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Pellet-plasma interaction is investigated both experimentally at ASDEX Upgrade tokamak and theoretically based on the obtained experimental data. For ELM triggering pellets were injected from the high field side at the ASDEX Upgrade tokamak into type-I ELMy H-mode plasma with a frequency much smaller than the natural ELM frequency. Every pellet triggered an ELM. In order to gain information about the triggering mechanism the delay between the time the pellet crossing the separatrix and the ELM onset was investigated by injecting pellets with two different velocities (240m/s, 600m/s). It was found that the delay time is in the order of 100μs and is shorter at higher pellet velocity. Pellets trigger ELMs when only a minor part the mass is ablated in the plasma: about 2 · 1018 (vp = 240m/s) and 8 · 1017 (vp = 600m/s) particles are deposited along the pellet path until the ELM onset is detected. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]