arXiv subjects

E. Martines

Publications and source records attributed to E. Martines.

14 recordsLinked to original sources

arXiv2014

Spatiotemporal synchronization of drift waves in a magnetron sputtering plasma

A feedforward scheme is applied for drift waves control in a magnetized magnetron sputtering plasma. A system of driven electrodes collecting electron current in a limited region of the explored plasma is used to interact with unstable drift waves. Drift waves actually appear as electrostatic modes characterized by discrete wavelengths of the order of few centimeters and frequencies of about 100 kHz. The effect of external quasi-periodic, both in time and space, travelling perturbations is studied. Particular emphasis is given to the role played by the phase relation between the natural and the imposed fluctuations. It is observed that it is possible by means of localized electrodes, collecting currents which are negligible with respect to those flowing in the plasma, to transfer energy to one single mode and to reduce that associated to the others. Due to the weakness of the external action, only partial control has been achieved.

physics.plasm-ph↗

arXiv2012

Transition from order to chaos, and density limit, in magnetized plasmas

It is known that a plasma in a magnetic field, conceived microscopically as a system of point charges, can exist in a magnetized state, and thus remain confined, inasmuch as it is in an ordered state of motion, with the charged particles performing gyrational motions transverse to the field. Here we give an estimate of a threshold, beyond which transverse motion become chaotic, the electrons being unable to perform even one gyration, so that a breakdown should occur, with complete loss of confinement. The estimate is obtained by the methods of perturbation theory, taking as perturbing force acting on each electron that due to the so--called microfield, i.e., the electric field produced by all the other charges. We first obtain a general relation for the threshold, which involves the fluctuations of the microfield. Then, taking for such fluctuations the fomula given by Iglesias, Lebowitz and MacGowan for the model of a one component plasma with neutralizing background, we obtain a definite formula for the threshold, which corresponds to a density limit increasing as the square of the imposed magnetic field. Such a theoretical density limit is found to fit pretty well the empirical data for collapses of fusion machines.

math-ph↗

arXiv2011

Density limit in a first principles model of a magnetized plasma in the Debye--Hückel approximation

A crucial problem concerning a large variety of fusion devices is that the confinement due to an external magnetic field is lost above a critical density, while a widely accepted first principles explanation of such a fact is apparently lacking. In the present paper, making use of standard methods of statistical mechanics in the Debye--Hückel approximation, we give indications that for a plasma there exists a density threshold corresponding to a transition from order to chaos, the ordered motions being those in which the confining Lorentz force on a single electron prevails over the diffusive effect of the Coulomb forces. The density limit, which is proportional to the square of the magnetic field, turns out to fit not too badly the empirical data for plasma collapses in a large set of fusion devices.

cond-mat.stat-mech↗

arXiv2010

A microscopic instability in neutral magnetized plasmas

We show that in a neutral magnetized plasma there exist microscopic oscillatory modes, with wavelengths of the order of magnitude of the mean interparticle distance, which become unstable when the electron density exceeds a limit proportional to the square of the magnetic field. The model we consider is just a linearization of the classical one for neutral plasmas, namely a system of electrons subjected to Coulomb interactions among themselves and with a uniform positive neutralizing background. This model is here dealt with as an actual many-body problem, without introducing any averaging over the individual particles. The expression of the density limit coincides, apart possibly from a numerical factor of order one, with the well-known Brillouin density limit for a nonneutral plasma, which has however a macroscopic origin. The density limit here found has the same order of magnitude as the operational density limit observed in several conventional tokamak devices. We finally show that, when the full electromagnetic interactions are taken into account, dispersion relations are obtained which for short wavelengths reduce to those obtained here in the purely Coulomb case, and for long wavelengths reproduce the familiar ones of MHD.

physics.plasm-ph↗

arXiv2010

A first principles explanation for the density limit in magnetized plasmas

Fusion research on magnetic confinement is confronted with a severe problem concerning the electron densities ne to be used in fusion devices. Indeed, high densities are mandatory for obtaining large efficiencies, whereas it is empirically found that catastrophic disruptive events occur for densities exceeding a maximal one n^M_e. On the other hand, despite the large theoretical work there is no widely accepted, first principles model for the density limit (see [1], abstract). Here, we propose a simple microscopic model of a magnetized plasma suited for a tokamak, for which the existence of a density limit is proven. This property turns out to be a general collective feature of electrodynamics of point charges, which is lost in the continuum approximation. The law we find is n^M_e = 1.74 1/(me c^2) B^2/ μ_0, (1), where μ_0 is the vacuum permeability, c the speed of light, me the electron mass, and B the magnetic field. As shown in Fig. 1, the theoretical limit (big circles) is in rather good agreement with the empirical data, actually a surprisingly good one for a model based on first principles, with no adjustable parameter.

physics.plasm-ph↗

arXiv2009

The plasma boundary in Single Helical Axis RFP plasmas

Single Helical Axis (SHAx) states obtained in high current reversed field pinch (RFP) plasmas display, aside from a dominant mode in the m=1 spectrum, also a dominant m=0 mode, with the same toroidal mode number as the m=1 one. The two modes have a fixed phase relationship. The island chain created by the m=0 mode across the reversal surface gives rise, at shallow reversal of the toroidal field, to an X-point structure which separates the last closed flux surface from the first wall, creating a divertor-like configuration. The plasma-wall interaction is found to be related to the connection length of the field lines intercepting the wall, which displays a pattern modulated by the dominant mode toroidal periodicity. This configuration, which occurs only for shallow toroidal field reversal, could be exploited to realize an island divertor in analogy to stellarators.

