Non-Coronal radiation enhancement in advanced divertor geometries with power scaling to ITER

Through SOLPS-ITER simulations of DIII-D and MAST-U, an X-divertor (XD) on DIII-D and a super X-divertor (SXD) on MAST-U were shown to have increased emissivity P_Rad/(n_en_I) than corresponding standard divertors (SD) at similar degrees of partial detachment, specifically a target temperature of 2eV just outside the separatrix. The XD has a higher emissivity at higher input powers, unlike the SD which has a lower emissivity at higher input powers. This comparison of emissivity and its dependence on field line angles and input power is extended to ITER and DEMO (with nitrogen as the impurity) using a 1D model, where low field line angles (such as seen in an XD) continue to perform better. The reasons behind the increased emissivity in the DIII-D XD and SXD are analyzed using a simple 0D transport model. From the transport model, it is seen that a major cause of the increased emissivity in the XD and SXD over the SDs is a greater neutral deuterium density, which allows for greater charge exchange with carbon.