arXiv subjects

D. I. Palade

Publications and source records attributed to D. I. Palade.

17 recordsLinked to original sources

arXiv2025

Lagrangian features of turbulent transport in tokamak plasmas

This study investigates the Lagrangian properties of ion turbulent transport driven by drift-type turbulence in tokamak plasmas. Despite the compressible and inhomogeneous nature of Eulerian gyrocenter drifts, numerical simulations with the T3ST code reveal approximate ergodicity, stationarity, and time-symmetry. These characteristics are attributed to broad initial phase-space distributions that support ergodic mixing. Moreover, relatively minor constraints on the initial distributions are found to have negligible effects on transport levels.

physics.plasm-ph↗

arXiv2025

T3ST code: Turbulent Transport in Tokamaks via Stochastic Trajectories

We introduce the Turbulent Transport in Tokamaks via Stochastic Trajectories (T3ST) code, designed to address the problem of turbulent transport using a statistical approach complementary to gyrokinetics. The code employs test-particle methods to track the dynamics of charged particles in axisymmetric magnetic equilibria, accounting for both turbulence and Coulomb collisions. The turbulence is decoupled from plasma dynamics and represented through a statistical ensemble of synthetic random fields with specified spectral properties. This approach enables T3ST to compute transport coefficients as Lagrangian correlations - orders of magnitude faster than gyrokinetic codes.

physics.plasm-ph↗

arXiv2023

Predicting the turbulent transport of cosmic rays via neural networks

A fast artificial neural network is developed for the prediction of cosmic ray transport in turbulent astrophysical magnetic fields. The setup is trained and tested on bespoke datasets that are constructed with the aid of test-particle numerical simulations of relativistic cosmic ray dynamics in synthetic stochastic fields. The neural network uses, as input, particle and field properties and estimates transport coefficients 10^7 faster than standard numerical simulations with an overall error of ~5% .

astro-ph.HE↗

arXiv2023

Approximations of the modified Bessel functions of the second kind $K_ν$. Applications in random field generation

We propose an analytical approximation for the modified Bessel function of the second kind $K_ν$. The approximation is derived from an exponential ansatz imposing global constrains. It yields local and global errors of less than one percent and a speed-up in the computing time of $3$ orders in magnitude in comparison with traditional approaches. We demonstrate the validity of our approximation for the task of generating long-range correlated random fields.

physics.comp-ph↗

arXiv2023

Scaling laws of two-dimensional incompressible turbulent transport

The diffusive transport in two-dimensional incompressible turbulent fields is investigated with the aid of high-quality direct numerical simulations. Three classes of turbulence spectra that are able to capture both short and long-range time-space correlations and oscillating features are employed. We report novel scaling laws that depart from the $γ=7/10$ paradigm of percolative exponents and are dependent on the features of turbulence. A simple relation between diffusion in the percolative and frozen regimes is found. The importance of discerning between differential and integral characteristic scales is emphasized.

physics.flu-dyn↗

arXiv2021

Effects of intermittency on turbulent transport in magnetized plasmas

We analyze how the turbulent transport of $\mathbf{E}\times \mathbf{B}$ type in magnetically confined plasmas is affected by intermittent features of turbulence. The latter are captured by the non-Gaussian distribution $P(ϕ)$ of the turbulent electric potential $ϕ$. Our analysis is performed at an analytical level and confirmed numerically using two statistical approaches. We have found that the diffusion is inhibited linearly by intermittency, mainly via the kurtosis of the distribution $P(ϕ)$. The associated susceptibility for this linear process is shown to be dependent on the poloidal velocity $V_p$ and on the correlation time $τ_c$ with a maxima at the time-of-flight $τ_{fl}$. Intermittency does not affect the Kubo number scaling in the strong regime.

physics.plasm-ph↗

arXiv2020

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.

physics.plasm-ph↗

arXiv2020

Fast generation of Gaussian random fields for direct numerical simulations of stochastic transport

We propose a novel discrete method of constructing Gaussian Random Fields (GRF) based on a combination of modified spectral representations, Fourier and Blob. The method is intended for Direct Numerical Simulations of the V-Langevin equations. The latter are stereotypical descriptions of anomalous stochastic transport in various physical systems. From an Eulerian perspective, our method is designed to exhibit improved convergence rates. From a Lagrangian perspective, our method others a pertinent description of particle trajectories in turbulent velocity fields: the exact Lagrangian invariant laws are well reproduced. From a computational perspective, our method is twice as fast as standard numerical representations.

physics.comp-ph↗

arXiv2020

The isoscalar features of Pygmy Dipole Resonance: a subtle game of symmetry energy

