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 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. In this Letter, we investigate experimentally the dynamics of blobs and identify a generation mechanism on the TORPEX toroidal device [9]. In TORPEX, blobs are observed to originate at the crest of a coherent interchange wave [10]. A simple magnetic configuration is used with both toroidal and vertical field components. Low temperature and density plasmas allow measurements of local parameters with high spatial and temporal resolution across the entire plasma cross section. The generation mechanism is studied using time-resolved two-dimensional (2D) profiles of electron density and temperature, plasma potential, and E B velocity, which are conditionally sampled over many blob events. Blobs are observed to form from radially elongated structures that are sheared off by the E B flow. These structures develop from an interchange wave that increases in amplitude and extends radially in response to a decrease of the radial pressure scale length. We discuss the dependence of the blob amplitude upon the pressure radial scale length. These results detail a fundamental phenomenon in plasmas and can be used to validate theories and numerical simulations of blob dynamics [11]. Similarly to the tokamak scrape-off layer (SOL), the magnetic configuration features open field lines, a rB, and magnetic field curvature. Blobs in TORPEX exhibit universal statistical properties with strong similarities with observations in the tokamak SOL [12]. Thus, the observed dynamics may shed light on the blob ejection mechanism in tokamaks, where there are strong indications that blobs result from interchange instabilities in the SOL [13].In the TORPEX (major radius R 1 m, minor radius a 0:2 m) experiments investigated here, hydrogen plasmas are produced and sustained by microwaves in the electron cyclotron range of frequencies [14]. A microwave power of 400 W is used in a toroidal magnetic field of 76 mT on axis and a vertical magnetic field B z 2:3 mT, which results in a vertically elongated plasma configuration. The plasma source is localized on the high field side (HFS), with negligible plasma production for r 5 cm [14]. Figure 1 shows 2D profiles of the time-averaged (indicated by the overbar symbol) electron pressure p e n e T e , plasma potential V pl , and the E B velocity, v EB , obtained from Langmuir probe measurements. The plasma potential is computed from V pl V fl T e =e, where V fl and T e are floating potential and electron temperature, and the coefficient 3:1 0:6 is determined exp...

Gradient driven electrostatic instabilities are investigated in TORPEX ͓A. Fasoli, B. Labit, M. McGrath, S. H. Müller, M. Podestà, and F. M. Poli, Bull. Am. Phys. Soc. 48, 119 ͑2003͔͒, a toroidal device ͑R =1 m, a = 0.2 m͒ in which plasmas are produced by microwaves ͑P ഛ 20 kW͒ with f rf = 2.45 GHz, in the electron cyclotron frequency range. Typical density and temperature are n e ഛ 10 17 m −3 and T e Ӎ 5 eV, respectively. The magnetic field is mainly toroidal ͑ഛ0.1 T͒, with a small vertical component ͑ഛ4 mT͒. Instabilities that can be generally identified as drift-interchange waves are observed and characterized for different levels of collisionality with neutrals. The frequency spectrum and the spatial profile of the fluctuation-induced flux are measured. An 86-tip probe is used to reconstruct the spatio-temporal evolution of density structures across the plasma cross section. The measured structures are characterized statistically, and related quantitative observables are constructed.

The resistive wall mode ͑RWM͒ instability in high-beta tokamaks is stabilized by energy dissipation mechanisms that depend on plasma rotation and kinetic effects. Kinetic modification of ideal stability calculated with the "MISK" code ͓B. Hu et al., Phys. Plasmas 12, 057301 ͑2005͔͒ is outlined. For an advanced scenario ITER ͓R. Aymar et al., Nucl. Fusion 41, 1301 ͑2001͔͒ plasma, the present calculation finds that alpha particles are required for RWM stability at presently expected levels of plasma rotation. Kinetic stabilization theory is tested in an experiment in the National Spherical Torus Experiment ͑NSTX͒ ͓M. Ono et al., Nucl. Fusion 40, 557 ͑2000͔͒ that produced marginally stable plasmas with various energetic particle contents. Plasmas with the highest and lowest energetic particle content agree with calculations predicting that increased energetic particle pressure is stabilizing but does not alter the nonmonotonic dependence of stability on plasma rotation due to thermal particle resonances. Presently, the full MISK model, including thermal particles and an isotropic slowing-down distribution function for energetic particles, overpredicts stability in NSTX experiments. Minor alteration of either effect in the theory may yield agreement; several possibilities are discussed.

A unique parabolic relation is observed to link skewness and kurtosis of around ten thousand density fluctuation signals, measured over the whole cross section of a toroidal magnetized plasma for a broad range of experimental conditions. All the probability density functions of the measured signals, including those characterized by a negative skewness, are universally described by a special case of the Beta distribution. Fluctuations in the drift-interchange frequency range are necessary and sufficient to assure that probability density functions can be described by this specific Beta distribution.