Study of L-H transition triggered by pellet injection based on a power threshold model

Simulations of ITER plasma during an L-H transition triggered by pellet injection are carried out using the 1.5D BALDUR integrated predictive modeling code. In these simulations, plasma core transport is predicted using a combination of the Multimode (MMM95) turbulent transport model and neoclassical transport (NCLASS) model. The pellet ablation behavior is described using the neutral gas shielding pellet model with the grad- B drift effect also included. Because of the increase of the plasma density and the reduction of the temperature, the plasma self-heating powers (ohmic heating and alpha heating) increase, whereas the plasma energy loss (through radiation) decreases. As a result, the total power across the separatrix increases. On the other hand, the L-H power threshold decreases. With all the changes of heating, an L-H transition can be induced even though the auxiliary heating is not enough for the L-H transition before pellet injection. It is also found that the L-H transition triggered by pellet injection depends sensitively on the pellet radius, but only moderately on the pellet velocity.