Rosenbluth Dissertation Award: Quasisymmetry: a modern perspective on the stellarator concept*

Quasisymmetry (QS) is a property of magnetic field-plasma systems that enables the containment of the plasma, necessary to undergo thermonuclear fusion. The concept originated in the 80s, when a generalisation of the working principle of the axisymmetric tokamak was sought. Here we revisit the concept from the ground up, providing a renewed perspective and exploring previously unexplored questions.

In this presentation we shall start by building QS from the most fundamental level: the behaviour of charged particles, and the attempt to confine them. This allows the conceptualisation of QS without any assumptions on the nature of the underlying equilibrium. This will make it clear an underlying tension between the symmetry and force balance, which makes the construction of QS equilibrium fields in a typical isotropic pressure equilibrium challenging. Anisotropic pressure solutions appear as a more natural option.

Under a more practical prism, we are interested in finding explicit examples of QS fields. Solving the governing partial differential equations governing the problem directly is too complex. Thus, the problem is often addressed through optimisation. To gain additional insight and control,we consider an asymptotic description of solutions around their core, the so-called near-axis expansion. We focus on this in this talk. A systematic construction of quasisymmetric fields within such a framework is here presented, which unveils an ordered structured topological phase space of quasisymmetric fields, providing an opportunity to understand properties and exhaust possibilities of quasisymmetric configurations.