Intermittent fluctuations in the Alcator C-Mod scrape-off layer for ohmic and high confinement mode plasmas

TL;DR

This study analyzes plasma fluctuations in the Alcator C-Mod tokamak's scrape-off layer, revealing that bursty filament structures dominate the fluctuations across different confinement modes, and supports a stochastic model for these phenomena.

Contribution

It demonstrates that plasma fluctuations are well described by a super-position of exponential pulses, consistent across various confinement modes, and validates a stochastic model for these fluctuations.

Findings

Fluctuation statistics are similar in ohmic and high confinement modes.

Burst amplitudes and waiting times follow exponential distributions.

The stochastic model accurately predicts experimental data.

Abstract

Plasma fluctuations in the scrape-off layer of the Alcator C-Mod tokamak in ohmic and high confinement modes have been analyzed using gas puff imaging data. In all cases investigated, the time series of emission from a single spatially-resolved view into the gas puff are dominated by large-amplitude bursts, attributed to blob-like filament structures moving radially outwards and poloidally. There is a remarkable similarity of the fluctuation statistics in ohmic plasmas and in edge localized mode-free and enhanced D-alpha high confinement mode plasmas. Conditionally averaged wave forms have a two-sided exponential shape with comparable temporal scales and asymmetry, while the burst amplitudes and the waiting times between them are exponentially distributed. The probability density functions and the frequency power spectral densities are self-similar for all these confinement modes. These results are strong evidence in support of a stochastic model describing the plasma fluctuations in the scrape-off layer as a super-position of uncorrelated exponential pulses. Predictions of this model are in excellent agreement with experimental measurements in both ohmic and high confinement mode plasmas. The stochastic model thus provides a valuable tool for predicting fluctuation-induced plasma-wall interactions in magnetically confined fusion plasmas.