  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
($\Phi$ 1.0mm) from 0.5 to 3 mm by using the special "length restriction
rod".&#12288;&#12288;
