STUDIES ON BASIC PHENOMENA DURING THE PELLET INJECTION INTO HIGH-TEMPERATURE PLASMAS

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. Immediat ely after the injection, different types of oscillations are excited. On TEXTOR, the first type immediately follows the injection and the se cond one is excited with a delay of more than IO ms. The oscillations show a 'snake-like' structure and occur close to the q = 1 surface wit h a frequency of 0.7-2 kHz. The radial location of the second oscillat ion is slightly shifted with respect to the first one. A fast-cooling phenomenon ('pre-cooling') in the core region of a plasma is often obs erved at pellet ablation phase. A study on the relation between the 'p re-cooling' and sawtooth oscillations suggests that the central value of safety factor of plasmas, q(0), is kept sufficiently below unity ev en just after the sawtooth crash. During pellet injection, the ablatio n rate is strongly modulated; these modulations cause so-called 'stria tions' in the ablation cloud. One model relates the striations to the energy reservoir on the plasma flux surfaces and describes the possibi lity of deriving the q-profile; the question of whether this method pr ovides reliable results cannot yet be answered conclusively. The traje ctory of the pellet in the plasma is in general not straight but defle cted in the electron drift direction (OH discharges) or in the ion dri ft direction (CO-NBI discharges). The cloud develops a helically struc tured tail in the electron flow direction (toroidally) and in the elec tron diamagnetic drift direction (poloidally). The tail structure is a ttributed to charge-exchange processes and to plasma rotation.