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

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.

21 pages, 16 figures

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• Piecewise omnigenous stellarators with zero bootstrap current
• Flexible Stellarator Physics Facility
• Critical gradient optimization for quasi-isodynamic stellarators
• Near-axis quasi-isodynamic database