Plasma Physics Colloquium
Friday,
October 13, 2017
2:00 PM - 3:00 PM
"Control system-plasma synchronization and naturally occurring edge localized tokamak modes"
Sandra C Chapman
University of Warwick/Boston University/2017-18 Fulbright Lloyd’s of London Scholar
A ubiquitous aspect of strongly connected, many component systems is the potential for self- organisation to synchronous states, in the sense of nonlinear active feedback between global and local scales which leads to the emergence of global spatio-temporal 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. Emergent phenomenology is found in these large scale plasma systems: they self-organise to generate large scale structures and flows with enhanced high confinement, known as H-mode. Edge localized modes (ELMs) are intense, short duration relaxation events observed in tokamak H-mode regimes. Typically, in present day devices a few hundred ELMs occur naturally in the quasi-stationary phase of H-mode plasmas. Each ELM releases particles and energy which load the plasma facing components; scaled up to ITER, the largest such loads would be unacceptable.
Sandra C Chapman
University of Warwick/Boston University/2017-18 Fulbright Lloyd’s of London Scholar
A ubiquitous aspect of strongly connected, many component systems is the potential for self- organisation to synchronous states, in the sense of nonlinear active feedback between global and local scales which leads to the emergence of global spatio-temporal 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. Emergent phenomenology is found in these large scale plasma systems: they self-organise to generate large scale structures and flows with enhanced high confinement, known as H-mode. Edge localized modes (ELMs) are intense, short duration relaxation events observed in tokamak H-mode regimes. Typically, in present day devices a few hundred ELMs occur naturally in the quasi-stationary phase of H-mode plasmas. Each ELM releases particles and energy which load the plasma facing components; scaled up to ITER, the largest such loads would be unacceptable.
In addition, ELMs are key in removing impurities from the plasma which must also be achieved in a controllable manner. Thus ELM prediction, mitigation and control are central to MCF research. The peeling-ballooning MHD instability of the plasma edge is believed to underlie ELM initiation, however a comprehensive model for the birth-to- death ELM cycle is not yet available. In order to maintain a steady state, the plasma is continually stabilized by an active control system. This talk will present time domain time series analysis on the JET and ASDEX Upgrade experiments that investigates the role that the control system plays in the global plasma dynamics associated with natural ELMing. Under certain conditions, the temporal phase of the vertical field coil current is found to contain information on when the next natural ELM 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 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 phase synchronous states in which nonlinear feedback between plasma and control system is intrinsic to natural ELMing.
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