Theoretical studies of equilibrium beta limit in heliotron plasmas

TL;DR

This paper investigates the maximum achievable plasma pressure (beta limit) in heliotron devices using 3D MHD equilibrium calculations, revealing how stochastic regions influence the equilibrium and proposing a new index for the beta limit.

Contribution

It introduces a novel approach to determine the equilibrium beta limit in heliotron plasmas by analyzing force balance in stochastic regions using the HINT code.

Findings

Force balance breaks in stochastic regions at high beta.

Pressure gradient decreases to maintain equilibrium.

Proposes a new index for the beta limit based on force balance.

Abstract

In this study, the MHD equilibrium limit of LHD plasmas is studied using a 3D MHD equilibrium calculation code, HINT, which is an initial value solver based on the relaxation method without the assumption of nested flux surfaces. For finite beta equilibria, flux surfaces become stochastic in the peripheral region. However, though the axis shifts until the conventional limit, the separatrix to limit the equilibrium does not appear. For high beta equilibria, the force balance starts breaking in the stochastic region. To keep the force balance, the pressure gradient in the stochastic region decreases and the fixed profile is reduced. As the result, the volume averaged beta saturates. The beta value, where the force balance starts breaking inside the stochastic region, is proposed as an index of equilibrium beta limit in heliotron plasmas.