Fluctuation statistics in the scrape-off layer of Alcator C-Mod

TL;DR

This study analyzes fluctuation statistics in the scrape-off layer of Alcator C-Mod, revealing large-amplitude burst events, their statistical properties, and how blob dynamics vary with plasma conditions.

Contribution

It provides a detailed statistical characterization of fluctuation events and blob dynamics in Alcator C-Mod's scrape-off layer, linking these to plasma density and divertor conditions.

Findings

Ion saturation current exhibits large burst events with specific statistical distributions.

Blob velocity increases with burst amplitude in the scrape-off layer.

Waveform shapes of bursts change under different divertor conditions.

Abstract

We study long time series of the ion saturation current and floating potential, sampled by Langmuir probes dwelled in the outboard mid-plane scrape off layer and embedded in the lower divertor baffle of Alcator C-Mod. A series of ohmically heated L-mode plasma discharges is investigated with line-averaged plasma density ranging from n_e/n_G = 0.15 to 0.42, where n_G is the Greenwald density. All ion saturation current time series that are sampled in the far scrape-off layer are characterized by large-amplitude burst events. Coefficients of skewness and excess kurtosis of the time series obey a quadratic relationship and their histograms coincide partially upon proper normalization. Histograms of the ion saturation current time series are found to agree well with a prediction of a stochastic model for the particle density fluctuations in scrape-off layer plasmas. The distribution of the waiting times between successive large-amplitude burst events and of the burst amplitudes are approximately described by exponential distributions. The average waiting time and burst amplitude are found to vary weakly with the line-averaged plasma density. Conditional averaging reveals that the radial blob velocity, estimated from floating potential measurements, increases with the normalized burst amplitude in the outboard mid-plane scrape-off layer. For low density discharges, the conditionally averaged waveform of the floating potential associated with large amplitude bursts at the divertor probes has a dipolar shape. In detached divertor conditions the average waveform is random, indicating electrical disconnection of blobs from the sheaths at the divertor targets.