Modelling of impurity pellet ablation in ASDEX Upgrade (neon) and Wendelstein W7-AS (carbon) by means of a radiative (`killer') pellet code

The penetration depths of different impurity pellets, such as carbon and neon, injected into different thermonuclear devices were reproduced by means of a single numerical code with the same set of assumptions, only the atom physical data being changed. All major characteristics of the ablation process were calculated: the spatial variation of the ablation rate, the depositon of ablated particles at a succession of magnetic flux surfaces, the expansion of deposited particles in the directions both parallel and perpendicular to the magnetic field lines, and the temporal and spatial variations of the radiant power emitted by the expanding impurity cloud. The calculations were done by means of a time dependent quasi-three-dimensional code consisting of three modules accounting for the B⊥ and B|| expansions of the cloud and the traversing motion of the pellet, operated interactively and, when needed, iteratively. The radiation characteristics were computed by a collisional-radiative loss model, developed for low temperature light impurities, without the usual equilibrium assumptions. With some modifications, the code is adaptable to predictive pre-disruptive `killer pellet' scenario calculations for future large scale machines, such as ITER.