Statistical analysis of fluctuations in the Alcator C-Mod scrape-off layer

Abstract

Experimental data from Alcator C-Mod have been used to analyze large density events occurring at the edge of fusion reactors. The data acquisition methods used are gas puff imaging and Langmuir probes. The data consist of long time series sampled at high frequency, excellent for statistical analysis. Large density events have been analyzed where blobs are found to be consistent with high skewness and kurtosis. The skewness and kurtosis are found to increase with both radius and decreasing line-averaged density. For small kurtosis and skewness the distributions are well fitted by a gamma distribution. The conditionally averaged waveforms in the scrape-off layer have a sharp exponential rise with slower exponential decay. The waveform is independent of line-averaged density, major radius and amplitude. In addition the waiting times between events are found to be exponentially distributed, consistent with independent events in a Poisson process. Self-similarity and long range correlations have been analyzed by using the Hurst exponent. The Hurst exponents have been estimated by four methods, power specter, rescaled range, structure functions and wavelets. The Hurst exponents are found to decrease with major radius, where only weak long range correlations are found in the blob dominated area. The average blob velocities are found to be 1-3km/s both in radial and vertical direction, consistent with a mixture of kink and ballooning magnetohydrodynamic instabilities. The blobs have a poloidal size of 1-2cm, independent of radial position and line averaged density. The stochastic model proposed by Garcia is found to be a good fit for signals close to the separatrix, where the blobs are created. The model can not be used to explain long range correlations.