Self-consistent simulations of plasma turbulence and neutral dynamics in detachment regime

Simulations of high-density deuterium plasma turbulence in a lower single-null magnetic configuration based on a TCV discharge are presented. We reconstruct plasma dynamics of three charged species, electrons, atomic deuterium ions and molecular deuterium ions, interacting with two neutrals species, atomic and molecular deuterium, through ionization, charge-exchange, recombination and molecular dissociation processes. The plasma is modelled using drift-reduced fluid Braginskii equations, while neutral dynamics is described by a kinetic model solved without the use of Montecarlo methods. Increasing the D2 puffing we are able to reach a turbulent steady state where the inner divertor target is detached, presenting lower particle and heat flux for higher scrape-off layer density. The analysis of transport balance in the divertor volume shows that the decrease of particle flux is caused by a decrease of local neutrals ionization and parallel velocity. The effect of molecular reactions is to increase atomic neutral density, emitted by molecular activated recombination, increasing ionization and charge-exchange reactions above the X-point, see Fig. 1. Plasma energy losses are dominated by ionization reactions for temperatures higher than 3 eV, by dissociation reactions for lower temperatures and by charge-exchange reactions for high neutrals density. The presence of strong electric fields in high-density plasma is also investigated through the analysis of Ohm's law, determining the importance of increased resistivity in the establishment of electrostatic potential gradients. A comparison between two simulations with different toroidal field direction leads to the assessment of the role of the ExB drift in the access of detachment conditions, in the asymmetries of the divertor targets and in the increase of turbulent transport with higher density at the mid-plane. The trends observed in the simulations are in agreement with experimental observations of increased density decay length, together with an increase of plasma blob size and radial velocity for increased plasma density.