Analysis and modelling of plasma response to pellet injection in RFX

Abstract

Pellet injection experiments have been performed on the RFX reversed field pinch (RFP). The main aim of the experimental campaign was the study of the target plasma response to get information both on the possibility of an efficient pellet fuelling of the discharge and on the associated transport modification. In this paper, we present an analysis of different experimental scenarios. The tool employed is a one-dimensional transport code coupled with an ablation calculation that yields the pellet particle source and trajectory. It is seen that, in standard plasmas, the pellets that reach the plasma centre trigger a phase of rapid profile flattening and poorer particle and energy confinement, and an increase of the transport coefficients is required to reproduce the post-pellet behaviour of the profiles. On the other hand, the slow pellets that suffer large deflection and miss the plasma centre do not have such a deleterious effect and, in this case, the unperturbed transport coefficients are able to reproduce the post-pellet evolution of the profiles. Finally, when the pellets are launched in combination with some method that induces an improved confinement, the transport degradation is less evident. In our case, pellet injection has been combined with pulsed poloidal current drive (PPCD), a technique that reduces magnetic field stochasticization and transport in the RFP core. When the effect of the PPCD is strong enough, a synergy between pellet injection and PPCD is observed and plasma fuelling without dramatic confinement reduction is possible.