Exploration of stellarator configuration space with global search methods

An exploration of stellarator configuration space z for quasi-axisymmetric stellarator (QAS) designs is discussed, using methods which provide a more global view of that space. To this end, a “differential evolution” search algorithm has been implemented in an existing stellarator optimizer, which is much less prone to become trapped in local, suboptimal minima of the cost function χ than the local search methods used previously. This search algorithm is complemented by mapping studies of χ over z aimed at gaining insight into the results of the automated searches. It is found that a wide range of the attractive QAS configurations previously found fall into a small number of classes, with each class corresponding to a basin of χ(z). Maps are developed on which these earlier stellarators can be placed, the relations among them seen, and understanding gained into the physics differences between them. It is also found that, while still large, the region of z space containing practically realizable QAS configurations is much smaller than earlier supposed.