Level-crossing function in the analysis of edge plasma turbulence

The amplitude variability of the confined fusion plasma turbulence is neglected in order to study solely its zero-crossing information. The retained zero-crossing set enables insight into various aspects of edge plasma turbulence. Certain common features with neutral fluid turbulence are recognized in the low confinement regime (L-mode). Spectral characteristics of the zero-crossing set are compared between L-mode, dithering H-mode and H-mode plasma edge turbulence and a scaling exponent characterizing the tendency of small scales to cluster is introduced. A transformation invariant level-crossing function is shown to be a more reliable indicator of stable equilibrium states than the probability density function. In particular the zero-crossing function detects L–H transitions and recognizes edge localized modes as catastrophic events in the sense of stochastic catastrophe theory. The method may be of significant importance in the analysis and control of edge plasma trubulence.