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Detecting changes in plasmas is compulsory for control and the detection of novelties.
Moreover, automated novelty detection allows one to investigate large data sets to substantially
enhance the efficiency of data mining approaches. To this end we introduce permutation entropy
(PE) for the detection of changes in plasmas. PE is an information-theoretic complexity measure
based in fluctuation analysis that quantifies the degree of randomness (resp. disorder,
unpredictability) of the ordering of time series data. This method is computationally fast and
robust against noise, which allows the evaluation of large data sets in an automated procedure.
PE is applied on electron cyclotron emission and soft x-ray measurements in different
Wendelstein 7-X low-iota configuration plasmas. A spontaneous transition to high core-electron
temperature (Te) was detected, as well as a localized low-coherent intermittent oscillation which
ceased when Te increased in the transition. The results are validated with spectrogram analysis
and provide evidence that a complexity measure such as PE is a method to support in-situ
monitoring of plasma parameters and for novelty detection in plasma data. Moreover, the
acceleration in processing time offers implementations of plasma-state-detection that provides
results fast enough to induce control actions even during the experiment.
AbstractFluctuations of electron cyclotron emission (ECE) signals are analyzed for differently heated Wendelstein 7-X plasmas. The fluctuations appear to travel predominantly on flux surfaces and are used as ‘tracers’ in multivariate time series. Different statistical techniques are assessed to reveal the coupling and information entropy-based coupling analysis are conducted. All these techniques provide evidence that the fluctuation analysis allows one to check the consistency of magneto-hydrodynamic (MHD) equilibrium calculations. Expanding the suite of techniques applied in fusion data analysis, partial mutual information (PMI) analysis is introduced. PMI generalizes traditional partial correlation (Frenzel and Pompe Phys. Rev. Lett.
99 204101) and also Schreiber’s transfer entropy (Schreiber 2000 Phys. Rev. Lett.
85 461). The main additional capability of PMI is to allow one to discount for specific spurious data. Since PMI analysis allows one to study the effect of common drivers, the influence of the electron cyclotron resonance heating on the mutual dependencies of simultaneous ECE measurements was assessed. Additionally, MHD mode activity was found to be coupled in a limited volume in the plasma core for different plasmas. The study reveals an experimental test for equilibrium calculations and ECE radiation transport.