Intermittent electron density and temperature fluctuations and associated fluxes in the Alcator C-Mod scrape-off layer

TL;DR

This study analyzes intermittent fluctuations of electron density, temperature, and fluxes in the Alcator C-Mod scrape-off layer, revealing bursty behavior and validating a stochastic pulse model for plasma turbulence.

Contribution

It provides a detailed statistical analysis of plasma fluctuations and confirms the applicability of a superposition of exponential pulses model to describe these phenomena.

Findings

Electron density and temperature fluctuations are dominated by large-amplitude bursts.

The characteristic time scale of bursts is approximately 15 microseconds.

Radial electron heat flux is mainly driven by convection, with blobs contributing about 6%."

Abstract

The Alcator C-Mod mirror Langmuir probe system has been used to sample data time series of fluctuating plasma parameters in the outboard mid-plane far scrape-off layer. We present a statistical analysis of one second long time series of electron density, temperature, radial electric drift velocity and the corresponding particle and electron heat fluxes. These are sampled during stationary plasma conditions in an ohmically heated, lower single null diverted discharge. The electron density and temperature are strongly correlated and feature fluctuation statistics similar to the ion saturation current. Both electron density and temperature time series are dominated by intermittent, large-amplitude burst with an exponential distribution of both burst amplitudes and waiting times between them. The characteristic time scale of the large-amplitude bursts is approximately 15{\mu}s. Large-amplitude velocity fluctuations feature a slightly faster characteristic time scale and appear at a faster rate than electron density and temperature fluctuations. Describing these time series as a superposition of uncorrelated exponential pulses, we find that probability distribution functions, power spectral densities as well as auto-correlation functions of the data time series agree well with predictions from the stochastic model. The electron particle and heat fluxes present large-amplitude fluctuations. For this low-density plasma, the radial electron heat flux is dominated by convection, that is, correlations of fluctuations in the electron density and radial velocity. Hot and dense blobs contribute approximately 6% of the total fluctuation driven heat flux.