Improvements to the DESC code for finding and optimizing stellarator equilibria

Abstract: DESC is a pseudo-spectral code that computes 3D MHD equilibria by directly solving the force balance equations JxB=grad(p). We present a number of recent improvements to the code for both finding and optimizing stellarator equilibria. Automatic differentiation allows fast and accurate computation of derivatives used for solving the force balance equations and determining sensitivity to inputs. These calculations are further accelerated by leveraging GPUs. Accurate derivatives are used in a perturbation method to explore the phase space of stellarators and efficiently compute high resolution equilibria. We also demonstrate computing free boundary equilibria with DESC.