The global build-up to intrinsic edge localized mode bursts seen in divertor full flux loops in JET

Abstract

A global signature of the build-up to an intrinsic edge localized mode (ELM) is found in the temporal analytic phase of signals measured in full flux azimuthal loops in the divertor region of JET. Toroidally integrating, full flux loop signals provide a global measurement proportional to the voltage induced by changes in poloidal magnetic flux; they are electromagnetically induced by the dynamics of spatially integrated current density. We perform direct time-domain analysis of the high time-resolution full flux loop signals VLD2 and VLD3. We analyze plasmas where a steady H-mode is sustained over several seconds during which all the observed ELMs are intrinsic; there is no deliberate intent to pace the ELMing process by external means. ELM occurrence times are determined from the Be II emission at the divertor. We previously [Chapman et al., Phys. Plasmas 21, 062302 (2014); Chapman et al., in 41st EPS Conference on Plasma Physics, Europhysics Conference Abstracts (European Physical Society, 2014), Vol. 38F, ISBN 2-914771-90-8] found that the occurrence times of intrinsic ELMs correlate with specific temporal analytic phases of the VLD2 and VLD3 signals. Here, we investigate how the VLD2 and VLD3 temporal analytic phases vary with time in advance of the ELM occurrence time. We identify a build-up to the ELM in which the VLD2 and VLD3 signals progressively align to the temporal analytic phase at which ELMs preferentially occur, on a ∼2−5ms timescale. At the same time, the VLD2 and VLD3 signals become temporally phase synchronized with each other, consistent with the emergence of coherent global dynamics in the integrated current density. In a plasma that remains close to a global magnetic equilibrium, this can reflect bulk displacement or motion of the plasma. This build-up signature to an intrinsic ELM can be extracted from a time interval of data that does not extend beyond the ELM occurrence time, so that these full flux loop signals could assist in ELM prediction or mitigation.

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

“…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][2][3][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.…”