Speaker
Description
The stellarator concept offers advantages for a fusion reactor compared to the tokamak, but stellarator magnetic fields require careful optimization to achieve a confinement quality comparable to that of tokamaks. The numerical optimization for reduced neoclassical transport has already been experimentally validated in W7-X [1] and HSX [2], and great improvements have been made in the last few years including fast-ion confinement and turbulent transport in the set of optimization criteria [3,4,5]. However, insufficient attention is usually paid to the rotational transform profile during the optimization process. In the optimization of quasi-symmetric configurations, few restrictions are commonly set to the rotational transform profile. In the optimization of quasi-isodynamic (QI) configurations, if an island divertor is pursued, the rotational transform is usually constrained only to avoid the lowest-order rational values in the plasma column and to approach a low-order rational value at the edge [3,4]. However, if the rotational transform profile is not optimized more carefully than this, several issues, such as the formation of low-order islands, the overlapping of neighboring islands, and the insufficient quality of the edge island for a divertor, can appear, which can be worsened by finite b effects [6]. Furthermore, these issues can be very sensitive to coil design details and construction errors.
In this work, we show how including a strict control of the rotational transform profile and the magnetic shear in the optimization process can improve the flux surface quality and the structure of the divertor island, and propose new metrics to be included in the optimization with this purpose. Using this strategy we have obtained a new reactor-relevant QI configuration, named CIEMAT-QI4X, which keeps the physics properties of the QI configuration presented in [3] (reduced neoclassical and turbulent transport, low bootstrap current, and good fast ion confinement), while at the same time improves the flux surface quality and shows a prominent low-order island at the edge suited to design an island divertor. Furthermore, its optimized rotational transform profile makes it more robust to coil construction errors. A set of optimized magnetic coils for this configuration is also presented.
