M. I. Mikhailov

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.

Pressure insensitivity of a high-β quasi-isodynamic stellarator

The sensitivity of a high-β quasi-isodynamic stellarator equilibrium with respect to changes of the plasma pressure is investigated. It is shown that a plasma boundary near to the high-β boundary exists with the property that the magnetohydrodynamic and drift-kinetic properties of the associated low-β plasma are similar.

The pseudo-symmetric optimization of the National Compact Stellarator Experiment

A new experiment, the National Compact Stellarator Experiment (NCSX) [Monticello et al. “Physics Consideration for the Design of NCSX,” Proceedings of 25th EPS Conference on Controlled Fusion and Plasma Physics, Prague, 1998 (European Physical Society, Petit-Lancy), paper 1.187], hopes to overcome the deleterious ripple transport usually associated with stellarators by creating a quasi-axisymmetric configuration. A quasi-axisymmetric configuration is one in which the Fourier spectrum of the magnetic field strength in so-called Boozer coordinates is dominated by the toroidal angle averaged (n=0) components. In this article the concept of pseudosymmetry is used to improve ripple transport in a four-period variant of NCSX. By definition, pseudosymmetric magnetic configurations have no locally trapped particles. To obtain a pseudosymmetric configuration, different target functions are considered. It is found that a target function equal to the area of ripple of the magnetic field magnitude along the field lin...

Global magnetohydrodynamic instabilities in the L-2M stellarator

Analysis of global magnetohydrodynamic (MHD) instabilities in the L-2M stellarator (Prokhorov General Physics Institute, Russian Academy of Sciences) is presented. The properties of free-boundary equilibria states are outlined, the stability conditions for small-scale modes are briefly discussed, and the number of trapped particles is estimated. All the magnetic configurations under study are stable against ballooning modes. It is shown that global ideal internal MHD modes can be found reliably only in Mercier unstable plasmas. In plasma that is stable with respect to the Mercier criterion, global unstable modes that are localized in the vicinity of the free plasma boundary and are not associated with any rational magnetic surface inside the plasma (the so-called peeling modes) can be found. The radial structure of all perturbations under study is almost entirely determined by the poloidal coupling of harmonics. The results of calculations are compared with the available experimental data.

Stability of global magnetohydrodynamic modes in the L-5 compact torsatron

Global magnetohydrodynamic (MHD) modes in free-boundary plasma equilibrium configurations of the L-5 six-period compact torsatron (project of the Prokhorov General Physics Institute, Russian Academy of Sciences; G. M. Batanov et al., Plasma Dev. Operat. 11, 161 (2003)) have been investigated using the CAS3D code. It is shown that global internal ideal MHD modes can be found reliably only in Mercier unstable plasmas. In all cases under study, they are intrinsically ballooning in nature, i.e., the disturbance at the inner side of the torus is significantly lower in value than that at the outer side. Besides internal modes localized near low-order rational magnetic surfaces, external modes that are localized near the free plasma boundary and are not associated with any rational magnetic surface inside the plasma (the so-called peeling modes) have been found. In contrast to a tokamak with the same aspect ratio, in which pressure-gradient edge-localized modes are ballooning in nature, the peeling modes in the 3D currentless magnetic configuration investigated in the present work are quasi-cylindrical in the flux coordinate system. The problem of plasma boundary destruction under the action of magnetic fields generated by plasma currents is also briefly discussed.

Improvement of collisionless particle confinement in a non-quasi-symmetric stellarator vacuum magnetic field

A non-quasi-symmetric stellarator vacuum magnetic field with an aspect ratio of about 11 is found in which collisionless particles are confined up to about 2/5 of the minor radius.

Quasi-helical symmetry at finite aspect ratio

Computational stellarator optimization is used to create a configuration which is quasi-helically symmetric at finite aspect ratio. For the aspect ratio per period chosen (∼2), this procedure results in benign properties throughout the plasma volume.

A new class of quasi-omnigenous configurations

An approximation to a quasi-omnigenous structure of the magnetic field strength has been numerically obtained by optimization of the magnetic configuration of a conventional heliotron/torsatron. This configuration shows very good collisionless confinement of the guiding centre orbits of fusion alpha-particles for values of up to 0.05.

Ideal MHD stability calculations for compact stellarators

Stability results for high-n ideal local pressure-driven instabilities (ballooning and interchange modes) are calculated from the COBRA and TERPSICHORE codes and compared for several low aspect ratio stellarators. Such a comparison is important because of the predominant roles that these codes are playing in the design of compact stellarators at several laboratories around the world. The code development required to reach the levels of convergence and accuracy needed for reliable operation at low aspect ratios is also described.

Approximate quasi-isodynamicity at a finite aspect ratio in a stellarator vacuum magnetic field

A stellarator vacuum field is found in which, at a finite aspect ratio (A ≈ 40), the contours of the second adiabatic invariant of nearly all particles reflected inside that surface are poloidally closed.