Abstract
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 from fast radial field coils and toroidal full flux azimuthal loops. These toroidally integrating signals 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. Pellet rates were sufficiently low to prevent sustained changes of the underlying plasma conditions and natural ELM behaviour. We find a global signature of the build-up to natural ELMs in the temporal analytic phase of both the full flux loops and fast radial field coil signals. Before a natural ELM, the signal phases align to the same value on a 2–5 ms timescale. This global build up to a natural ELM occurs whilst the amplitude of the full flux loop and fast radial field coil signals are at their background value: it precedes the response seen in these signals to the onset of ELMing. In contrast these signals do not clearly phase align before the ELM 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.
Highlights
Intense, short duration relaxation events known as edge localized modes (ELMs) [5,6,7,8,9] generally accompany enhanced confinement (H-mode) regimes in tokamak plasmas
This result further supports the idea of a global build up phase to naturally occuring ELMs: pellets can trigger ELMs even when the signal phase is at a value when a natural ELM is unlikely to occur
We analysed the signature of the global build-up to ELMs in JET plasmas where ELMs are triggered by pellet injection in the presence of ELMs that are naturally occurring; the natural ELM frequency exceeds that of the injected pellets
Summary
Short duration relaxation events known as edge localized modes (ELMs) [5,6,7,8,9] generally accompany enhanced confinement (H-mode) regimes in tokamak plasmas. [1,2,3,4] we found that for naturally occurring ELMs, the phase of the signals from a system scale diagnostic, the toroidally integrating full flux loops in the divertor region of JET, contains statistically significant information on the ELM occurrence times. We investigate the time dynamics of the signal temporal analytic phases, and directly test whether these signal phases contain information on the build-up to an ELM We find that these signals do not clearly phase align before the pellet-triggered ELMs in these signals, whereas they do align for the natural ELMs within the same plasmas. Low amplitude background fluctuations in the active control system field coil current and passive full flux loops could become phase synchronized [38,39,40], through their individual interactions without the need of coupling between them
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