Description
The non-axisymmetric toroidal magnetic geometry of a stellarator presents a vast space, in which one can find designs of different shapes and properties, filling a number of different stellarator classes. This talk focuses on the quasi-isodynamic (QI) stellarator, which promises a number of uniquely attractive properties that make it a front-runner for future stellarator power plants. Although the Wendelstein 7-X stellarator, whose design can be thought of as an early precursor of the QI variant, has validated a number of these expectations, it is only over recent years that advances in stellarator optimization have demonstrated that the full potential of QI stellarators could be realized in practice. We will discuss the physics of QI stellarators, and how this understanding is being used to design the next generation of experiments and fusion reactor designs. These designs combine intrinsic stability, excellent confinement of fusion-born alpha particles, and the resilience in shape needed for reliable and predictable operation. Our deepening understanding of the turbulence in QI stellarators has also enabled us to incorporate key physics into the optimization, such as zonal flows, flux compression, and maximum-$J$ stabilization. Understanding the strong coupling of heat and particle transport channels has lead to designs with a tendency to draw fuel inward, with strong consequences for performance and economics of future devices.