Macroscopic trends of linear tearing stability in cylindrical current profiles

Abstract

Abstract The likelihood of realising tokamak power-plants will be greatly improved by the
discovery of high-gain equilibria that resist the formation of small islands and hence
avoid the disruptive neoclassical tearing mode. We propose a series of studies to
understand how simple tokamak design can leverage aspects of tearing onset physics
to maximise passive resistance to island formation. Here we investigate the variation
that current profiles can bring about in preventing tearing onset through the cylindrical
linear tearing stability parameter ∆′. A database of 159148 realistic pilot-plant current
profiles was generated with Monte Carlo sampling, and the distribution of ∆′ values was
linked with interpretable profile characteristics. In agreement with prior theoretical
and experimental studies, ∆′ was found to be strongly correlated with the existence
and steepness of a local toroidal current well or hill, with the former destabilising and
the latter stabilising. In the absence of these two cases, the remaining ∆′ values were
linearly bounded by the toroidal current gradient at the rational surface.