Ionization-recombination processes and ablation cloud structure for a carbon pellet

Modelling of carbon pellet injection into hot hydrogen plasma is performed. Models of ionization and radiation processes previously used for descriptions of the ablation clouds are discussed. It is demonstrated that the assumption made by other authors that the plasma is transparent significantly overestimated ionization lengths for carbon. The optical thickness of the cloud is estimated. With typical ablation rates, the cloud in the vicinity of the pellet is opaque for resonant radiation in lines, in contrast to the visual transparency for non-resonant photons. The ionization by incident electrons is not sufficient to provide the short experimental cloud sizes. A reduced model for the ion stripping dynamics and energy losses in optically thick plasmas is proposed. Two very important phenomena are taken into account: partial trapping of resonant photons in the cloud and ionization from the excited state. The model is included into the MHD pellet code LLP. Calculations with the new model show at least a qualitative agreement with the experimental sizes of the carbon clouds, in contrast to the models used earlier.