Numerical computations of turbulent motions in magnetized plasmas

Abstract

Intermittent fluctuations and turbulence-induced transport of magnetically confined fusion plasmas are investigated by numerical computations. A reduced fluid model describing the evolution of plasma pressure and electric drift vorticity in a two-dimensional plane perpendicular to the magnetic field is derived. A numerical simulation code implemented on graphical processing units is presented. We observe significant speedup compared to sequential Fortran implementations.

The convective motions are driven by a constant incoming heat flux at the inner radial boundary of the domain. We identify different transport and confinement states. We observe stationary convection and self-sustained shared flows for low heat flux drive. For increasing drive, oscillatory motion with sheared flows arise until the system enters a state of turbulent convection.

At the onset of turbulent convection we observe that the probability density functions of the normalized radial velocity, pressure and flux fluctuations show nearly Gaussian form. This distributions get increasingly non-Gaussian and develop exponential tails for increasing heat flux drive. We observe quasi-periodic bursts at the state of intermittent convection, separated by quiescent periods. The waiting time and amplitude distribution of these bursts take a nearly exponential form. The conditionally averaged waveform and the autocorrelation function of the normalized pressure fluctuations are discussed. We further compare those results to experimental measurements and predictions from stochastic modelling and find very good agreement.