Ahmed Diallo

Mechanism for blob generation in the TORPEX toroidal plasma

Reference CRPP-CONF-2009-011Afficher la publication dans Web of Science Notice creee le 2009-01-23, modifiee le 2017-05-12

Experimental characterization of drift-interchange instabilities in a simple toroidal plasma

Low frequency electrostatic instabilities are investigated on TORPEX [Fasoli, Labit, McGrath, Muller, Podesta, and Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device for basic plasma physics experiments with a toroidal magnetic field 100mT and a small vertical magnetic field (⩽4mT). A two-dimensional (2D) profile of the frequency and amplitude of density and potential fluctuations is reconstructed using electrostatic probes with high space and time resolution. The measured phase velocity, corrected for the Doppler shift induced by the E×B drift, is consistent with the electron diamagnetic drift velocity. The local dispersion relation, measured along and across the magnetic field, is in agreement with the predictions of a linear kinetic slab model for drift waves. Unstable modes are generated in regions of unfavorable curvature, where the pressure gradient is colinear with the magnetic field gradient. It is demonstrated that the curvature of the magnetic field lines is essential for driving the ...

Plasma blobs in a basic toroidal experiment: Origin, dynamics, and induced transport

Detaching plasma blobs with very similar properties to tokamaks are observed in the basic toroidal plasma experiment TORPEX [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)]. The blobs originate from the breaking of wave crests of a drift-interchange wave, which span over regions characterized by strongly inhomogeneous background parameters. Once decoupled from the wave, the blobs follow a predominantly radial trajectory pattern. The blob-induced cross-field transport can instantaneously exceed the steady-state parallel fluxes by one order of magnitude, while accounting for only 10% of the time-average device losses. If the particles were confined in the parallel direction, as is the case in tokamaks, blobs would constitute the dominant loss mechanism in TORPEX. The presented results show that the presence of gradB is sufficient and neither a magnetic-topology change nor the presence of limiters, both absent in TORPEX, are necessary for the generation of blobs.

Probabilistic analysis of turbulent structures from two-dimensional plasma imaging

A method is presented to construct object-related structure observables, such as size, mass, shape, and trajectories from two-dimensional plasma imaging data. The probability distributions of these observables, deduced from measurements of many realizations, provide a robust framework in which the fluctuations, the turbulence, and the related transport are characterized. The results for imaging data recorded in the presence of drift-interchange instabilities and turbulence on the TORPEX toroidal plasma experiment [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)] are discussed.

The role of the density gradient on intermittent cross-field transport events in a simple magnetized toroidal plasma

Intermittent cross-field particle transport events (ITEs) are studied in the basic toroidal device TORPEX [TORoidal Plasma EXperiment, A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], with focus on the role of the density gradient. ITEs are due to the intermittent radial elongation of an interchange mode. The elongating positive wave crests can break apart and form blobs. This is not necessary, however, for plasma particles to be convected a considerable distance across the magnetic field lines. Conditionally sampled data reveal two different scenarios leading to ITEs. In the first case, the interchange mode grows radially from a slab-like density profile and leads to the ITE. A novel analysis technique reveals a monotonic dependence between the vertically averaged inverse radial density scale length and the probability for a subsequent ITE. In the second case, the mode is already observed before the start of the ITE. It does not elongate radially in a first stage, but at a later time. It is shown that this elongation is preceded by a steepening of the density profile as well. ©2008 American Institute of Physics

Statistical properties of electrostatic turbulence in toroidal magnetized plasmas

Note: invited paper Reference CRPP-CONF-2007-059 URL: http://www.eps2007.ifpilm.waw.pl/ Record created on 2008-05-13, modified on 2017-05-12

Fast ion source and detector for investigating the interaction of turbulence with suprathermal ions in a low temperature toroidal plasma

A specific experimental apparatus consisting of an ion source and a detector for the investigation of the interaction between suprathermal ions and drift-wave turbulence is developed on the toroidal plasma experiment. Due to the low plasma temperature (similar to 5 eV), a spatially localized, small-size ion source (similar to 4 cm) mounted inside the vacuum vessel with relatively low ion energy (similar to 100 eV-1 keV) can be used. The source consists of an aluminosilicate Li-6 ion emitter (6 mm diameter, 10-30 mu A current) installed on a two-dimensional (2D) poloidally moving system. The location, energy, and current density profile of the ion beam will be measured using a 2D movable gridded energy analyzer. (c) 2006 American Institute of Physics.

Fast visible imaging of turbulent plasma in TORPEX

Fast framing cameras constitute an important recent diagnostic development aimed at monitoring light emission from magnetically confined plasmas, and are now commonly used to study turbulence in plasmas. In the TORPEX toroidal device [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], low frequency electrostatic fluctuations associated with drift-interchange waves are routinely measured by means of extensive sets of Langmuir probes. A Photron Ultima APX-RS fast framing camera has recently been acquired to complement Langmuir probe measurements, which allows comparing statistical and spectral properties of visible light and electrostatic fluctuations. A direct imaging system has been developed, which allows viewing the light, emitted from microwave-produced plasmas tangentially and perpendicularly to the toroidal direction. The comparison of the probability density function, power spectral density, and autoconditional average of the camera data to those obtained using a multiple head electrostatic probe covering the plasma cross section shows reasonable agreement in the case of perpendicular view and in the plasma region where interchange modes dominate.

Characterization of the electron distribution function in an electron-cyclotron driven toroidal plasma

The local properties of the electron energy distribution function are investigated in the electron-cyclotron wave driven toroidal device TORPEX (Fasoli et al 2006 Phys. Plasmas 13 055902). The electron energy distribution function is measured by means of a time-resolved electrostatic energy analyser. The measurements indicate that electrons are accelerated to different energies at the electron-cyclotron and upper-hybrid resonances. Electrons from the upper-hybrid resonance have higher energies and can be described by a secondary maxwellian energy distribution function. The electron energy distribution function is found to be invariant with respect to experimental parameters such as the neutral gas pressure, the magnetic field configuration and the injected microwave power. The implications for a correct modelling of the particle source term for TORPEX plasmas are discussed.

Driving Low Frequency Breathing Oscillations in a Hall Thruster

Coherent m = 0 breathing oscillations in a cylindrical Hall thruster are driven by imposing a sinusoidal modulation of the applied anode potential. Using high speed imaging and total discharge current measurements to monitor fluctuations, 11 kHz oscillations are driven at 13 VAC. The resulting discharge provides a test-bed to perform laser-induced fluorescence measurements of the time-dependent ion velocity distribution using a novel heterodyne approach. Development of the diagnostic and characterization of induced oscillations could be relevant steps to understand the naturally occurring breathing mode and rotating spoke oscillations in Hall thrusters.