Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

Abstract

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM.

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

“…There is a lower cutoff in the ELM waiting time at Dt $ 10ms, and there are time intervals where ELMs occur less often (Ref. 16, and in other plasmas 12 ). Large ensemble statistical studies across many JET plasmas have also revealed 16,17 that the ELM waiting time probability distribution shows time structure, that is, some ELM waiting times are more likely than others.…”

“…We determine the ELM occurrence times t k by identifying the peak of the Be II signal within each ELM using the method described in Ref. 16.…”

“…Since the full flux loop signals capture aspects of the global plasma dynamics including large scale plasma motion, this may suggest, as first proposed in Ref. 17, a nonlinear feedback on a global scale where the control system and plasma behave as a single nonlinearly coupled system, rather than as driver and response. This feedback may act to pace the intrinsic ELMs.…”

“…The organization of the paper is as follows: in Section II we introduce the data used in this study, in Section III we describe how the full flux loop instantaneous temporal analytic phases are determined, our main results are given in Section IV, in Section V we quantify the statistical significance of these results, and in Section VI we present a possible interpretation of these results following the scenario of Ref. 17. We provide significance tests against null hypotheses, that is, phase alignment by chance coincidence, in the Appendix.…”

“…Recently, 16,17 we found that the signals from a system scale diagnostic, the toroidally integrating full flux loops in the divertor region of JET, contain statistically significant information on the occurrence times of intrinsic ELMs: the ELMs tend to preferentially occur when the full flux loop signals are at a specific analytic phase of their timeseries. Since the full flux loop signals capture aspects of the global plasma dynamics including large scale plasma motion, this may suggest, as first proposed in Ref.…”