Particle balance modelling in ergodic divertor experiments on Tore Supra

The particle balance and the evaluation of the impact of the plasma edge characteristics in the open divertor configuration (no neutral baffling) is performed with a three reservoir model. The control of the plasma density in the divertor volume is analyzed with respect to the wall behaviour and to the screening capability of the stochastic boundary. The present modelling particularly shows the important role of both the edge electron temperature and the characteristic width of the ergodic layer. Active pumping in conjunction with the establishment of a stochastic boundary is demonstrated to be essential for the precise control of the boundary density, required both to obtain high radiation conditions and good coupling of the auxiliary heating power. This model is compared to a typical ergodic divertor experiment with and without ICRH auxiliary heating and characterised by about 70% of radiated power and achieved with strong D2 gas injections balanced by active pumping. The radiation enhancement by a factor 3.5 from the Ohmic phase to the ICRH phase is achieved with an increase by 15% of the bulk carbon contamination deduced from the increase in Zeff. The qualitative agreement between the three reservoir model and the experimental results is presented. (orig.)