Stellarator vacuum fields are designed to have a dense set of nested flux surfaces, with only small islands and stochastic regions in the interior of the confinement region. As the plasma pressure is increased, pressure driven currents appear, and they can significantly modify the vacuum field. The question arises whether pressure driven currents can cause significant flux surface breakage. Both the Wendelstein 7-AS and large helical device stellarators had dedicated experimental campaigns to study the limit in those devices. This paper reviews the evidence that the flux surfaces in a significant fraction of the plasma volume were stochastized by the pressure-driven currents in the highest experiments. That phenomenon appears to have been the dominant source of pressure-driven flux surface loss in these experiments.
Pressure-induced breaking of equilibrium flux surfaces in the W7AS stellarator