Pellet injection experiments on the TFR tokamak

Single pellet injection experiments have been carried out on TFR in Ohmic conditions, aiming at an improvement of the experimental knowledge of ablation physical processes and also at a better description of the heat and matter transport during and after pellet evaporation. Ablation clouds have been photographed, providing experimental penetration depths in rough agreement with the neutral shielding model. Observation of striations in the cloud has led to an experimental determination of the safety factor profile. The plasma parameters in the ablation cloud have been determined spectroscopically. Fast heat transport has been evidenced during pellet evaporation (∼ 100 μs), which exhibits some features of minor or major disruptions (appearance of an m = 1, n = 1 island on the q = 1 surface, bursts of density fluctuations, comparable heat diffusivity, etc.). Matter transport takes place on a larger time-scale (∼10 ms). This long temporal relaxation is well accounted for by the MAKOKOT 1-D transport code with the same dependence of the transport coefficients on the plasma parameters before and after pellet injection. Heat and matter transports are affected by the presence of the m = 1, n = 1 island on the q = 1 surface.