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

Abstract

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.

Auto-correlation function and frequency spectrum due to a super-position of uncorrelated exponential pulses

“…The case of a one-sided exponential pulse function is readily generalized to a pulse shape that is continuous by introducing a finite pulse rise time, 79,82,88 …”

“…[78][79][80][81][82][83][84] From single-point measurements it has been demonstrated that there is an exponential distribution of both the peak amplitudes and the waiting times between them. Moreover, the average duration time does not depend on the amplitude and also appears to be robust against changes in plasma parameters.…”

Stochastic modelling of intermittent fluctuations in the scrape-off layer: Correlations, distributions, level crossings, and moment estimation

“…The statistical properties of these bursts in the SOL of several tokamak plasmas have recently been elucidated by the analysis of long data time series from probe measurements and gas puff imaging. [11][12][13][14][15] Conditional averaging has revealed exponentially distributed burst amplitudes and waiting times, while the burst duration is found to be constant. The wave-form for largeamplitude bursts in the time series is well described by an exponential function.…”

“…[6][7][8][9][10] The average SOL plasma density and the particle and heat fluxes due to the radial motion of plasma filaments depend on their velocity, amplitude, and frequency of occurrence. [11][12][13][14][15] The turbulence-driven transport results in broad plasma profiles in the SOL and enhanced levels of plasma--wall interactions that may be an issue for the next generation plasma confinement experiments and future fusion power reactors. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] The radial motion of blob-like structures leads to largeamplitude bursts in single-point measurements.…”

“…The wave-form for largeamplitude bursts in the time series is well described by an exponential function. [11][12][13][14][15][23][24][25][26][27][28][29][30][31][32] The present study incorporates these features in a stochastic model for intermittent plasma fluctuations in the SOL, described as a super-position of uncorrelated pulses. [34][35][36][37][38][39] This model explains many of the salient experimental findings and empirical scaling relations, including broad plasma profiles and large fluctuation levels, skewed and flattened probability density functions, and a parabolic relation between the skewness and flatness moments.…”