arXiv subjects

Matteo Valerio Falessi

Publications and source records attributed to Matteo Valerio Falessi.

8 recordsLinked to original sources

arXiv2025

Excitation of toroidal Alfvén eigenmode by energetic particles in DTT and effect of negative triangularity

A linear gyrokinetic eigenvalue code is developed to study the stability of toroidal Alfvén eigenmode (TAE) in general axisymmetric toroidal geometry, with the self-consistent treatment of energetic particle drive and core plasma Landau damping in a non-perturbative way. The general particle responses of both circulating and trapped particles are incorporated in the calculation by means of the action-angle approach, and, particularly, the finite Larmor radius and orbit width effects of energetic particles are fully taken into account. The ballooning-mode representation is adopted to solve the eigenmode equations in order to reduce the computational resource while obtaining a high resolution of the fine radial structure. Furthermore, the code is able to study the physics of wave-particle interaction in great detail, thanks to the development of systematic theory-based numerical diagnostics, including effective mode structure and phase space resonance structure. As an application of the code, we perform an in-depth study of the triangularity effect on TAE stability based on the reference equilibrium of the Divertor Tokamak Test facility. It is demonstrated that TAE growth rate can be affected by the triangularity through the modifications of geometric couplings, resonance condition, as well as mode frequency and mode structure. As a result, negative triangularity can either stabilize or destabilize the energetic particle driven TAE depending on the dominant mechanism. The relative importance of these mechanisms under different circumstances is systematically analyzed, providing clear physical insights. The overall effect of negative triangularity for a specific tokamak scenario can be assessed based on these studies.

physics.plasm-ph↗

arXiv2024

Calculation of toroidal Alfvén eigenmode mode structure in general axisymmetric toroidal geometry

A workflow is developed based on the ideal MHD model to investigate the linear physics of various Alfvén eigenmodes in general axisymmetric toroidal geometry, by solving the coupled shear Alfvén wave (SAW) and ion sound wave (ISW) equations in ballooning space. The model equations are solved by the FALCON code in the singular layer, and the corresponding solutions are then taken as the boundary conditions for calculating parallel mode structures in the whole ballooning space. As an application of the code, the frequencies and mode structures of toroidal Alfvén eigenmode (TAE) are calculated in the reference equilibria of the Divertor Tokamak Test facility (DTT) with positive and negative triangularities, respectively. By properly handling the boundary conditions, we demonstrate finite TAE damping due to coupling with the local acoustic continuum, and find that the damping rate is small for typical plasma parameters.

physics.plasm-ph↗

arXiv2023

Nonlinear equilibria and transport processes in burning plasmas

In this work, we put forward a general phase-space transport theory in axisymmetric tokamak plasmas based upon the concept of zonal state (ZS). Within this theoretical framework, the ZS corresponds to a renormalized plasma nonlinear equilibrium consisting of phase-space zonal structures (PSZS) and zonal electromagnetic fields (ZFs) which evolve self-consistently with symmetry breaking fluctuations and sources/collisions. More specifically, our approach involves deriving governing equations for the evolution of particle distribution functions (i.e, PSZS), which can be used to compute the corresponding macro-/meso-scale evolving magnetized plasma equilibrium adopting the Chew Goldberger Low (CGL) description, separating the spatiotemporal microscale structures. The nonlinear physics of ZFs and of geodesic acoustic modes/energetic particle driven geodesic acoustic modes is then analyzed to illustrate the implications of our theory.

physics.plasm-ph↗

arXiv2022

Energetic particle transport: diffusion vs convection and phase-space barriers

Energetic particle redistribution in the presence of multiple Alfvén eigenmodes is analyzed in [PPCF 58, 014019 (2016)] for the ITER 15MA baseline scenario: non-linear hybrid simulations (within their well known limits) point out that transport can be dominated by avalanches under certain conditions. These phenomena are properly reproduced by the 1D reduced description of [PPCF 64, 035010 (2022)]. Here, using this simplified 1D model, we define the transport character (convective/diffusive) of self-consistent energetic particle redistribution. Transport barriers in phase space are studied using the Lagrangian Coherent Structures technique.

