Collins Conference Room
Seminar
Speaker: 
Sandra C. Chapman (University of Warwick)

This event is closed.

Abstract. A ubiquitous aspect of strongly connected, many component systems is the potential for self-organization to synchronous states, in the sense of nonlinear active feedback between global and local scales which leads to the emergence of global spatiotemporal coherent dynamics. Tokamak experiments for magnetically confined fusion (MCF) support large-scale plasmas with nonlinear coupling of plasma physics processes over several orders of magnitude in length and timescales- they span a similar range of scales as other complex systems such as earth’s climate and stock markets.

Emergent phenomenology is found in these plasma systems: they self-organize to generate large-scale structures and flows with enhanced high confinement, known as H-mode. Edge localized modes (ELMs) are intense bursty energy release events observed to occur naturally in these tokamak H-mode regimes. Each ELM releases particles and energy which load the plasma facing components. When these are scaled up to the next generation of tokamaks that are large enough to achieve fusion (c.f. ITER) these loads would be unacceptably large, ‘solving’ the ELM problem is thus in the critical path the achieving fusion energy using tokamaks.

In order to maintain a steady state, tokamak plasmas are continually stabilized by an active control system. This talk will present high time-resolution time domain time series analysis on the JET and ASDEX Upgrade experiments. We recently found evidence that under certain conditions, the temporal phase of the control system vertical field coil current contains precursor information on when the next natural ELM  burst will occur; importantly, even when the amplitude of the field coil current is small. In some cases, the plasma can transition into a state in which the control system field coil currents continually synchronously oscillate with the global plasma. 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 phase synchronous states in which nonlinear feedback between plasma and control system is intrinsic to natural ELMing. It is a new paradigm which raises questions about how one can test for correlation and causality in real world systems.

Purpose: 
Research Collaboration
SFI Host: 
Sid Redner