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

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.