Adjoint-based shape optimization of the ITER divertor using advanced fluid neutral models

The extremely high heat fluxes exhausted onto the plasma-facing components of magnetically confined fusion devices pose a fundamental challenge for the design of a viable nuclear fusion power plant. Neutral particles play an essential role in mitigating these heat loads, and they must therefore be simulated accurately in plasma boundary codes. However, the accurate simulation of these neutral particles is a computational challenge. This work focuses on the development of fluid and hybrid fluid-kinetic models which provide cheaper alternatives to the fully kinetic reference simulations. A kinetic model resolves the velocity distributions of individual atoms and molecules, while fluid models assume an equilibrium distribution. The newly developed fluid models clearly outperform the existing fluid models in terms of agreement with the kinetic reference model. The hybrid models further reduce the remaining discrepancies, and they succeed in the reducing the computational time per particle in the kinetic simulations. However, the global statistical performance of the hybrid methods needs to be further studied and optimized.