physics.plasm-ph↗

arXiv2009

Resistive g-modes in a reversed field pinch plasma

First direct experimental evidence of high frequency, high toroidal mode number (n>20), magnetic fluctuations due to unstable resistive interchange modes (g-modes) resonant in the edge region of a reversed field pinch (RFP) plasma is presented. Experimental characterization of time and space periodicities of the modes is provided by means of highly resolved in-vessel edge and insertable magnetic diagnostics. It is found that the spectral mode properties are in good agreement with the predictions of the theoretical linear resistive magnetohydrodynamic stability analysis. A simple model is proposed for the observed saturation levels of the modes.

physics.plasm-ph↗

arXiv2009

Drift-Alfven vortex structures in the edge region of a fusion relevant plasma

Edge turbulent structures are commonly observed in fusion devices and are generally believed to be responsible for confinement degradation. Among their origin Drift-Alfven turbulence is one of the most commonly suggested. Drift-Alfven paradigm allows the existence of localized vortex-like structures observed also in various systems. Here we present the evidence of the presence of drift-Alfvén vortices in the edge region of RFX-Mod RFP device, showing how these structures are responsible for electromagnetic turbulence at the edge and its intermittent nature.

physics.plasm-ph↗

arXiv2004

Formation of convective cells in the scrape-off layer of the CASTOR tokamak

Understanding of the scrape-off layer (SOL) physics in tokamaks requires diagnostics with sufficient temporal and spatial resolution. This contribution describes results of experiments performed in the SOL of the CASTOR tokamak (R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding the whole poloidal cross section. The individual probes measure either the ion saturation current of the floating potential with the spatial resolution up to 3 mm. Experiments are performed in a particular magnetic configuration, characterized by a long parallel connection length in the SOL, L_par ~q2piR. We report on measurements in discharges, where the edge electric field is modified by inserting a biased electrode into the edge plasma. In particular, a complex picture is observed, if the biased electrode is located inside the SOL. The poloidal distribution of the floating potential appears to be strongly non-uniform at biasing. The peaks of potential are observed at particular poloidal angles. This is interpreted as formation of a biased flux tube, which emanates from the electrode along the magnetic field lines and snakes q times around the torus. The resulting electric field in the SOL is 2-dimensional, having the radial as well as the poloidal component. It is demonstrated that the poloidal electric field E_pol convects the edge plasma radially due to the E_pol x B_T drift either inward or outward depending on its sign. The convective particle flux is by two orders of magnitude larger than the fluctuation-induced one and consequently dominates.

physics.plasm-ph↗

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↗

arXiv2002

To what extent can dynamical models describe statistical features of turbulent flows?

Statistical features of "bursty" behaviour in charged and neutral fluid turbulence, are compared to statistics of intermittent events in a GOY shell model, and avalanches in different models of Self Organized Criticality (SOC). It is found that inter-burst times show a power law distribution for turbulent samples and for the shell model, a property which is shared only in a particular case of the running sandpile model. The breakdown of self-similarity generated by isolated events observed in the turbulent samples, is well reproduced by the shell model, while it is absent in all SOC models considered. On this base, we conclude that SOC models are not adequate to mimic fluid turbulence, while the GOY shell model constitutes a better candidate to describe the gross features of turbulence.

nlin.CD↗

arXiv2001

Intermittency in plasma turbulence

Intermittency in fluid turbulence can be evidentiated through the analysis of Probability Distribution Functions (PDF) of velocity fluctuations, which display a strong non-gaussian behavior at small scales. In this paper we investigate the occurrence of intermittency in plasma turbulence by studying the departure from the gaussian distribution of PDF for both velocity and magnetic fluctuations. We use data coming from two different experiments, namely in situ satellite observations of the inner solar wind and turbulent fluctuations in a magnetically confined fusion plasma. Moreover we investigate also time intermittency observed in a simplified shell model which mimics 3D MHD equations. We found that the departure from a gaussian distribution is the main characteristic of all cases. The scaling behaviour of PDFs are then investigated by using two different models built up in the past years, in order to capture the essence of intermittency in turbulence.

nlin.CD↗

arXiv2001

Intermittency and turbulence in a magnetically confined fusion plasma

We investigate the intermittency of magnetic turbulence as measured in Reversed Field Pinch plasmas. We show that the Probability Distribution Functions of magnetic field differences are not scale invariant, that is the wings of these functions are more important at the smallest scales, a classical signature of intermittency. We show that scaling laws appear also in a region very close to the external wall of the confinement device, and we present evidences that the observed intermittency increases moving towards the wall.

physics.plasm-ph↗

arXiv1998

Search for low-dimensional chaos in a reversed field pinch plasma

An analysis of experimental data from the RFX (Reversed Field eXperiment) reversed field pinch machine [L. Fellin, P. Kusstatscher and G. Rostagni, Fusion Eng. Des. {\bf 25}, 315 (1995)] is carried out to investigate the possible existence of deterministic chaos in the edge plasma region. The mathematical tools used include Lyapunov exponents, Kaplan-Yorke dimension, minimum embedding dimension estimates and nonlinear forecasting. The whole analysis agrees in ruling out the possibility of low-dimensional chaos: The dimension of the underlying dynamical system is estimated to be $ > 10$. From a critical re-reading of the literature it emerges that the findings of this work are likely to be common to all reversed field pinches.

physics.plasm-ph↗