Studies on basic phenomena during the pellet injection into high-temperature plasmas

Abstract

The injection of hydrogen/deuterium pellets into a tokamak leads to a sudden increase in the electron density, and subsequently to a profile peaking of the density and an increase in the stored energy. Immediately after the injection, different types of oscillations are excited. On TEXTOR, the first type immediately follows the injection and the second one is excited with a delay of more than 10 ms. The oscillations show a `snake-like' structure and occur close to the q = 1 surface with a frequency of 0.7 - 2 kHz. The radial location of the second oscillation is slightly shifted with respect to the first one. A fast-cooling phenomenon (`pre-cooling') in the core region of a plasma is often observed at pellet ablation phase. A study on the relation between the `pre-cooling' and sawtooth oscillations suggests that the central value of safety factor of plasmas, q(0), is kept sufficiently below unity even just after the sawtooth crash. During pellet injection, the ablation rate is strongly modulated; these modulations cause so-called `striations' in the ablation cloud. One model relates the striations to the energy reservoir on the plasma flux surfaces and describes the possibility of deriving the q-profile; the question of whether this method provides reliable results cannot yet be answered conclusively. The trajectory of the pellet in the plasma is in general not straight but deflected in the electron drift direction (OH discharges) or in the ion drift direction (CO-NBI discharges). The cloud develops a helically structured tail in the electron flow direction (toroidally) and in the electron diamagnetic drift direction (poloidally). The tail structure is attributed to charge-exchange processes and to plasma rotation.