Efficient Stellarator Optimization and Analysis with DESC

We present an overview of the new stellarator design and analysis capabilities made possible by the DESC code suite [1-4]. This software package couples equilibrium and optimization codes together to efficiently solve the numerical optimization problems required for next-generation stellarator designs. Unlike finite differences or adjoint methods, automatic differentiation provides access to exact derivatives of any objective function and allows the inclusion of more physics constraints, such as metrics for particle confinement and stability. Boundary conditions specifying a Poincaré section instead of the last closed flux surface help to understand the existence of solutions with nested flux surfaces and reveal the evolution of tokamaks into stellarators. This approach to stellarator optimization is valid for finite-β solutions throughout the full plasma volume and has been extended to free-boundary equilibria. DESC is the first stellarator optimization code to use only a single equilibrium solution at each iteration, which reduces the computation time by three orders of magnitude in tests compared to STELLOPT and enables exploration over a higher-dimensional parameter space. Examples of these computational advances are demonstrated along with a discussion of the novel physics insights they provide.