A quasi-isodynamic configuration with good confinement of fast ions at low plasma $\beta$

TL;DR

This paper introduces a new quasi-isodynamic stellarator design optimized for effective fast ion confinement at low plasma beta, demonstrating stability and reduced bootstrap current, with practical coil configurations.

Contribution

The paper presents a novel quasi-isodynamic stellarator configuration optimized for low-beta fast ion confinement, including stability analysis and coil design.

Findings

Achieves ideal and ballooning MHD stability up to 5% beta.

Shows reduced effective ripple and good fast ion confinement at low beta.

Supports reduced bootstrap current in the configuration.

Abstract

A new quasi-isodynamic stellarator configuration optimized for the confinement of energetic ions at low plasma $\beta$ is obtained. The numerical optimization is carried out using the STELLOPT suite of codes. New proxies to measure closeness to quasi-isodynamicity and quality of fast ion confinement have been included. The new configuration has poloidally closed contours of magnetic field strength, low magnetic shear and a rotational transform profile allowing an island divertor. It shows ideal and ballooning magnetohydrodynamic stability up to $\beta = 5%$, reduced effective ripple, with $\epsilon_{eff} < 0.5%$ in the plasma core. Even at low $\beta$, the configuration approximately satisfies the maximum-$J$ property, and the confinement of fast ions is good at $\beta \sim 1.5%$ and becomes excellent at reactor values, $\beta \sim 4%$. An evaluation of the $D_{31}$ neoclassical mono-energetic coefficient supports the expectation of a reduced bootstrap current for plasmas confined in quasi-isodynamic configurations. A set of filamentary coils that preserve the good confinement of fast ions in the core is presented.