A simulation-data derived reduced model for near-wall molecule-plasma interactions

Molecular hydrogen plays an important role in divertor detachment by providing additional momentum and energy sink mechanisms. For plasma temperatures greater than the Franck-Condon energy, molecules exist in a narrow region adjacent to the wall. The limited spatial extent of this "molecular presheath" permits the use of simplified models to characterize its effects on the plasma. Here, a reduced model for the momentum and energy sink of the plasma due to molecular interactions, as well as an effective atomic production energy, is informed by fitting to thousands of kinetic-neutral DEGAS2 simulations. Free parameters include: the plasma density and temperature, the angle the magnetic field makes with the wall, the wall composition, and its recycling coefficient. This model is implemented in XGC to provide a simple but accurate estimate of the effects of desorbed molecular hydrogen. Implications for plasma boundary conditions are also discussed.