Burst statistics in Alcator C-Mod SOL turbulence

TL;DR

This paper analyzes bursty fluctuations in the Alcator C-Mod SOL turbulence, revealing statistical properties similar to probe measurements and validating a stochastic model predicting Gamma-distributed amplitudes.

Contribution

It introduces a stochastic model of burst dynamics in SOL turbulence and validates it against gas puff imaging data, linking burst statistics to Gamma distributions.

Findings

Fluctuations are normally distributed near the SOL and burst-dominated farther out.

Burst waveforms have a fast rise and slow decay.

Fluctuation amplitudes follow a Gamma distribution, matching the model.

Abstract

Bursty fluctuations in the scrape-off layer (SOL) of Alcator C-Mod have been analyzed using gas puff imaging data. This reveals many of the same fluctuation properties as Langmuir probe measurements, including normal distributed fluctuations in the near SOL region while the far SOL plasma is dominated by large amplitude bursts due to radial motion of blob-like structures. Conditional averaging reveals burst wave forms with a fast rise and slow decay and exponentially distributed waiting times. Based on this, a stochastic model of burst dynamics is constructed. The model predicts that fluctuation amplitudes should follow a Gamma distribution. This is shown to be a good description of the gas puff imaging data, validating this aspect of the model.