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According to recent suggestions, turbulent transport in fusion plasmas could possibly be characterized by Self-Organized Criticality (SOC) [1, 2, 3]. Models based on SOC predict transport by avalanches, which would generate self-similar fractal behaviour in space and time of the turbulent qualities. The attractiveness of the hypothesis lies in several predictions that seem to be in accord with observations of Fusion plasmas: (1) the existance of a critical gradient that is robust to a certain extent (the well-known "profile consistency" of fusion plasmas that causes the profiles to be rather insensitive to the power deposition profile, (2) long-range interactions (the observed radial cold and heat pulses that travel faster than diffusivity), and (3) Bohm scaling of the diffusivities.

To test the SOC hypothesis, first one needs to demonstrate the self-similar nature of the turbulence. This can in principal be tested by accurately determining the shape of the autocorrelational function (ACF) of turbulent signals. The ability to discern between an algebraic or exponential decay of the ACF at large lags would provide an indication of whether SOC models could be appropriate descriptions of the turbulence. Unfortunately, the critical information is present in the tail of the distribution where statistics are generally poor, making this approach inviable for experimental data analysis.

In this paper we will focus on an alternative technique, the "Rescaled Range" analysis, discuss its merits and provide experimental results regarding the analysis of data obtained with Langmuir probes from plasma edge turbulence in a wide range of plasma devices. We find significant and similar values of the Hurst exponent in all devices, pointing towards a possible universality of the turbulence. ce.

Additional information

Authors: MARTINES E, Consorzio RFX, Padova (IT);ANTONI V, Consorzio RFX, Padova (IT);LATTEN A, Institut f³r Experimentalphysik, Kiel (DE);KLINGER T, Institut f³r Experimentalphysik, Kiel (DE);VAN MILLIGEN B ET AL, Asociaci¾n Euratom-Ciemat, Madrid (ES);CARRERAS B A, Oak Ridge National Laboratory, Oak Ridge, Tennessee (US);NEWMAN D E, Oak Ridge National Laboratory, Oak Ridge, Tennessee (US);BLEUEL J, EURATOM-IPP, Garching (DE);ENDLER M, EURATOM-IPP, Garching (DE);DAVIES S, JET Joint Undertaking, Abingdon (GB);MATTHEWS G F, JET Joint Undertaking, Abingdon (GB);MARTINES, JET Joint Undertaking, Abingdon (GB);RICCARDI C, Dipartimento di Fisica, Universita di Milano (IT)
Bibliographic Reference: Article: Theory of Fusion Plasmas (2000) pp. 333-346
Availability: Theory of Fusion Plasmas (Journal)
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