Analysis of pellet ablation and density build-up in the RFX reversed-fieldpinch

New data from the Reversed Field eXperiment (RFX) are presented and analyse d, which provide a deeper understanding of pellet experiments in a reversed field pinch. In particular, evidence on the ablation cloud density, ablati on rate and homogenization process are given by the measurements of two mid -infra-red interferometers located at different toroidal locations, one of which is at the same poloidal plane as the pellet injector. For each pellet , the measurement of the latter interferometer displays a huge peak, which is due to the crossing of the interferometer chord by the ablation cloud. I ts analysis yields information on the cloud dimension and radial density di stribution. The typical cloud density is much lower than that measured in t okamaks. Due to such a low density, the stopping power of the ionized part of the ablation cloud is weak. As a result the ablation rate is higher than in tokamaks. Another characteristic of the density increase measured by bo th of the interferometers is the absence of dense plasma structures propaga ting for long distances along field lines. This proves that the distance ne cessary for the ablated material to become incorporated into the plasma is less than about 4 m in the core of the discharge, due to a rapid mixing of the ablated material. Despite the different features displayed by pellet in jection experiments in reversed-field pinches and tokamaks, the ablation an d homogenization of the deposited material can be described within the same general frame for both magnetic configurations. The differences arise main ly because of the different magnetic field topologies and values of the tra nsport coefficients in the two machines.