The vibrational structure of the Pygmy Dipole Resonance (PDR) is investigated within a quantum many-body treatment with extended separable interactions able to encode the dependence of nuclear symmetry energy on density. A new picture of PDR is unveiled in terms of a combined dynamics of the neutron skin and of the core isovector polarization, which determines the isoscalar features of PDR while reproducing the isovector properties of Giant Dipole Resonance. The key role played by the variation with density of the symmetry energy on shaping the low-lying dipole response and its isoscalar-isovector structure is underlined. Our results provide insights for the challenge of clarifying the transition from skin oscillation to a highly bulk collective dynamics.

nucl-th↗

arXiv2018

Nonlocal Orbital-Free kinetic pressure tensors for the Fermi gas

A novel nonlocal density functional for the kinetic pressure tensor of a Fermi gas is derived. The functional is designed to reconcile the Quantum Hydrodynamic Model with the microscopic approaches, both for homogeneous equilibrium and dynamical regime. The derivation opens new ways to improve and implement further time-nonlocal functionals. The present proposal is systematically tested in and beyond the linear regime for the Fermi gas, as well as for some small sodium clusters, proving that it is quantitative superior to other existing functionals.

physics.plasm-ph↗

arXiv2016

Multiple surface plasmons on an unbounded quantum plasma half-space

The propagation of surface plasmons on a quantum plasma half-space in the absence of any external confinement is investigated. By means of Quantum Hydrodynamic Model in the electrostatic limit it is found that the equilibrium density profile is a smooth continuous function which, in the linear regime, supports multiple non-normal surface modes. Defining a spectrum function and using a cutting condition, the dispersion relations of these modes and their relevance for realistic dynamics are computed. It is found that the multiple surface plasmons present a significant red-shift with respect to the case of fully bounded quantum plasmas.

physics.plasm-ph↗

arXiv2016

Turbulent transport of alpha particles in tokamak plasmas

We investigate the ExB diffusion of fusion born αparticles 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 αparticle. According to our results, signficant turbulent transport of the αparticles can appear only at energies of the order of 100KeV. We determine the corresponding conditions.

physics.plasm-ph↗

arXiv2015

N-Block Separable Random Phase Approximation: Application to metal clusters and C60 fullerene

Starting from the Random Phase Approximation (RPA), we generalize the schematic model of separable interaction defning subspaces of ph excitations with different coupling constants between them. This ansatz simplifies the RPA eigenvalue problem to a finite, small dimensional system of equations which reduces the numerical effort. Associated dispersion relation and the normalization condition are derived for the new defined unknowns of the system. In contrast with the standard separable approach, the present formalism is able to describe more than one collective excitation even in the degenerate limit. The theoretical framework is applied to neutral and singly charged spherical sodium clusters and C60 fullerene with results in good agreement with full RPA calculations and experimental data.

physics.atm-clus↗

arXiv2014

On the collectivity of Pygmy Dipole Resonance within schematic TDA and RPA models

Within schematic models based on the Tamm-Dancoff Approximation and the Random-Phase Approximation with separable interactions, we investigate the physical conditions which determine the emergence of the Pygmy Dipole Resonance in the E1 response of atomic nuclei. We find that if some particle-hole excitation manifests a different, weaker residual interaction, an additional mode will appear, with an energy centroid closer to the distance between two major shells and therefore well below the Giant Dipole Resonance. This state, together with Giant Dipole Resonance, exhausts all the transition strength in the Tamm-Dancoff Approximation and all the Energy Weighted Sum Rule in the Random-Phase Approximation. Thus, within our scheme, this mode, which could be associated with the Pygmy Dipole Resonance, is of collective nature. By relating the coupling constants appearing in the separable interaction to the symmetry energy value at and below saturation density we explore the role of the density dependence of the symmetry energy on the low energy dipole response.

nucl-th↗

arXiv2014

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.

physics.atm-clus↗

arXiv2014

Monte Carlo tests of Orbital-Free Density Functional Theory

The relationship between the exact kinetic energy density in a quantum system in the frame of Density Functional Theory and the semiclassical functional expression for the same quantity is investigated. The analysis is performed with Monte Carlo simulations of the Kohn-Sham potentials. We find that the semiclassical form represents the statistical expectation value of the quantum nature. Based on the numerical results, we propose an empirical correction to the existing functional and an associated method to improve the Orbital-Free results.

physics.atm-clus↗

arXiv2014

General static polarizability in spherical neutral metal clusters and fullerenes within Thomas-Fermi theory

We study the static linear response in spherical Thomas-Fermi systems deriving a simple diferen- tial equation for general multipolar moments and associated polarizabilities. We test the equation on sodium clusters between 20 and 100 atoms and on fullerenes between C60 and C240 and propose it for general Thomas-Fermi systems. Our simple method provides results which deviates from experimental data with less then 15%.

physics.atm-clus↗