CIEMAT-QI4X: a reactor-relevant quasi-isodynamic stellarator configuration compatible with an island divertor

TL;DR

CIEMAT-QI4X advances reactor-relevant stellarator design by enforcing strict rotational-transform control and Mercier stability to simultaneously suppress low-order magnetic islands and cultivate a robust edge 4/4 island divertor. The configuration employs a 48-filament coil set, achieving a practical coil–plasma distance and high flux-surface fidelity, preserving nested surfaces up to $\beta \sim 4\%$ and maintaining favorable MHD, neoclassical, fast-ion, and turbulent transport properties. Key results include a total bootstrap current of about $54\ \mathrm{kA}$ that barely perturbs the rotational transform, strong fast-ion confinement with $\alpha$-energy losses $<8\%$, and reduced electrostatic turbulent transport relative to W7-X. Overall, CIEMAT-QI4X presents a viable physics basis for a reactor-scale island-divertor stellarator, with ongoing work on blanket integration and coil feasibility to support eventual engineering realization.

Abstract

A four-field-period quasi-isodynamic stellarator configuration is presented that exhibits small neoclassical and electrostatic turbulent transport, good fast-ion confinement over a wide range of $β$ values, small bootstrap current and an edge island structure compatible with an island divertor. This configuration, called CIEMAT-QI4X, has been obtained by building on the optimization strategy and sophisticating the methods employed in [Sánchez E. et al 2023 Nucl. Fusion 63 066037]. The optimization has been improved by incorporating metrics to control Mercier stability and by enforcing strict constraints on the rotational transform profile to achieve nested toroidal surfaces in the confinement region and a divertor island structure at the plasma edge. Specifically, CIEMAT-QI4X has a 4/4 island chain at the edge that is resilient at least up to $β=4\%$, even when the bootstrap current is included. A corresponding set of filamentary coils is presented that generates the configuration with enough accuracy to preserve the aforementioned physics properties. In terms of physics performance, CIEMAT-QI4X establishes as a candidate for a stellarator fusion reactor design.

Figures (14)

• Figure 1: Rotational transform profiles for the CIEMAT-QI4 (green) and the CIEMAT-QI4X (red) optimized configurations. Dashed lines are shown for the values of the lowest-order rational values crossed by the profiles. The $\iota$ profiles are extrapolated outside the last closed flux surface until they reach the $\frac{4}{4}$ rational, which is pursused to form an island divertor structure.
• Figure 2: Poincaré plots at toroidal angle $\phi=\pi/4$ for the CIEMAT-QI4 configuration at $\beta=0.5\%$ obtained with HINT-3D calculations, using an optimized set of coils producing the configuration with good fidelity (left) SanchezEFTC23 and with a coil set optimized to minimize the $\frac{8}{9}$ island width (right) sanchezSimons24. The last closed flux surface for a VMEC calculation at same $\beta$ value is shown in red for reference. The $\frac{8}{9}$ , $\frac{12}{13}$, and $\frac{16}{17}$ islands are seen in the plasma column with some island overlapping at the plasma edge in the left case.
• Figure 3: Magnetic field strength at the last closed flux surface (left) and Poincaré plots for the CIEMAT-QI4X configuration at several toroidal angles (right)
• Figure 4: Modular coils designed for the CIEMAT-QI4X configuration. The coils are filamentary and the volume is only shown for visualization purposes. This coil design is the one producing the flux surfaces in figures \ref{['fig:CQI4XPoincare']} and the results shown in section \ref{['FIMetrics']}
• Figure 5: Poincaré plots obtained with HINT-3D for the CIEMAT-QI4X configuration at toroidal angles $\phi=\pi/4$ and $\phi=0$, and for three values of the plasma pressure, $\beta=0.05\%, 2\%$, and $4\%$.