Turbulence intermittency and burst properties in tokamak scrape-off layer

Density fluctuation measured by a reciprocating Langmuir probe on the Tore Supra tokamak [Garbet , Nucl. Fusion 32, 2147 (1992)] is investigated. The purpose of this article is to give a rather comprehensive analysis of intermittency by using several data analyses to compare the bursts properties to that of coherent structures and avalanches. The probability distribution of the density fluctuations is found positively skewed, while a Gaussian shape for the negative values is recorded. It is shown that the fluctuation spectra possess one scaling region with a power law close to the one predicted by a Kolmogorov-Kraichnan model in the inverse cascade subrange. However, a net deviation from this law at higher moment orders is demonstrated. The deviation from the mono-fractal model is investigated by the multifractal analysis that reveals the variety of the dissipative structures similar to what is found in fully developed fluid turbulence. The spectra are found asymmetric, indicating the presence of structures not generated by a multiplicative process. Using conditional analysis, a detailed study of the intermittent bursts independently of the background is performed. The typical form of the intermittent structures is asymmetric. Furthermore, they do not conserve mass for only positive density fluctuations are recorded. Their poloidal velocity is estimated to be 70% greater than the background turbulence, suggesting that they may not result from a diffusive process. (C) 2001 American Institute of Physics.