Scrape-off layer turbulence in TCV: evidence in support of stochastic modelling

TL;DR

This paper investigates turbulence in the TCV scrape-off layer, analyzing probe data to characterize fluctuations and support stochastic modeling of filament-driven plasma behavior.

Contribution

It provides detailed statistical analysis of fluctuation data and offers evidence supporting stochastic models for scrape-off layer turbulence.

Findings

Large-amplitude bursts are frequent and follow exponential distributions.

Burst shapes are well described by double-exponential waveforms.

Fluctuation distributions exhibit exponential tails for large deviations.

Abstract

Intermittent fluctuations in the TCV scrape-off layer have been investigated by analysing long Langmuir probe data time series under stationary conditions, allowing calculation of fluctuation statistics with high accuracy. The ion saturation current signal is dominated by the frequent occurrence of large-amplitude bursts attributed to filament structures moving through the scrape-off layer. The average burst shape is well described by a double-exponential wave-form with constant duration, while the waiting times and peak amplitudes of the bursts both have an exponential distribution. Associated with bursts in the ion saturation current is a dipole shaped floating potential structure and radially outwards directed electric drift velocity and particle flux, with average peak values increasing with the saturation current burst amplitude. The floating potential fluctuations have a normal probability density function while the distributions for the ion saturation current and estimated radial velocity have exponential tails for large fluctuations. These findings are discussed in the light of prevailing theories for filament motion and a stochastic model for intermittent scrape-off layer plasma fluctuations.