Linear stability studies for a quasi-axisymmetric stellarator configuration including effects of parallel viscosity, plasma flow, and resistive walls

We present linear stability studies performed with the newly developed CASTOR3D code. This code is a very flexible and versatilely applicable numerical tool to study the stability properties of 3D equilibria. Ideal external kink modes of a quasi-axisymmetric (QA) equilibrium are investigated taking into account effects of parallel viscosity, plasma flow, and resistive walls. The results are compared with those obtained for an axisymmetric approximation of this equilibrium. Two major differences in the stability properties are found: (i) the QA equilibrium is vertically stable, while the axisymmetric equilibrium is vertically unstable; (ii) a mode of the QA equilibrium only oscillates when the plasma rotates with a minimum rotation frequency, or faster. This mininum rotation frequency depends on the n ^*-type of mode (with n ^* being the dominant toroidal Fourier harmonic contributing to the mode). Resistive wall structures in combination with plasma flow, however, lead to resistive wall modes and/or plasma modes for both types of equilibria. (paper)