha-particle confinement in stellarators with poloidally closed contours of the magnetic field strength

The possibility of fulfilling the condition of poloidal closure of the contours of the second adiabatic invariant for all reflected particles is studied for systems with poloidally closed contours of the magnetic field B on the magnetic surfaces through computational stellarator optimization. It is shown that by adjusting the geometry this is possible in a major fraction of the plasma volume. The most salient characteristic (as compared to previous quasi-isodynamic configurations) is a magnetic axis whose curvature vanishes in all cross sections with an extremum of B on the magnetic axis and renders possible a three-dimensional structure of B with unprecedentedly high collisionless α-particle confinement.

Ideal magnetohydrodynamic stability of the NCSX

Abstract The ideal magnetohydrodynamic (MHD) stability of the National Compact Stellarator Experiment (NCSX) is extensively analyzed using the most advanced three-dimensional MHD codes. It is shown that the NCSX is stable to finite-n MHD modes, including the vertical mode, external kink modes and ballooning modes. However, high-n external kink modes that peak near the plasma edge are found to be weakly unstable. A global calculation shows that finite-n ballooning modes are significantly more stable than the local infinite-n modes.

Bootstrap current and quasi-symmetry in reactor-size stellarators

Abstract The impact of the bootstrap current (BC) has become an important issue in the modeling of quasi-symmetric stellarator devices. Magnetohydrodynamic equilibria have been calculated with self-consistent BC in the collisionless 1/ε regime for characteristic quasi-symmetric configurations: a three-period quasi-axisymmetric and a four-period quasi-helically symmetric stellarator. The relationship between magnetic geometry and BC is shown along with its effect on the equilibrium when β is increased. The relevance of the nonsymmetric modes is also investigated for both configurations. In each case, the effect on stability has been studied.

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

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

Quasi‐isodynamic Configuration With N = 12 and High β

Results of an optimization toward quasi‐isodynamicity for a stellarator with a comparatively large number of periods, N = 12, are presented. The following set of physics properties to be achieved was used: 1) good long‐time collisionless confinement of α‐particles; 2) small neoclassical transport in the 1/ν regime; 3) small bootstrap current; 4) high stability‐β limit. As a result, the boundary magnetic surface of a configuration is found that satisfies the above requirements for 〈β〉 ≈ 20%.