Investigations on the ELM cycle by local 3D perturbation experiments

An experimental study is performed to investigate the underlying physics of ELM triggering by imposing local perturbations at the plasma edge. Deuterium is injected during type-I ELMy H-mode phases by solid small pellets or as a supersonic gas jet. In both cases the repetition rate is small compared to the intrinsic ELM frequency, aiming for a small perturbation of the intrinsic ELM cycle. Active triggering of ELMs requires a density perturbation in the gradient region about 1 cm inside the separatrix (measured at horizontal mid plane), a condition that is achieved by pellets of any size and speed but not even by the strongest available gas jets. The pellet-induced triggering of ELMs occurs always when pellets reach the H-mode barrier region. A density perturbation produced by a gas jet remains localized at the separatrix, and even if larger by a factor of at least 100, it is not sufficient to trigger an ELM. No significant dependence was found of the ELM size on the ELM onset time, measured from the onset of the previous ELM. 1 1