Role of hydrogenic molecules in fusion-relevant divertor plasmas

Power production using magnetic confinement fusion is technically challenging and necessitates operation under detached divertor conditions. Under detached conditions the power loads to the plasma-facing components are reduced within their thermo-mechanical properties. Molecular processes are critical at temperatures relevant to onset of plasma detachment. Therefore, molecules are expected to affect the local plasma conditions and the onset of detachment. This dissertation evaluates the role of molecular effects on the onset of detachment using a fluid model for molecules implemented in the edge-fluid code UEDGE coupled to the collisional-radiative (CR) code CRUMPET. The applicability of the molecular fluid model is assessed using the kinetic neutral Monte-Carlo code EIRENE. To assess the validity of the predictions, and to quantify the role of molecular particle and power sinks on the onset of detachment, the numerical UEDGE predictions are compared to Divertor Thomson Scattering (DTS) and Langmuir probe measurements in DIII–D low-confinement (L-mode) plasmas. Including fluid molecules in UEDGE simulations of deuterium plasmas in L-mode conditions improves the qualitative code-experiment agreement for detachment onset compared to UEDGE simulations considering atoms only. Accounting for molecularly-induced plasma particle and power sinks in the simulations reduces the plasma temperatures sufficiently for detachment onset to occur. The role of electronic and vibrational CR transitions are shown to be more important for the effective CR dissociation rates than the assumption of ion-electron thermal equilibration and non-local transport effect of vibrationally excited molecules. Omitting vibrational and electronic transitions from EIRENE simulations decrease effective dissociation rates by up to 65% compared to when they are included. UEDGE fluid predictions of molecular content and mean energy are shown to lie within 15% and a factor of 2 of EIRENE kinetic predictions, respectively. These findings indicate that a fluid treatment of the molecules in divertor plasmas is applicable for divertor-relevant conditions. Using fluid molecular models in plasma-edge simulations reduces the computational times compared to kinetic molecular models, especially under highly collisional conditions encountered in detached divertor plasmas. The effect of approximating the molecules as a fluid is found to be smaller than that of other processes affecting molecular predictions, such as the CR processes included in the effective rates. The dissertation suggests a number of improvements to the implemented molecular fluid model to further reduce the difference between kinetic and fluid molecular models.