Plasma density buildup after pellet injection

Details of the increase of the plasma density following a pellet injection have been measured with high temporal resolution (16 mu s) and the parallel expansion of ablated matter modeled with a four-fluid hydrodynamic code. The driving force of the expansion is the parallel pressure gradient, progressively balanced by the compression of the background plasma. It is shown that the ablated material experiences a strong transient poloidal motion (approximate to 5x10(3) ms(-1), approximate to 100 mu s) as it expands along the field lines. This motion, which is induced by the pellet itself, results from the conservation of kinetic momentum: the convective motion in the sheet of ablated material (due to its positive potential with respect to the plasma) is compensated by a global drift of the whole magnetic surface. This model reproduces the main observations concerning the parallel propagation of the ablatant in the discharge. In particular, it shows that the stretching of the sheet of ablated material by the magnetic shear and the poloidal rotation is responsible for the homogenization of the density in a characteristic time of approximate to 1 ms. The plasma rotation measured immediately after a pellet injection is therefore not, in general, simply linked to the background radial electric field. (C) 1996 American Institute of Physics.