Intense, short duration relaxation events known as edge localized modes (ELMs) generally accompany enhanced confinement (H-mode) regimes in tokamak plasmas. Mitigation of large amplitude ELMs is essential for ITER because each ELM releases particles and energy which load the plasma facing components; scaled up to ITER, the largest such loads would be unacceptable. ELMs also play a role in removing impurities from the plasma, which should also be achieved in a controllable manner. We focus on JET plasmas in which ELMs are triggered by pellets in the presence of ELMs which occur naturally. We perform direct time domain analysis of signals that provide simultaneous high time resolution measurements of global plasma dynamics and its coupling to the control system. We examine the time dynamics of these signals in plasmas where pellet injection is used to trigger ELMs in the presence of naturally occurring ELMs. Pellets whose size and speed are intended to provide maximum local perturbation for ELM triggering are launched at pre-programmed times, without correlation to the occurrence times of intrinsic ELMs. We find a global signature of the build-up to natural ELMs but not for ELMs which are the first to occur following pellet injection. This provides a direct test that can distinguish when an ELM is triggered by a pellet as opposed to occurring naturally. It further supports the idea of a global build up phase that precedes natural ELMs; pellets can trigger ELMs even when the signal phase is at a value when a natural ELM is unlikely to occur.

• Caption to figure: The inside of the JET tokamak, the glowing plasma indicates energy loss.
• Publication: S. C. Chapman, R. O. Dendy, P. T. Lang, N. W. Watkins, F. Calderon, M. Romanelli, T. N. Todd and JET Contributors, The global build-up to intrinsic ELM bursts and comparison with pellet precipitated ELMs seen in JET, Nucl. Fusion, 57, 022017, (2017) (appeared 2016)
• DOI: http://dx.doi.org/10.1088/0029-5515/57/2/022017