In this study, the stochastization of the magnetic field in a simplified stellarator, which has a strong magnetic shear in the peripheral region, is studied using a nonlinear 3D equilibrium calculation code, HINT, which calculates the nonlinear resistive steady state using the relaxation method without the assumption of nested flux surfaces. For the finite beta equilibrium, the pressure-induced perturbed field breaks the nested flux surfaces, and the stochastization of the magnetic field appears in the region of the strong magnetic shear. The stochastization penetrates into the plasma core due to the increased beta and the effective plasma confinement degrades; in other words, the effective plasma volume shrinks. The stochastization strongly depends on the profile shape of the plasma pressure. Since the pressure gradient drives the Pfirsch–Schlüter current, the pressure profile affects the stochastization of the magnetic field. In order to understand the equilibrium beta limit due to the stochastization of the magnetic field, the profile shape of the plasma pressure is an important factor.
Effect of transverse magnetic field on axial magnetic surfaces of a three-turn stellarator perturbed by an equilibrium plasma