Investigation of three-dimensional effects in stellarator configurations using extended MHD

The maximum achievable beta in stellarators is thought to be due to equilibrium considerations, rather than instability-induced disruption processes that limit tokamak operation. That is, the equilibrium magnetic topology changes in response to increasing pressure, resulting in production of regions of stochasticity that envelop the plasma rather than the growth of pressure-driven instabilities which cause major disruptions. Furthermore, time-dependent effects can be important as the equilibrium evolves. The primary aim of the present work is to investigate how the equilibrium evolves using the self-consistent initial value MHD code NIMROD. Here the NIMROD code is used to investigate two broad classes of 3-D magnetic topology evolution. In the first class, helically-symmetric straight stellarator configurations are heated and the deterioration of flux surfaces is observed. These cases are compared with their spoiled-symmetry counterparts. Especially interesting are cases where a pressure-driven instability appears but nonlinearly saturates as the equilibrium magnetic structure evolves. Next, in the second class of problems studied, current is driven by application of a toroidal loop voltage in a model of the Compact Toroidal Hybrid. The resulting gradients drive island formation which depends on the periodicity of the device and also on the rotational transform profile. In addition to these numerical simulations, 3-D pressure-induced magnetic islands are analytically investigated in the context of finite parallel thermal transport. The inclusion of finite parallel thermal transport has the effect of attenuating pressure effects and neoclassical effects, but this additional physics does not affect island size driven by Pfirsch-Schluter currents.