Abstract

Edge Localised Modes (ELMs) naturally occur in tokamak plasmas in high confinement mode. We find in ASDEX Upgrade that the plasma can transition into a state in which the control system field coil currents, required to continually stabilize the plasma, continually oscillate with the plasma edge position and total MHD energy. These synchronous oscillations are one-to-one correlated with the occurrence of natural ELMs; the ELMs all occur when the control system coil current is around a specific phase. This suggests a phase synchronous state in which nonlinear feedback between plasma and control system is intrinsic to natural ELMing, and in which the occurrence time of a natural ELM is conditional on the phase of the control system field coil current.

Highlights

  • Large-scale tokamak experiments self-organise to generate large scale structures and flow with enhanced confinement, known as H-mode.1 Edge localized modes.2–5 (ELMs) are intense, short duration relaxation events observed in tokamak H-mode regimes

  • Oscillations coinciding with the occurrence times of all the natural Edge Localised Modes (ELMs). In such a synchronous state, continual nonlinear feedback between global plasma dynamics and control system is intrinsic to natural ELMing

  • We report observational support for a new hypothesis29–32 that naturally occurring ELMs can result from phase coherent nonlinear feedback between plasma and the control system that is required to stabilize the plasma

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Summary

INTRODUCTION

Large-scale tokamak experiments self-organise to generate large scale structures and flow with enhanced confinement, known as H-mode. Edge localized modes. (ELMs) are intense, short duration relaxation events observed in tokamak H-mode regimes. We can apply a simple algorithm across the entire timeseries to identify the time just before the ELM, at which the stored energy WMHD and outboard edge Rout are both at peak values just before the ELM. The control system field coil current instantaneous phase /ðICu Þ is obtained from the analytic signal SðtÞ þ iHðtÞ 1⁄4 A exp 1⁄2i/ðtފ (H(t) is the Hilbert transform of S(t)). Phases are given relative to the average h/ðtBÞi over all ELMs in the interval

OVERVIEW OF INTERVALS OF SYNCHRONOUS DYNAMICS IN PLASMAS 30416 AND 30930
Plasma 30416 overview and time domain behaviour
Plasma 30930 overview and time domain behaviour
STATISTICAL QUANTIFICATION OF PHASE ALIGNMENT
SYNCHRONOUS DYNAMICS
Findings
DISCUSSION
CONCLUSIONS
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