Quasi-isodynamic stellarator optimisation

Abstract

Stellarators, a promising class of fusion reactor candidates, use geometrically complex magnetic fields to confine plasma in donut-shaped geometries. This complexity means that the vast majority of stellarator designs are nonviable, and any reactor-relevant stellarator design requires an extremely carefully sculpted geometry -- typically achieved through numerical optimisation. Such optimisation requires the use of "target functions", which should quantitatively assess the quality of a stellarator magnetic field. In this work, we present several novel target functions for stellarator optimisation, which we used to guide the design of magnetohydrodynamic (MHD) plasma equilibria. These equilibria are quasi-isodynamic (QI) --- a property of stellarator magnetic fields --- to an extent never before seen, granting them unrivalled stability and low neoclassical transport. These results were made possible by enhancements in computational power and an improved theoretical understanding of plasma physics. We additionally present other target functions which grant additional important properties to the plasma, notably magnetohydrodynamic stability and a reduction in plasma turbulence, whose presence hinders the performance of magnetic confinement fusion reactors. These target functions, while often at-odds with one another, were made sufficiently compatible that a wide variety of extremely enticing properties can be simultaneously achieved in a single plasma equilibrium. These equilibria, known as SQuIDs (Stable Quasi-Isodynamic Designs) are now a leading candidate for stellarator fusion reactors