3D MHD simulations of stellarator plasmas with SPECYL and PIXIE3D codes

Preliminary simulations of stellarator-like configurations with the nonlinear 3D MHD codes SPECYL and PIXIE3D are reported in this work. SPECYL [1] solves the zero-b visco-resistive MHD equations in cylindrical geometry, whereas PIXIE3D [2] can run in toroidal geometry as well and with finite plasma pressure. The mathematical correctness of the two codes was proven by a nonlinear crossbenchmark study [3]. Both codes have been used to study the nonlinear MHD dynamics of reversed-field pinch (RFP) and tokamak plasmas. Qualitative agreement with respect to experimental observations in the RFX-mod device, operated both in RFP and tokamak modes, has been demonstrated, in particular with the application of external magnetic perturbations [4,5,6]. More recently, helical perturbations of the magnetic boundary have been used in both codes to produce stellarator fields within an axisymmetric computational domain [6]. Here, we first focus on zero-b stellarator configurations in both cylindrical and toroidal geometry. The equilibrium properties such as the i profile and the magnetic field topology are discussed. Magnetic islands and stochastic regions appear when the helical symmetry provided by the dominant helical perturbation is violated. As expected, this occurs in the cylindrical case when secondary magnetic perturbations are applied, and in the toroidal case due to geometric effects, even without secondary perturbations. The connection length of field lines to the wall provides a measure of edge stochasticity. Finite-b stellarator simulations are then considered with the PIXIE3D code in cylindrical geometry. The finite-b helical equilibria obtained with increasing heating sources are described and compared with the zero-b solution.