arXiv subjects

F. Sattin

Publications and source records attributed to F. Sattin.

At least 37 records · Page 2Linked to original sources

arXiv2009

On the statistics of edge fluctuations: comparative study between various fusion devices

In this paper we present a statistical study of edge fluctuations taken with the Gas Puffing Imaging (GPI) diagnostics. We carry out a comparison of GPI signal from an extensive database including four devices (two Tokamaks and two Reversed Field Pinches). The data are analyzed in terms of their statistical moments Skewness and Kurtosis, as done in [B. Labit, et al, Phys. Rev. Lett. \textbf{98}, 255002 (2007)]. The data align along parabolic curves, although different from machine to machine, with some spread around the best-fitting curve. A discussion about the meaning of the parabolic trend as well as the departure of real data from it is provided. A phenomenological model is finally provided, attempting to accomodate experimental evidence.

physics.plasm-ph↗

arXiv2008

Fick's law and Fokker-Planck Equation in inhomogeneous environments

In inhomogeneous environments, the correct expression of the diffusive flux is often not given by the Fick's law $Γ= - D \nabla n $. The most general hydrodynamic equation modelling diffusion is indeed the Fokker-Planck Equation (FPE). The microscopic dynamics of each specific system may affect the form of the FPE, either establishing connections between the diffusion and the convection term, as well as providing supplementary terms. In particular, the Fick's form for the Diffusion Equation may arise only in consequence of a specific kind of microscopic dynamics. It is also shown how, in the presence of sharp inhomogeneities, even the hydrodynamic FPE limit may becomes inaccurate and mask some features of the true solution, as computed from the Master Equation.

physics.plasm-ph↗

arXiv2007

When can Fokker-Planck Equation describe anomalous or chaotic transport?

The Fokker-Planck Equation, applied to transport processes in fusion plasmas, can model several anomalous features, including uphill transport, scaling of confinement time with system size, and convective propagation of externally induced perturbations. It can be justified for generic particle transport provided that there is enough randomness in the Hamiltonian describing the dynamics. Then, except for 1 degree-of-freedom, the two transport coefficients are largely independent. Depending on the statistics of interest, the same dynamical system may be found diffusive or dominated by its Lévy flights.

physics.plasm-ph↗

arXiv2006

Statistical features of edge turbulence in RFX-mod from Gas Puffing Imaging

Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the Gas Puffing Imaging Diagnostics. Statistical features of the signal are quantified in terms of the Probability Distribution Function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al [J.P. Graves, et al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand, pulses with external methods for plasma control feature modified PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDFs is common to other devices as well, and has been suggested to be due to the simultaneous presence of different mechanisms driving respectively coherent bursts and gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed.

physics.plasm-ph↗

arXiv2005

Statistical model for intermittent plasma edge turbulence

The Probability Distribution Function of plasma density fluctuations at the edge of fusion devices is known to be skewed and strongly non-Gaussian. The causes of this peculiar behaviour are, up to now, largely unexplored. On the other hand, understanding the origin and the properties of edge turbulence is a key issue in magnetic fusion research. In this work we show that a stochastic fragmentation model, already successfully applied to fluid turbulence, is able to predict an asymmetric distribution that closely matches experimental data. The asymmetry is found to be a direct consequence of intermittency. A discussion of our results in terms of recently suggested BHP universal curve [S.T. Bramwell, P.C.W. Holdsworth, J.-F. Pinton, Nature (London) 396, 552 (1998)], that should hold for strongly correlated and critical systems, is also proposed

physics.plasm-ph↗

arXiv2005

Signatures of non-Markovian turbulent transport in Reversed Field Pinch plasmas

Transport of field lines is studied for a realistic model of magnetic field configuration in a Reversed Field Pinch. It is shown that transport is anomalous, i.e., it cannot be described within the standard diffusive paradigm. To fit numerical results we present a transport model based upon the Continuous Time Random Walk formalism. Fairly good quantitative agreement appears for exponential memory functions.

cond-mat.stat-mech↗

arXiv2004

Non-extensive entropy from incomplete knowledge of Shannon entropy?

