Optical response of C60 fullerene from a Time Dependent Thomas Fermi approach

We study the collective electron dynamics in C60 clusters within the Time Dependent Thomas Fermi method in the frame of jellium model. The results regarding the optical spectrum are in good agreement with the experimental data, our simulations being able to reproduce both resonances from 20eV and 40eV . We compare also, the results with those from other theoretical approaches and investigate the implications of quantum effects including exchange-correlation corrections, or gradient corrections from a Weizsacker term. The nature of the second resonance is studied using transition densities and analysing the probability current amplitudes.Comment: 9 pages, 5 figure

Turbulent transport of alpha particles in tokamak plasmas

We investigate the ExB diffusion of fusion born \alpha particles in tokamak plasmas. We determine the transport regimes for a realistic model that has the characteristics of the ion temperature gradient (ITG) or of the trapped electron modes (TEM) driven turbulence. It includes a spectrum of potential fluctuations that is modeled using the results of the numerical simulations, the drift of the potential with the effective diamagnetic velocity and the parallel motion. Our semi-analytical statistical approach is based on the decorrelation trajectory method (DTM), which is adapted to the gyrokinetic approximation. We obtain the transport coefficients as a function of the parameters of the turbulence and of the energy of the \alpha particle. According to our results, signficant turbulent transport of the \alpha particles can appear only at energies of the order of 100KeV. We determine the corresponding conditions.Comment: 11 pages, 8 figure

Regimes of self-organized criticality in the atmospheric convection

Large scale organization in ensembles of events of atmospheric convection can be generated by the combined effect of forcing and of the interaction between the raising plumes and the environment. Here the "large scale" refers to the space extension that is larger or comparable with the basic resolved cell of a numerical weather prediction system. Under the action of external forcing like heating individual events of convection respond to the slow accumulation of vapor by a threshold-type dynamics. This is due to the a time-scale separation, between the slow drive and the fast convective response, expressed as the "quasi-equilibrium". When there is interaction between the convection plumes, the effect is a correlated response. We show that the correlated response have many of the characteristics of the self-organized criticality (SOC). It is suggested that from the SOC perspective, a description of the specific dynamics induced by "quasi-equilibrium" can be provided by models of "punctuated equilibrium". Indeed the Bak-Sneppen model is able to reproduce (within reasonable approximation) two of the statistical results that have been obtained in observations on the organized convection. We also give detailed derivation of the equations connecting the probabilities of the states in the update sequence of the Bak-Sneppen model with $K=2$ random neighbors. This analytical framework allows the derivation of scaling laws for the size of avalanches, a result that gives support to the SOC interpretation of the observational data.Comment: Text prepared for the Report of COST ES0905 collaboration (2014). Latex 45 page

Turbulent transport of fast ions in tokamak plasmas in the presence of resonant magnetic perturbations

The effects of resonant magnetic perturbations on the turbulent transport of fast ions in tokamak devices are investigated using a theoretical transport model of test-particle type. The direct numerical simulation method is used to compute, via the transport model, the diffusion coefficients. The numerical results are in good agreement with other, analytically derived, estimations. It is found that finite Larmor radius effects decrease algebraically the transport, while the amplitude of magnetic perturbations has an opposite effect. In the presence of stochastic dynamics, the asymmetric toroidal magnetic field induces a small, radial, outward pinch. A synergistic mechanism of non-linear coupling between turbulence and magnetic perturbations enhances the radial diffusion. General scaling laws are proposed for the transport coefficients