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Andrea Di Vita

Publications and source records attributed to Andrea Di Vita.

7 recordsLinked to original sources
arXiv2026

A possible approach to overcome the saturation of the neutron yield in a Plasma Focus and to achieve breakeven

Saturation of the neutron yield with increasing energy of the condenser bank in a Plasma Focus led to the shutdown of PF research focussed on controlled nuclear fusion in the past. We review available models of saturation and develop further the model of Lee S., Applied Phys. Lett. 95, 151503, 2009. This model relies on the well-known and generally accepted model of Lee S., J. Fusion Energy 2014, 33, 319 of Plasma Focus discharges and describes saturation in terms of the dynamic resistance, i.e. the rate of change of PF inductance due to the motion of the plasma sheath during rundown. A model of this sheath discussed in Di Vita A., J. Plasma Physics, 1993, 50, 1 shows that its spontaneous filamentation rules the dynamic resistance, spoiling the power supply from the condenser bank to the plasma at the values of condenser bank energy above 0.5 MJ values which are relevant to a fusion reactor. Together, these two models lead to the conclusion that suppression of such filamentation prevents saturation, multiplies the PF drive parameter by a factor 3 at least and allows breakeven in a 224 kV, 10 MJ Plasma Focus working with DT. We can suppress filamentation by superimposing a radial magnetic field to the interelectrode region of the Plasma Focus where rundown occurs. A conservative estimate shows that a 1.4 T radial magnetic field is enough to suppress many known filamentation instabilities. Suitably located magnets can generate this field. Their layout resembles the layout of the sources of radial magnetic field in the cylindrical geometry of a Hall thruster for space propulsion. For high temperature superconducting magnets, the required current density is too small to trigger quenching.

physics.plasm-ph↗
arXiv2021

The persistent, the anti-persistent and the Brownian: when does the Hurst exponent warn us of impending catastrophes?

The analogy between self-similar time series with given Hurst exponent H and Markovian, Gaussian stochastic processes with multiplicative noise and entropic index q (Borland, PRE 57, 6, 6634-6642, 1998) allows us to explain the empirical results reported in (Pavithran et al., EPL, 129 2020 24004) and (Pavithran et al. Sci. Reports 10.1 (2020) 1-8) with the help of the properties of the nonextensive entropy Sq of index q: a dominant oscillating mode arises as H goes to zero in many different systems and its amplitude is proportional to 1/ H^2 . Thus, a decrease of H acts as precursor of large oscillations of the state variable, which corresponds to catastrophic events in many problems of practical interest. In contrast, if H goes to 1 then the time series is strongly intermittent, fluctuations of the state variable follow a power law whose exponent depends on H, and exceedingly large event are basically unpredictable. These predictions agree with observations in problems of aeroacoustics, aeroelasticity, electric engineering, hydrology, laser physics, meteorology, plasma physics, plasticity, polemology, seismology and thermoacoustics.

physics.class-ph↗
arXiv2018

On super-elastic collisions between large-scale magnetized plasmoids in the heliosphere

Recently, a unique collision between two large-scale magnetized plasmoids produced by coronal mass ejections in the heliosphere has been observed (C. Shen et al. Nature 8, 2012). Results suggest that the collision is super-elastic, i.e. the total linear kinetic energy of the two plasmoids after the collision is larger than before the collision, and that an anti-correlation exists, i.e. the lower the initial relative velocity of the plasmoids, the larger the relative increase in total kinetic energy. Following an old suggestion of W. H. Bostick, IEEE Trans. Plasma Science PS-14 703-717 (1986), here we start from first principles, retrieve some results of D. Kagan et al., Mon. Not. R. Astron. Soc. 406 1140-1145 (2010) and S. Ohsaki et al., Ap. J. Lett. 559 L61 (2001) and show that the anti-correlation is just a consequence of Hall magnetohydrodynamics.

