Aunai Memorial Journal Club on Turbulence
<calendar> name=jclub view=month </calendar>
|10/10 Thu 2013, Judith de Patoul|
| 10 Oct 2013
|19/9 Thu 2013, Özgür D. Gürcan|
| 19 Sep 2013
Prestented at the festival de théorie
|9 Sep 2011, Evgeniy Gusakov|
| 9 Sep 2011
Abstract: Geodesic acoustic mode (GAM), which manifests itself as a symmetric oscillation of the potential of toroidal plasma, as well as low-frequency zonal flows, is considered nowadays as a factor of micro-turbulence induced self-control. Experimental information on GAM is usually obtained either via direct measurement of the plasma potential by probes or HIBP technique or using measurements of oscillating component of plasma rotation velocity by Doppler reflectometry. In the present paper the Doppler frequency shift fD of the signal, backscattered (BS) in the upper hybrid resonance (UHR), is utilized for investigation of the GAM. The Doppler UHR BS technique utilizes X-mode microwave plasma probing out of the tokamak’s equatorial plane from high magnetic field side. It is benefiting from the microwave electric field and both poloidal and radial wave numbers growth in the UHR resulting in enhancement of the scattering signal, sub-mm radial resolution and substantial fD increase. The measurements were carried out at FT-2 tokamak (R = 55 cm; a = 7.9 cm; Bt = 2.2 T) in hydrogen plasma and with additional intensive helium puffing in regimes with different plasma currents 19-32 kA. The GAM oscillations of plasma poloidal rotation possessing surprisingly large amplitude comparable to the average rotation velocity were observed in both regimes. The drop of the GAM frequency with the helium component density growth was clearly observed in the experiment in accordance with theoretical predictions. The GAM spectral line is shown to be Lorenzian, possessing frequency width determined by the GAM damping rate. The radial profiles of GAM’s spectral line amplitude were investigated and compared with damping estimated from theoretical values of Landau and collision damping rates. The radial spatial structure of the GAM was investigated with dual-frequency UHR BS correlative approach possessing sub-mm spatial resolution and with combined UHR BS and reflectometry correlation technique. The GAM was shown to propagate in the plasma edge direction. The corresponding wave-length and correlation length are estimated. The obtained experimental data was shown to agree to results of FT-2 tokamak gyro-kinetic modelling using ELMFIRE code.
|13 May 2011, Roland Grappin|
| 13 May 2011
|25 Mar 2011, Marie-Christine Firpo|
| 25 Mar 2011
Abstract: The issue of magnetic confinement in magnetic fusion devices is addressed within a purely magnetic approach. Using some Hamiltonian models for the magnetic field lines, the dual impact of low magnetic shear is shown in a unified way. Away from resonances, it induces a drastic enhancement of magnetic confinement that favors robust internal transport barriers (ITBs) and stochastic transport reduction. When low shear occurs for values of the winding of the magnetic field lines close to low-order rationals, the amplitude thresholds of the resonant modes that break internal transport barriers by allowing a radial stochastic transport of the magnetic field lines may be quite low. The approach can be applied to assess the robustness versus magnetic perturbations of general (almost) integrable magnetic steady states, including nonaxisymmetric ones such as the important single-helicity steady states. This analysis puts a constraint on the tolerable mode amplitudes compatible with ITBs and may be proposed as a possible explanation of diverse experimental and numerical signatures of their collapses.
|11 Feb 2011, Roch Smets|
| 11 Feb 2011
Abstract: We address the problem of particle diffusion in real space, as a consequence of electromagnetic fluctuations. We present original numerical results obtained with a self-consistent hybrid code, and we propose a method to build perpendicular diffusion coefficient. We conducted two types of simulations. The first type (associated to an agyrotropic instability) is stationary, wide band white noise, and associated to Gaussian probability distribution function for the magnetic fluctuations. The second type (associated to a Kelvin-Helmholtz instabilityi) is non-stationary, with a power-law spectrum, and a non-Maxwellian probability distribution function. We emphasize that the Kelvin-Helmholtz instability is not playing a peculiar role in mass loading process from the solar wind to the Earth magnetosphere, associated to diffusive transport.
|21 Jan 2011, Sedina Tsikata|
| 21 Jan 2011
|3 Dec 2010, Ozgur Gurcan|
| 3 Dec 2010