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

New data from the Reversed Field eXperiment (RFX) are presented and analysed, which provide a deeper understanding of pellet experiments in a reversed field pinch. In particular, evidence on the ablation cloud density, ablation 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. Its analysis yields information on the cloud dimension and radial density distribution. The typical cloud density is much lower than that measured in tokamaks. 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 both of the interferometers is the absence of dense plasma structures propagating for long distances along field lines. This proves that the distance necessary 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 injection experiments in reversed-field pinches and tokamaks, the ablation and homogenization of the deposited material can be described within the same general frame for both magnetic configurations. The differences arise mainly because of the different magnetic field topologies and values of the transport coefficients in the two machines.