A single-field-period quasi-isodynamic stellarator

TL;DR

This paper introduces a novel single-field-period quasi-isodynamic stellarator configuration optimized for excellent confinement and fewer coils, achieved through a rapid numerical method and near-axis expansion.

Contribution

It presents a new stellarator design combining direct construction and numerical optimization, resulting in improved confinement with fewer coils at a modest aspect ratio.

Findings

Achieved effective ripple comparable to W7-X

Optimization completed in 30 seconds on a single CPU core

Device requires fewer coils due to reduced field periods

Abstract

A single-field-period quasi-isodynamic stellarator configuration is presented. This configuration, which resembles a twisted strip, is obtained by the method of direct construction, that is, it is found via an expansion in the distance from the magnetic axis. Its discovery, however, relied on an additional step involving numerical optimization, performed within the space of near-axis configurations defined by a set of adjustable magnetic-field parameters. This optimization, completed in 30 seconds on a single cpu core using the SIMSOPT code, yields a solution with excellent confinement, as measured by the conventional figure of merit for neoclassical transport, effective ripple, at a modest aspect ratio of eight. The optimization parameters that led to this configuration are described, its confinement properties are assessed, and a set of magnetic-field coils is found. The resulting transport at low collisionality is much smaller than that of W7-X, and the device needs significantly fewer coils thanks to the reduced number of field periods.