The global build-up to intrinsic ELM bursts and comparison with pellet triggered ELMs seen in JET

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 [1-4] 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.

Control system-plasma synchronization and naturally occurring edge localized modes in a tokamak

“…29 In these JET plasmas, we found that the phase of toroidal full flux loop signals became aligned around a single value just before the onset of each natural ELM. [29][30][31][32] In particular, 30 we found a class of prompt natural ELMs which are at distinct short inter-ELM time intervals that occur at a specific phase of the plasma's own response to the previous ELM. For these ELMs, the initial ELM and its successor form a linked pair, in that the second ELM arises near the end of the first.…”

“…The vertical control coil current phase may provide a parameter that orders ELM cycle dynamics even if the ELM frequency is drifting. On JET, we found a class of prompt [29][30][31][32] natural ELMs that occur at a specific phase of the plasmas own response to the previous ELM. We would thus expect under some plasma conditions a global dynamics where all the ELMs are "prompt" with each ELM directly following the previous one.…”

“…Importantly, this phase coherent feedback which we propose here is distinct from a scenario, whereby the control system causes ELMs by direct destabilisation, and from the entrainment of triggered ELMs by externally applied vertical magnetic kicks [9][10][11][12] which relies on kicks of sufficiently large amplitude, typically much larger than that seen in the control system vertical field coil current during natural ELMing. Phase coherent feedback in natural ELMing is found on JET [29][30][31][32] in which the occurrence time of natural ELMs can be conditioned by the phase, rather than the amplitude, of the control system and global plasma dynamics. If such a relationship exists between the control system and naturally occurring ELMs, then we would anticipate that under certain conditions the coupled control system and a) Electronic mail: S.C.Chapman@warwick.ac.uk 1070-664X/2018/25(6)/062511/9…”

“…The control system is not a part of standard physical models for the ELM cycle. A new hypothesis [29][30][31][32] is that phase coherent nonlinear feedback between the plasma and the automatic control system is part of the observed natural ELM cycle. Importantly, this phase coherent feedback which we propose here is distinct from a scenario, whereby the control system causes ELMs by direct destabilisation, and from the entrainment of triggered ELMs by externally applied vertical magnetic kicks [9][10][11][12] which relies on kicks of sufficiently large amplitude, typically much larger than that seen in the control system vertical field coil current during natural ELMing.…”

Intrinsic ELMing in ASDEX Upgrade and global control system-plasma self-entrainment

“…This process modifies the current in the field coils, generating inductive magnetic fields that react back on the plasma. The question is then whether in the absence of any external driving, the non-linearly coupled global plasma and control system dynamics plays a role in the physics of naturally occurring ELMs [27][28][29][30][31]33].…”