Development of a precise size-controllable pellet injector for the detailed studies of ablation phenamena and mechanism

Abstract

From the viewpoint of performance of nuclear fusion plasmas, pellet injection experiments have been actively carried out in many toroidal devices in the sense of controlling density profile, obtaining high density or improved confinement, and diagnostic purposes. In order to have a common measure of pellet ablation, the regression study has been performed as an international cooperation activity, obtaining "IPAD" (International Pellet Ablation Database) >. However, these are an empirical scaling, and the mechanism of pellet ablation still remains to be studied. According to the code calculations based on a typical pellet ablation model (e. g., so-called the neutral gas shielding model), it is understood that the penetration depth into plasma is always quite sensitive to the pellet size. If the pellet size is too large, the pellet passes through the plasma, and if it is too small, it is trapped at the plasma surface. Also, an effective or suitable range of the pellet size for a certain plasma is generally very narrow, and this range largely varies depending on each plasma size and plasma parameters. Thus, the precise controllability of the pellet size, especially the size controllability with continuously variable system will be quite effective in order to carry out the detailed studies on pellet ablation and associated phenomena. A pellet injector of new type with precisely and continuously controllable system of pellet size is being developed. This has a unique mechanics and structure of producing a frozen pellet in extremely low temperature region. In the device presently developed in this research, we will precisely adjust the length of the cylindrical pellet ($Φ$ 1.0mm) from 0.5 to 3 mm by using the special "length restriction rod".