Calculation of the radiative cooling coefficient for molybdenum in a low density plasma

Abstract

The radiative cooling coefficient for molybdenum (Z=42) in a low density (ne ≲ 1015 cm-3) plasma is calculated. First, the molybdenum charge state distribution (CSD) is computed using the best available atomic physics data for ground state recombination and ionization, including the rates of excitation-autoionization for Mo6+ to Mo13+ and Mo23+ to Mo32+. The emissivities of Mo4+ to Mo41+ are then found using a collisional-radiative model such that the contributions from metastable levels to an ion's emissivity are taken into account. The CSD and the radiative emissivity for all molybdenum ions are combined to yield the total radiative cooling coefficient for molybdenum in a low density plasma. A radiative loss coefficient over 2 orders of magnitude smaller than that predicted by an `average ion' model for temperatures relevant to tokamak divertor and scrape-off layer plasmas (Te ≲ 50 eV) is found. The cooling coefficient of the present work varies from a factor of 2 smaller to a factor of 2 larger than that predicted by the `average ion' model for all other plasma temperatures. The coefficient calculated in the present work is benchmarked against the measured bolometric loss profile from a molybdenum dominated shot in the Frascati Tokamak Upgrade (FTU)