Flexible and precise equilibrium optimization with the Plasma Equilibrium Toolkit.

The optimization of stellarator performance by targeting particular confinement metrics has been demonstrated to be a fruitful avenue for device design, e.g. HSX & W7-X. The community has thus produced metrics targeting the various classes of instability or undesired transport channels, e.g., magenetohydrodynamic instability, neoclassical transport, turbulent transport, etc. Evaluating these metrics generally requires coordinate systems derived from solutions to the magnetohydrodynamic equilibrium problem. There are a number of equilibrium solvers which employ various different magnetic field representations such that applications relying on a particular representation are often incompatible. To address this challenge, the Plasma Equilibrium Toolkit (PET) is a Julia code created to interface physics applications and optimization metrics with numerical and analytic equilibria, in a rigorous, flexible, and convenient fashion. Recent changes to PET have enabled the use of automatic differentiation tools to provide machine-precision gradients in the geometrical parameter space, which in turn improve the optimization process through local gradient descent methods. Results will be presented on test cases showcasing the application PET to target various metrics, including linear stability and available energy.