A quasi-isodynamic configuration with good confinement of fast ions at low plasma ${\hat{I}}^2$

Abstract

A new quasi-isodynamic stellarator configuration optimized for the confinement of energetic ions at low plasma ${\hat{I}}^2$ 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 ${\hat{I}}^2=5$, reduced effective ripple, with $\hat{I}{\mu }_{eff}<0.5$ in the plasma core. Even at low ${\hat{I}}^2$, the configuration approximately satisfies the maximum-$J$ property, and the confinement of fast ions is good at ${\hat{I}}^2\sim 1.5$ and becomes excellent at reactor values, ${\hat{I}}^2\sim 4$. 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.

21 pages, 16 figures