Transition from no-ELM response to pellet ELM triggering during pedestal build-up -- insights from extended MHD simulations
Pellet ELM triggering is a well established scheme for decreasing the time between two successive ELM crashes below its natural value. Reliable ELM pacing has been demonstrated experimentally in several devices increasing the ELM frequency considerably. However, it was also shown that the frequency cannot be increased arbitrarily due to a so-called lag-time. During this time after a preceding natural or triggered ELM crash, neither a natural ELM crash occurs nor the triggering of an ELM crash by pellet injection is possible. For this article, pellet ELM triggering simulations are advanced beyond previous studies in two ways. Firstly, realistic ExB and diamagnetic background flows are included. And secondly, the pellet is injected at different stages of the pedestal build-up. This allows to recover the lag-time for the first time in simulations and investigate it in detail. A series of non-linear extended MHD simulations is performed to investigate the plasma dynamics resulting from an injection at different time points during the pedestal build-up. The experimentally observed lag-time is qualitatively reproduced well. In particular, a sharp transition is observed between the regime where no ELMs can be triggered and the regime where pellet injection causes an ELM crash. Via variations of pellet parameters and injection time, the two regimes are studied and compared in detail revealing pronounced differences in the non-linear dynamics. The toroidal mode spectrum is significantly broader when an ELM crash is triggered enhancing the stochasticity and therefore also the losses of thermal energy along magnetic field lines. In the heat fluxes to the divertor targets, pronounced toroidal asymmetries are observed. In case of high injection velocities leading to deep penetration, also the excitation of core modes like the $2/1$ neoclassical tearing mode is observed.