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Edge Localised Modes (ELMs) are momentary breakdowns in the edge transport barrier responsible for the H-mode in tokamak plasmas, and give rise to sharp spikes in the Da emission from the plasma edge, allowing their easy observation. An investigation of Da emission time-series from the TCV tokamak was carried out on a large set of ELMy discharges with the purpose of determining whether the apparently random variations in the time delay between ELMs is the result of noise only, or whether a deterministic process in a chaotic state exists, as suggested by results on JT - 60U [1]. This is an important question for two reasons: 1) Any candidate model of ELM dynamics must belong to the same behavioural family; 2) feedback control algorithms have been developed for stabilising low dimensional chaotic dynamical systems using small perturbations to the system via a control parameter. If ELMs are chaotic then it might be possible to apply such an algorithm to regulate ELM occurrences.

This investigation was carried out by searching the time-series of each discharge for transient sequences that are generic to chaotic systems, called Unstable Periodic Orbits (UPOs). A subset of discharges was shown to contain many more UPOs than could be expected if a random process was operating, and that the properties of VPOs exhibit systematic variations with plasma parameters. Therefore it is concluded that a deterministic, chaotic process is at work in these cases, confirming and extending the results from JT-60U. The implications for ELM models and the prospects of controlling ELMs in light of these findings are discussed. .

Additional information

Authors: DEGELING A.W, CRPP, EPF-Lausanne (CH);MARTIN Y.R, CRPP, EPF-Lausanne (CH);BAK P.E, CRPP, EPF-Lausanne (CH);LISTER J.B, CRPP, EPF-Lausanne (CH);LLOBET X, CRPP, EPF-Lausanne (CH)
Bibliographic Reference: An article published in: Plasma Physics and Controlled Fusion, (2001), pp.46
Record Number: 200013792 / Last updated on: 2001-10-03
Original language: en
Available languages: en