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 form...

/pdf/convective-transport-by-intermittent-blob-filaments-4wpj3hvowo.pdf

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

The nonlinear propagation of ion-acoustic waves is considered in a magnetized plasma, composed of kappa distributed electrons and an inertial ion fluid. The fluid-dynamical system of equations governing the dynamics of ion-acoustic waves is reduced to a pseudoenergy-balance equation. The properties of arbitrary amplitude, obliquely propagating ion-acoustic solitary waves are thus investigated via a mechanical-motion analog (Sagdeev potential) approach. The presence of excess superthermal electrons is shown to influence the nature of magnetized ion-acoustic solitons. The influence on the soliton characteristics of relevant physical parameters such as obliqueness (the angle between soliton propagation direction and magnetic field), the electron deviation from a Maxwellian (“superthermality”) and the soliton speed is investigated.

/pdf/oblique-electrostatic-excitations-in-a-magnetized-plasma-in-4y6wkqatc0.pdf

Oblique electrostatic excitations in a magnetized plasma in the presence of excess superthermal electrons

The mechanism for blob generation in a toroidal magnetized plasma is investigated using time-resolved measurements of two-dimensional structures of electron density, temperature, and plasma potential. The blobs are observed to form from a radially elongated structure that is sheared off by the $E \times B$ flow. The structure is generated by an interchange wave that increases in amplitude and extends radially in response to a decrease of the radial pressure scale length. The dependence of the blob amplitude upon the pressure radial scale length is discussed.

/pdf/experimental-observation-of-the-blob-generation-mechanism-50d8ru3agw.pdf

Experimental observation of the blob-generation mechanism from interchange waves in a plasma.

Dynamics of blobs described by two different models for the closures of parallel plasma current is studied numerically. One of these closures describes the blob sitting on magnetic lines going in the scrape off layer through the conducting surface, while the other one describes the blob with high plasma pressure where the bending of magnetic field lines becomes important.

Two-dimensional modelling of blob dynamics in tokamak edge plasmas

Recent experiments performed on the low β plasma device Mirabelle [T. Pierre, G. Leclert, and F. Braun, Rev. Sci. Instrum. 58, 6 (1987)] using a limiter have shown that transitions between various gradient driven instabilities occurred on increasing the magnetic field strength. New thorough measurements allow to identify unambiguously three instability regimes. At low magnetic field the strong E×B velocity shear drives a Kelvin–Helmholtz instability, whereas at high magnetic field drift waves are only observed. A centrifugal (Rayleigh–Taylor) instability is also observed in between when the E×B velocity is shearless and strong enough. A close connection is made between the ratio ρs∕L⊥ of the drift parameter to the radial density gradient length and each instability regime.

Transition from flute modes to drift waves in a magnetized plasma column

The turbulent regime of a rotating magnetized plasma column has been studied The detection and the spatiotemporal analysis of structures by means of conditional sampling techniques is performed Because of the overall rotation and centrifugal effects, the structures inside the turbulence move on average along a spiral trajectory leading to a net radial convection of the charged particles to the walls The development of a poloidal electric field inside the structures has been measured It leads to the observed outwards radial $\mathbf{E}\ifmmode\times\else\texttimes\fi{}\mathbf{B}$ drift in agreement with the expectations of recent theoretical works

Radial convection of plasma structures in a turbulent rotating magnetized-plasma column

The rotation of a cylindrical plasma column in a magnetic field has been studied in the linear section of the new plasma device Mistral. Under suitable conditions we observe a transition to a turbulent regime characterized by strong, bursty fluctuations at the edge of the column. The detection and the study of the spatio-temporal evolution of structures in the turbulent regime have been performed by means of a new enhanced conditional sampling technique. We have collected evidence of the development of a bent tail emanating from the plasma column. The charged particles inside the structure move along a spiral trajectory resulting in a net radial convection of the plasma to the walls. We show experimentally that a poloidal electric field is present inside the structures leading to the observed outwards radial E × B drift, in agreement with the expectations of recent and past theoretical works.

Formation of spiral structures and radial convection in the edge region of a magnetized rotating plasma

The turbulent regime of a rotating magnetized plasma column has been studied. The detection and the spatiotemporal analysis of structures by means of conditional sampling techniques is performed. Because of the overall rotation and centrifugal effects, the structures inside the turbulence move on average along a spiral trajectory leading to a net radial convection of the charged particles to the walls. The development of a poloidal electric field inside the structures has been measured. It leads to the observed outwards radial E x B drift in agreement with the expectations of recent theoretical works.

Radial convection of plasma structures in a turbulent rotating magnetized plasma column

A statistical analysis of density fluctuations in a cylindrical non-fusion device is performed. The experimental setup is implemented in order to reach a turbulent behavior of the linear plasma column. Two different turbulent regimes are obtained corresponding to two selected sets of values for the discharge parameters. The first regime displays a rotating column characterized by the presence of a shear layer separating the plasma bulk from the tenuous plasma in the shadow of the limiter, the latter showing a strong intermittent behavior and superdiffusion. The second regime corresponds to a weakly rotating column in which coherence is lost in the plasma bulk and a standard diffusive process takes place in the shadow region. These findings are supported by the calculation of the Hurst's exponent using wavelet-analysis techniques. Furthermore the intermittent behavior is characterized and related to the diffusive process. Finally the shape of the probability distribution function of density fluctuations seems to be well described by an analytical form suggested on the basis of Tsallis generalized statistics.

/pdf/statistical-analysis-of-correlations-and-intermittency-of-a-2xfsvmdhcg.pdf

Statistical analysis of correlations and intermittency of a turbulent rotating column in a magnetoplasma device.

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Radial convection of plasma structures in a turbulent rotating magnetized-plasma column

Formation of spiral structures and radial convection in the edge region of a magnetized rotating plasma

Radial convection of plasma structures in a turbulent rotating magnetized plasma column

Statistical analysis of correlations and intermittency of a turbulent rotating column in a magnetoplasma device.