In this paper we give an interpretation of Tsallis' nonextensive statistical mechanics based upon the information-theoretic point of view of Luzzi et al. [cond-mat/0306217; cond-mat/0306247; cond-mat/0307325], suggesting Tsallis' entropy to be not a fundamental concept but rather a derived one, stemming from an incomplete knowledge of the system, not taking properly into account its interaction with the environment. This interpretation seems to avoid some problems occurring with the original interpretation of Tsallis statistics.

cond-mat.stat-mech↗

arXiv2004

On the Probability distribution function of particle density at the edge of fusion devices

The Probability Distribution Function (PDF) Pn(n) of the particle density at the edge of several magnetic fusion devices, including tokamaks, stellarators, and linear devices, is known to be strongly non-gaussian. In this paper we present experimental results from RFX Reversed Field Pinch [G. Rostagni, Fus. Eng. Design 25, 301 (1995)], confirming the universal shape of Pn also for RFP's. An explanation for the form of Pn is attempted, on the basis of simple conservation equations. The model result is shown to fitted fairly well empirical data in a few different experimental scenarios.

physics.plasm-ph↗

arXiv2004

The electron capture cross section using the r-CTMC method: a review and a suggestion for improvement

In this work we review the classical recipe for estimating partial cross sections for electron capture into selected energy levels within the Classical Trajectory Monte Carlo (CTMC) method [R.L. Becker and A.D. MacKellar, J. Phys. B: At. Mol. Phys. 17 (1984) 3923], and point to a deficiency of the method when applied to the CTMC in its r-version, according to the terminology introduced by Cohen [J.S. Cohen, J. Phys. B: At. Mol. Phys. 18 (1985) 1759]. We suggest, in this case, an alternative recipe based on the generation of stationary classical spatial distributions for electrons in arbitrary levels. The output of the CTMC simulations is projected onto these classical distributions rather than on the quantal ones. A comparison between the two approaches is done.

physics.atom-ph↗

arXiv2004

Superstatistics from a different perspective

In this paper we elaborate on the recently proposed superstatistics formalism [C. Beck and E.G.D. Cohen, Physica A 322, 267 (2003)], used to interpret unconventional statistics. Their interpretation is that unconventional statistics in dynamical systems arise as weighted averages of the ordinary statistics obeyed by these systems over a statistical distribution of background configurations due to fluctuations intrinsic to the background. In this paper we suggest that the same picture can arise because of the intrinsic dynamics of the system. The dynamics of the system and the background, hence, concur together to determine the overall final statistics: differently evolving systems embedded within the same background can yield different statistical distributions. Some simple examples are provided; among them a toy model able to yield a power-law distribution. Also, some recent independent results are quoted, that appear to support this viewpoint.

cond-mat.stat-mech↗

arXiv2003

An equilibrium model for RFP plasmas in the presence of resonant tearing modes with an application to particle and heat transport

The equilibrium of a finite-beta cylindrical RFP plasma in the presence of saturated-amplitude tearing modes is investigated. The singularities arising from the perturbative analysis of the MHD force balance equation JxB=grad(p) are resolved through a proper regularization of the cylindrical symmetric component of the pressure and parallel current density profiles, by setting to zero their gradient at the modes rational surfaces. An equilibrium model, which satisfies the regularization rule at the various rational surfaces, is developed. The comparison with the experimental data from the Reversed Field eXperiment (RFX) gives encouraging results. The model provides an easy tool to extrapolate the magnetic data, ordinarily measured only at the plasma edge, inside the plasma. As an application, we present the computation of the particle and heat stochastic diffusion coefficient from the reconstructed magnetic field profile.

physics.plasm-ph↗

arXiv2003

Non-Gaussian PDFs from Maximum-Entropy-principle considerations

In this work we develop on the recently suggested concept of superstatistics [C. Beck and E.G.D. Cohen, Physica A {\bf 322}, 267 (2003)], face the problem of devising a viable way for estimating the correct statistics for a system in absence of sufficient knowledge of its microscopical dynamics, and suggest to solve it through the Maximum Entropy Principle. As an example, we deduce the Probability Distribution Function for velocity fluctuations in turbulent fluids, which is slightly different from the form suggested in [C. Beck, Phys. Rev. Lett. {\bf 87}, 180601 (2001)].

