Asymptotic quasisymmetric high-beta 3D MHD equilibria near axisymmetry

Abstract: Quasisymmetry (QS), a hidden symmetry of the magnetic field strength, is known to support nested flux surfaces and provide superior particle confinement in stellarators. In this work, we study the ideal MHD equilibrium and stability of high-beta plasma in a large aspect-ratio stellarator. In particular, we show that the lowest-order description of a near-axisymmetric equilibrium vastly simplifies the problem of 3D quasisymmetric MHD equilibria, which can be reduced to a standard elliptic Grad-Shafranov equation for the flux function. We show that any large aspect-ratio tokamak, deformed periodically in the vertical direction, is a stellarator with approximate volumetric QS. We discuss exact analytical solutions and numerical benchmarks. Finally, we discuss the ideal ballooning and interchange stability of some of our equilibrium configurations.