Optimization of quasisymmetric stellarators with self-consistent bootstrap current and energetic particle confinement

TL;DR

This paper introduces an efficient optimization method for quasisymmetric stellarators that self-consistently accounts for bootstrap current, leading to improved energetic particle confinement and reduced alpha particle energy losses.

Contribution

A novel optimization approach leveraging axisymmetry formulas for bootstrap current enables self-consistent stellarator design with enhanced confinement.

Findings

Configurations with significant pressure and self-consistent bootstrap current achieved.

New stellarators show substantially lower alpha energy losses.

Method reduces computational cost of bootstrap calculations in optimization.

Abstract

Quasisymmetry can greatly improve the confinement of energetic particles and thermal plasma in a stellarator. The magnetic field of a quasisymmetric stellarator at high plasma pressure is significantly affected by the bootstrap current, but the computational cost of accurate stellarator bootstrap calculations has precluded use inside optimization. Here, a new efficient method is demonstrated for optimization of quasisymmetric stellarator configurations such that the bootstrap current profile is consistent with the geometry. The approach is based on the fact that all neoclassical phenomena in quasisymmetry are isomorphic to those in axisymmetry. Therefore accurate formulae for the bootstrap current in tokamaks, which can be evaluated rapidly, can be applied also in stellarators. The deviation between this predicted parallel current and the actual parallel current in the magnetohydrodynamic equilibrium is penalized in the objective function, and the current profile of the equilibrium is included in the parameter space. Quasisymmetric configurations with significant pressure are thereby obtained with self-consistent bootstrap current and excellent confinement. In a comparison of fusion-produced alpha particle confinement across many stellarators, the new configurations have significantly lower alpha energy losses than many previous designs.