Abstract
It is well established that edge localized modes can be entrained to the frequency of applied global magnetic perturbations. These perturbations are delivered to the plasma using the vertical control system field coil currents. These field coils are part of an active control system that is required to maintain the plasma in a steady state. We perform time domain timeseries analysis of natural ELMing when there are no applied perturbations in the ASDEX Upgrade tokamak. We find that the plasma can transition into a state in which the control system field coil currents continually oscillate and are synchronized with oscillations in characteristic plasma parameters such as plasma edge position and total MHD energy. These synchronous oscillations have a one-to-one correlation with the naturally occurring ELMs; the ELMs all occur when the control system coil current is around a specific temporal phase. Large and small ELMs may be distinguished by the amplitude of inward movement of the edge following an ELM. Large ELMs are then found to occur preferentially around a specific temporal phase of the vertical position control coil current. Small ELMs are most likely in antiphase to this. The large and small natural ELMs occur at the opposite extrema of the oscillations in the control system vertical position control coil current. The control system coil current phase may thus provide a useful parameter to order the observed ELM dynamics. We have identified a class of natural ELMing which is a self-entrained state, in which there is a continual non-linear feedback between the global plasma dynamics and the active control system that is intrinsic to the cyclic dynamics of naturally occurring ELMs. Control system-plasma feedback thus becomes an essential component for integration into future models of natural ELM dynamics.
Highlights
Intense, short duration relaxation events known as edge localized modes (ELMs) [1,2,3,4,5] often accompany enhanced confinement (H-mode) regimes in tokamak plasmas
Time domain time series analysis in an ASDEX Upgrade plasma has identified an interval of natural ELMing in which current in the control system vertical position stabilization field coils continually oscillates, and is synchronized with oscillations in the plasma edge position and total MHD energy
Self-entrained state there is a one-to-one correlation between oscillations in the vertical position control coil current and ELM occurrence; ELMs occur preferentially when oscillations in the current in the control system vertical stabilization field coils are at a specific temporal phase
Summary
Short duration relaxation events known as edge localized modes (ELMs) [1,2,3,4,5] often accompany enhanced confinement (H-mode) regimes in tokamak plasmas. Forced large scale plasma perturbations are achieved via magnetic kicks by pulsing the current in coils that encircle the plasma toroidally to generate poloidal fields In these experiments, the ELM occurrence frequency can become entrained [24] to that of the externally applied driving current. We found in JET that the timeseries of system scale variables such as the current in full flux loops in the divertor region, [27,28,29,30] and in control system poloidal field coils, that all encircle the plasma toroidally [31, 33] contain statistically significant information on when ELMs will naturally occur. This mutually synchronous, or self-entrained dynamics is the topic of this paper
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