physics.plasm-ph↗

arXiv2020

Resonance overlap and non-linear features of the beam-plasma system

The beam-plasma instability can be addressed as a reduced model in several contexts of plasma physics, from space to fusion plasma. In this paper, we review and refine some non-linear features of this model. Specifically, by analyzing the dependence of the non-linear velocity spread as a function of the linear growth rate, we discuss the effective size of the resonance in view of its role in the spectral overlap at saturation. The relevance of this characterization relies on the necessity of a quantitative determination of the overlap degree to discriminate among different transport regimes of the self consistent dynamics. The analysis is enriched with a study of the phase-space dynamics by means of the Lagrangian Coherent Structure technique, in order to define the transport barriers of the system describing the relevant features of the overlap process. Finally, we discuss relevant features related to the mode saturation levels.

physics.plasm-ph↗

arXiv2019

Shear Alfvén and acoustic continuum in general axisymmetric toroidal geometry

The equations describing the continuous spectrum of shear Alfvén and ion sound waves propagating along magnetic field lines are introduced and solved in the ballooning space for general geometry in the ideal MHD limit. This approach is equivalent to earlier analyses by Chu et al. 1992 [Phys. Fluids B 4, 3713 (1992)] but the present formulation in the ballooning space allows to readily extend it to include gyrokinetic and three-dimensional equilibrium effects. In particular, following Chen and Zonca 2017 [Phys. Plasmas 24, 072511 (2017)], the MHD limit is adopted to illustrate the general methodology in a simple case, and the equations are solved within the framework of Floquet and Hill's equation theory. The connection of shear Alfvén and ion sound wave continuum structures to the generalized plasma inertia in the general fishbone like dispersion relation is also illustrated and discussed. As an application, the continuous frequency spectrum is calculated for a reference equilibrium of the Divertor Tokamak Test facility. The results are compared with those obtained by the MARS code adopting the standard methodology, demonstrating excellent agreement.

physics.plasm-ph↗

arXiv2017

Gyrokinetic theory for particle transport in fusion plasmas

A set of equations is derived describing the macroscopic transport of particles and energy in a thermonuclear plasma on the energy confinement time. The equations thus derived allow studying collisional and turbulent transport self-consistently, retaining the effect of magnetic field geometry without postulating any scale separation between the reference state and fluctuations. Previously, assuming scale separation, transport equations have been derived from kinetic equations by means of multiple-scale perturbation analysis and spatio-temporal averaging. In this work, the evolution equations for the moments of the distribution function are obtained following the standard approach; meanwhile, gyrokinetic theory has been used to explicitly express the fluctuation induced fluxes. In this way, equations for the transport of particles and energy up to the transport time scale can be derived using standard first order gyrokinetics.

physics.plasm-ph↗

arXiv2017

Gyrokinetic theory for particle transport in fusion plasmas

Predicting the dynamics of a thermonuclear plasma during a magnetic confinement experiment is fundamental in order to make nuclear fusion a reliable source of energy. The development of a set of equations describing the plasma evolution on a given time scale is the main requirement to reach this goal. A limited amount of works have studied in a self-consistent way collisional transport and fluctuation induced transport. The motivation of this work stems from the fundamental importance of the self-consistency of the adopted description in order to understand transport processes on the energy confinement (transport) time scale because of the mutual interaction between collisions and turbulence. In turn, this is crucial in order to predict fluxes of particle and energy and, ultimately, the overall plasma evolution. Using flux coordinates and the drift ordering we derive a set of evolutions equations for the number of particles and the energy density on the transport time scale. These equations show the interplay between collisions and fluctuations and, in particular, show that fluctuations may enhance collisional transport while the collisions can damp long lived structures formed by saturated instabilities, i.e zonal structures. Fluctuation induced fluxes are described using gyrokinetic field theory, which makes a comparison with the theory of phase space zonal structures possible, revealing that the fluctuations induced part of the transport equations can be obtained by taking the proper moment of the long length scale limit of the equation governing the evolution of phase space zonal structures. Finally, we show that plasma nonlinear evolution can yield to structures formation that are characterized by mesoscales, intermediate between the typical ones of plasma turbulence and those of the reference plasma equilibrium.

physics.plasm-ph↗