A model of the pellet deposition profile is presented, which describes in a self-consistent way the homogenization process and the simultaneous drift of the ablated material. Its main features are (i) that the drift is stopped by a parallel current that appears in the drifting flux tube and reduces the polarization of the expanding ablatant and (ii) that the pellet material does not move as a solid body but homogenizes in a radial interval of extent equal to its displacement. From the pellet and plasma pre-injection characteristics, the model yields the post-injection density and temperature profiles, allowing a quantitative comparison with measurements. The simulation results are compared with experimental data for both the homogenization phase and ∇B-induced displacement. In particular, (i) the calculated characteristics of the homogenization and drift (time constants and velocities) are in agreement with the measurements, (ii) for pellets launched from the low field side (LFS), the model reproduces the dependence of both the fuelling efficiency and the outward displacement on the pellet penetration and (iii) for pellets launched from the high field side (HFS), which are less documented, the calculated fuelling efficiency is always equal to 100%, larger than what is observed, suggesting a transient increase in the plasma (radial) transport. Practically, the main results are that the displacement is smaller for the HFS than for the LFS launched pellets and that, for deep fuelling, one must inject the pellet along the drift direction.

Pellet injection is planned to be the main fuelling method on ITER. The high temperature of the plasma during a fusion burn will limit the penetration of the pellet to the outer third of the minor radius. This limited penetration is expected to be compensated by a polarization drift, which will deposit the particles deeper in the plasma for the pellets injected from the high field side. In order to evaluate the expected depth of the fuelling on ITER, a good understanding of this drift effect is important. Experimental data acquired on the DIII-D (USA) and Tore Supra (France) tokamaks show that the polarization drift is influenced by the low order rational q surfaces. These surfaces appear to attenuate the polarization mechanism as the drifting particles cross them. In this paper, a correlation between the maximum of the pellet mass deposition profile and the positions of the q = 2 and q = 3 surfaces on DIII-D and Tore Supra is shown for high field side and low field side injection. A model is proposed to explain this effect and compared with the experimental results. To conclude, the possible consequences of this phenomenon on the fuelling in ITER are described.

In the last few decades, pellet injection has become an important tool of discharge operation and control. In ITER, plasma fuelling and edge localized mode (ELM) pacemaking will rely mainly on pellet injection (other applications will be impurity injection for diagnostic purposes and disruption mitigation). This paper describes our present understanding of the physics of ablation and ∇B-induced displacement of the pellet material, presents last experimental results on discharge fuelling and ELM pacemaking by pellet injection and discusses-on the basis of simulation results-how the experiments performed in present day machines can be extrapolated to ITER.