Numerical turbulence simulations of intermittent fluctuations in the scrape-off layer of magnetized plasmas

TL;DR

This paper uses numerical turbulence simulations to analyze intermittent plasma fluctuations in the scrape-off layer, revealing exponential profiles, burst statistics, and validating the stochastic model against experimental data.

Contribution

It introduces a comprehensive numerical study of plasma turbulence in the scrape-off layer, validating a stochastic model with experimental results and providing new insights into fluctuation statistics.

Findings

Radial density profile is exponential with constant scale length.

Particle density fluctuations have exponential tails in their PDF.

Burst amplitudes and waiting times follow exponential distributions.

Abstract

Intermittent fluctuations in the boundary of magnetically confined plasmas are investigated by numerical turbulence simulations of a reduced fluid model describing the evolution of the plasma density and electric drift vorticity in the two-dimensional plane perpendicular to the magnetic field. Two different cases are considered, one describing resistive drift waves in the edge region and another including only the interchange instability due to unfavorable magnetic field curvature in the scrape-off layer. Analysis of long data time series obtained by single-point recordings are compared to predictions of a stochastic model describing the plasma fluctuations as a super-position of uncorrelated pulses. For both cases investigated, the radial particle density profile in the scrape-off layer is exponential with a radially constant scale length. The probability density function for the particle density fluctuations in the far scrape-off layer has an exponential tail. Radial motion of blob-like structures leads to large-amplitude bursts with an exponential distribution of peak amplitudes and the waiting times between them. The average burst shape is well described by a two-sided exponential function. The frequency power spectral density of the particle density is simply that of the average burst shape and is the same for all radial positions in the scrape-off layer. The fluctuation statistics obtained from the numerical simulations are in excellent agreement with recent experimental measurements on magnetically confined plasmas. The statistical framework defines a new validation metric for boundary turbulence simulations.