  Injecting frozen deuterium pellets into an ELMy H-mode plasma is a well
established scheme for triggering edge localized modes (ELMs) before they
naturally occur. Based on an ASDEX Upgrade H-mode plasma, this article presents
a comparison of extended MHD simulations of spontaneous type-I ELMs and
pellet-triggered ELMs allowing to study their non-linear dynamics in detail. In
particular, pellet-triggered ELMs are simulated by injecting deuterium pellets
into different time points during the pedestal build-up described in [A. Cathey
et al. Nuclear Fusion 60, 124007 (2020)]. Realistic ExB and diamagnetic
background plasma flows as well as the time dependent bootstrap current
evolution are included during the build-up to capture the balance between
stabilising and destabilising terms for the edge instabilities accurately.
Dependencies on the pellet size and injection times are studied. The
spatio-temporal structures of the modes and the resulting divertor heat fluxes
are compared in detail between spontaneous and triggered ELMs. We observe that
the premature excitation of ELMs by means of pellet injection is caused by a
helical perturbation described by a toroidal mode number of n = 1. In
accordance with experimental observations, the pellet-triggered ELMs show
reduced thermal energy losses and narrower divertor wetted area with respect to
spontaneous ELMs. The peak divertor energy fluency is seen to decrease when
ELMs are triggered by pellets injected earlier during the pedestal build-up.
