Physics of compact stellarators

Abstract

Recent progress in the theoretical understanding and design of compact stellarators is described. Hybrid devices, which depart from canonical stellarators by deriving benefits from the bootstrap current which flows at finite beta, comprise a class of low aspect ratio AϽ4 stellarators. They possess external kink stability ͑at moderate beta͒ in the absence of a conducting wall, possible immunity to disruptions through external control of the transform and magnetic shear, and they achieve volume-averaged ballooning beta limits ͑4%-6%͒ similar to those in tokamaks. In addition, bootstrap currents can reduce the effects of magnetic islands ͑self-healing effect͒ and lead to simpler stellarator coils by reducing the required external transform. Powerful physics and coil optimization codes have been developed and integrated to design experiments aimed at exploring compact stellarators. The physics basis for designing the national compact stellarator will be discussed.

The weak effect of static, externally imposed, helical fields on fusion product confinement in the DIII-D tokamak

“…A conceptually related idea is to employ waves to extract energy from the alpha population prior to thermalization (called 'alpha channelling') [11]. The work reported here is also relevant to the confinement of alpha particles in compact stellarators [12], since some high beta stellarators combine the features of a tokamak with the external helical windings characteristic of a stellarator. This is not the first study of the effect of perturbation fields on the confinement of energetic ions.…”

Significance of MHD Effects in Stellarator Confinement

“…"Reverse engineering" has constituted an innovative approach to meet desired physics targets, as realized in the WENDELSTEIN W7-X (Helias concept) [9] and the compact quasi-symmetric stellarator (QS) design (e.g. NCSX, HSX) [10,11,12]. On the other hand, configuration optimization within the device flexibility has led to considerably improved plasma confinement in the Large Helical Device (LHD, Heliotron device) without compromising stability significantly [3,13].…”

Computational mathematics and physics of fusion reactors

“…They have played an important role in the design of the W7-X experiment at Greifswald in Europe and the National Compact Stellarator Experiment at the Princeton Plasma Physics Laboratory (10,11). The codes are mathematical tools that have enabled physicists to exploit a concept of two-dimensional quasisymmetry of the magnetic field providing good drift surfaces of trapped particles.…”