astro-ph.SR↗
arXiv2017

Exponential or power law? How to select a stable distribution of probability in a physical system

A mapping of nonextensive statistical mechanics into Gibbs' statistical mechanics exists, which leads to a generalization of Einstein's formula for fluctuations. A unified treatment of stability of relaxed states in nonextensive statistical mechanics and Gibbs' statistical mechanics follows. The former and the latter are endowed with probability distribution of microstates ruled by power laws and Boltzmann exponentials respectively. We apply our treatment to the relaxed states described by a 1D nonlinear FokkerPlanck equation. If the latter is associated to the stochastic differential equation obtained in the continuous limit from a 1D, autonomous, discrete map affected by noise, then we may ascertain whether if a relaxed state follow a power law distribution (and with which exponent) by looking at both map dynamics and noise level, with no assumptions concerning the additive or multiplicative nature of the noise and with no numerical computation of the orbits. Results agree with the simulations of Sanchez et al. EPJ 143.1 (2007) 141-143 concerning relaxation to a Pareto-like distribution.

physics.class-ph↗
arXiv2017

On the invariant quantities in an entropy balance and their impact on the stability of some physical systems

In a wide class of physical systems, diffeomorphisms in the state space leave the amount of entropy produced per unit time inside the bulk of the system unaffected [M. Polettini et al., 12th Joint European Thermodynamics Conference, Brescia, Italy, July 1-5, 2013]. This invariance implies that if relaxation towards some final ('relaxed') state occurs, then the necessary condition for the stability of the relaxed state against slowly evolving perturbations is the same for all the systems of this class, regardless of both detailed dynamics of the system, amplitude of fluctuations around mean values and possible occurrence of periodic oscillations in the relaxed state. Our discussion invokes no Onsager symmetry, no detailed model of heat transport and production, and no approximation of local thermodynamic equilibrium. This necessary condition of stability is the constrained minimization of time- and path-ensemble-averaged amount of entropy produced per unit time inside the bulk of the system; for each system, constraints are provided by the equations of motion. (Thermodynamic equilibrium corresponds to identically vanishing amount of entropy produced in the bulk, and satisfies trivially this condition). Our class of physical systems includes both macroscopic and mesoscopic systems: the problems of stability of these systems are dual to each other. As particular cases, we retrieve some necessary criteria for stability of relaxed states available in the literature concerning current filaments and plasmoids in a Dense Plasma Focus, flashing ratchets, Brownian motors and dynamo-affected, convection-ruled shells of Sun-like rotating stars - in the latter case, for example, we retrieve the scaling of the average magnetic field to the reciprocal of Rossby number. Finally, we argue that the validity of the popular "maximum entropy production principle" is questionable in these systems.

physics.plasm-ph↗
arXiv2014

On a non-linear sigma model of knotted relaxed states far from thermodynamic equilibrium in plasma physics and beyond

We show that a Faddeev-Niemi non-linear sigma model describes in the long wavelength limit a wide class of steady-state, knotted physical systems far from thermodynamic equilibrium which are stable against perturbations of temperature and interact weakly with the external world. In these systems temperature gradients are negligible, inertial effects are negligible in comparison with diffusion effects, entropy is mainly produced through Joule and-or viscous heating, the macroscopic state is described by specifying a unit vector at each point, and the Gauss linking number of this unit vector is lower than a threshold. In fluids and plasmas, the model describes filamentary structures which adjust themselves in order to offer minimum resistance to the medium embedding them and to the electric currents (if any) flowing across them; in the latter case, Gauss linking number is related to magnetic helicity. Both n and the relative velocity of the filament with respect to the medium are approximately Double Beltrami vector fields. We derive a stability criterion for a double helix. Moreover, a similar discussion describes the recently discovered writing process of skyrmions in a magnetic film with the help of a beam of polarised electrons. We derive a lower bound on the value of beam current required to write a skyrmion.

cond-mat.mtrl-sci↗