cond-mat.stat-mech↗

arXiv2002

Electron Impact Ionization Close to the Threshold: Classical Calculations

In this paper we present Classical Trajectory Monte Carlo (CTMC) calculations for single and multiple electron ionization of Argon atoms and ions in the threshold region. We are able to recover the Wannier exponents a for the power-law behavior of the cross section s versus excess energy: the exact value of the exponent as well as the existence of its saturation for multiple ionization appear to be related to how the total binding energy is shared between target electrons.

physics.atom-ph↗

arXiv2002

On the computation of the entropy for dissipative maps at the edge of chaos using non-extensive statistical mechanics

Tsallis' non-extensive statistical mechanics is claimed to be the correct tool to describe the behaviour of low-dimensional dissipative maps at the edge of chaos. Indeed, many different approaches confirm that, for those systems, the evolution is governed by power-laws, not exponential, trends; this coincides with predictions from generalized thermostatistics. In this work, however, we present some analytical considerations, supported also by some simple numerical examples, suggesting the existence of contradictions within this picture.

cond-mat.stat-mech↗

arXiv2002

An equilibrium model for RFP plasmas in the presence of resonant tearing modes

The equilibrium of a finite-beta RFP plasma in the presence of saturated-amplitude tearing modes is investigated. The singularities of the MHD force balance equation JXB=grad(p) at the modes rational surfaces are resolved through a proper regularization of the zeroth-order (equilibrium) profiles, by setting to zero there the gradient of the pressure and parallel current density. An equilibrium model, which satisfies the regularization rule at the various rational surfaces, is developed. The comparison with the experimental data from the Reversed Field eXperiment (RFX) gives encouraging results. The model provides an easy tool for magnetic analysis: many aspects of the perturbations can be analyzed and reconstructed.

physics.plasm-ph↗

arXiv2002

Derivation of Tsallis statistics from dynamical equations for a granular gas

In this work we present the explicit calculation of Probability Distribution Function for a model system of granular gas within the framework of Tsallis Non-Extensive Statistical Mechanics, using the stochastic approach by Beck [C. Beck, Phys. Rev. Lett. 87, 180601 (2001)], further generalized by Sattin and Salasnich [F. Sattin and L. Salasnich, Phys. Rev. E 65, 035106(R) (2002)]. The calculation is self-consistent in that the form of Probability Distribution Function is not given as an ansatz but is shown to necessarily arise from the known microscopic dynamics of the system.

cond-mat.stat-mech↗

arXiv2001

Semi-classical Over Barrier Model for low-velocity ion-atom charge exchange processes

We develop an Over Barrier Model for computing charge exchange between ions and one-active-electron atoms at low impact energies. The main feature of the model is the treatment of the barrier crossing process by the electron within a simplified quantum mechanical formulation which takes into account: (a) the probability of electron reflection even for over-barrier scattering, and (b) the discreteness of the receiving atom's quantum levels which strongly suppress captures far from the resonance condition. It is shown that inclusion of these effects yields a fairly good prediction of experimental data. We discuss also the probability of electron re-capture by the target.

physics.atom-ph↗

arXiv2001

Further study of the Over-Barrier Model to compute charge exchange processes

In this paper we study theoretically the process of electron capture between one-optical-electron atoms (e.g. hydrogenlike or alkali atoms) and ions at low-to-medium impact velocities ($v/v_e \approx 1$) working on a modification of an already developed classical In this work we present an improvement over the Over Barrier Model (OBM) described in a recent paper [F. Sattin, Phys. Rev. A {\bf 62}, 042711 (2000)]. We show that: i) one of the two free parameters there introduced actually comes out consistently from the starting assumptions underlying the model; ii) the modified model thus obtained is as much accurate as the former one. Furthermore, we show that OBMs are able to accurately predict some recent results of state selective electron capture, at odds with what previously supposed.

physics.atom-ph↗