Magnetohydodynamics stability of compact stellarators

Abstract

Recent stability results of external kink modes and vertical modes in compact stellarators are presented. The vertical mode is found to be stabilized by externally generated poloidal flux. A simple stability criterion is derived in the limit of large aspect ratio and constant current density. For a wall at infinite distance from the plasma, the amount of external flux needed for stabilization is given by F i = (κ 2 − κ)/(κ 2 + 1), where κ is the axisymmetric elongation and F i is the fraction of the external rotational transform. A systematic parameter study shows that the external kink mode in QAS can be stabilized at high beta (∼ 5%) without a conducting wall by magnetic shear via 3D shaping. It is found that external kinks are driven by both parallel current and pressure gradient. The pressure contributes significantly to the overall drive through the curvature term and the Pfirsch-Schluter current.

Robustness and flexibility in compact quasiaxial stellarators: Global ideal magnetohydrodynamic stability and energetic particle transport

“…While good transport and MHD stability are not anticorrelated in these thirty equilibria, stability only results from a delicate balance of the pressure and shear stabilization forces. Although the baseline design has been shown to be robustly stable relative to the tokamak vertical instability [15], we have shown in Secion III that variations in the pressure and ι profiles can lead to "vertical" (N=0 kink mode) instability, if the boundary and β are kept constant. It is important to note that this "vertical", N=0, instability arises only for plasmas which are also kink (N=1) mode unstable.…”

“…Recently CAS3D has been used to verify and extend calculations [14,15] of the TERPSICHORE code, showing stability of the kink (N=1) and periodicity-preserving (N=0) modes for the proposed stellarator, even without a conducting wall [15]. The two codes have been extensively benchmarked against tokamak and quasiaxial stellarator equilibia and have been found in good agreement [16].…”

Robustness and flexibility in compact quasiaxial stellarators: Global ideal MHD stability and energetic particle transport

“…While good transport and MHD stability are not anticorrelated in these thirty equilibria, stability only results from a delicate balance of the pressure and shear stabilization forces. Although the baseline design has been shown to be robustly stable relative to the tokamak vertical instability [13], we have shown in Secion III that variations in the pressure and ι profiles can lead to "vertical" (N=0 kink mode) instability, if the boundary and β are kept constant. It is important to note that this "vertical", N=0, instability arises only for plasmas which are also kink (N=1) mode unstable.…”

“…Recently CAS3D has been used to verify and extend calculations [12,13] of the TERPSICHORE code, showing stability of the kink (N=1) and periodicity-preserving (N=0) modes for the proposed stellarator, even without a conducting wall [14]. The two codes have been extensively benchmarked against tokamak and quasiaxial stellarator equilibia and have been found in good agreement [14].…”