Convective transport by intermittent blob-filaments: Comparison of theory and experiment

Abstract

A blob-filament (or simply “blob”) is a magnetic-field-aligned plasma structure which is considerably denser than the surrounding background plasma and highly localized in the directions perpendicular to the equilibrium magnetic field B. In experiments and simulations, these intermittent filaments are often formed near the boundary between open and closed field lines, and seem to arise in theory from the saturation process for the dominant edge instabilities and turbulence. Blobs become charge-polarized under the action of an external force which causes unequal drifts on ions and electrons; the resulting polarization-induced E × B drift moves the blobs radially outwards across the scrape-off-layer (SOL). Since confined plasmas generally are subject to radial or outwards expansion forces (e.g., curvature and ∇B forces in toroidal plasmas), blob transport is a general phenomenon occurring in nearly all plasmas. This paper reviews the relationship between the experimental and theoretical results on blob formation, dynamics and transport and assesses the degree to which blob theory and simulations can be compared and validated against experiments.

Physics of ultimate detachment of a tokamak divertor plasma

“…Umansky et al 1998;LaBombard et al 2000;Greenwald 2002;Rudakov et al 2005;Wiesen et al 2011. Usually it is related to the increased contribution of intermittent blobby plasma transport (see Zweben et al 2007;Krasheninnikov et al 2008;D'Ippolito et al 2011, and the references therein). However, recent experimental data (Sun et al 2015) suggest that even in H-mode, ∆ SOL increases during evolution to detachment.…”

X-Point Effect on Plasma Blob Dynamics

“…However, experimental investigations close to X points are difficult, limiting the progress in the understanding of the X-point dynamics and simulationexperiment comparisons. In fusion plasmas, the diagnostic accessibility is challenged by the high power flux so that the generation and propagation of intermittent plasma blobs in the vicinity of an X point is largely unexplored [8][9][10].In this Letter, we present the first spatial and temporaldependent in situ measurements of turbulence-generated plasma blob dynamics around the X-point region. The blob motion towards the X point is tracked and analyzed, showing an acceleration in the initial phase that can be directly linked to the background radial flow and to the measured blob electric potential dipole.…”

Stochastic Modeling of Intermittent Scrape-Off Layer Plasma Fluctuations

“…Under very general conditions, this model predicts a parabolic relation between the skewness and kurtosis moments of the plasma fluctuations. In the case of exponentially distributed burst amplitudes and waiting times, the probability density function for the fluctuation amplitudes is shown to be a Gamma distribution with the scale parameter given by the average burst amplitude and the shape parameter given by the ratio of the burst duration and waiting times.Cross-field transport of particles and heat in the scrape-off layer (SOL) of non-uniformly magnetized plasmas is caused by radial motion of blob-like structures [1][2][3][4][5][6]. This results in singlepoint recordings dominated by large-amplitude bursts, which have an asymmetric wave form with a fast rise and a slow decay, and positively skewed and flattened amplitude probability density functions [7][8][9][10][11][12][13][14].…”