1,320 research outputs found
Heating in collisionless plasmas: Do collisions play an effective role?
High temperature and low density plasmas are ubiquitous in the Universe. These systems often exhibit a turbulent dynamics, characterized by the cross-scale coupling of fluid and kinetic scales and by the inhomogeneous development of coherent spatial and temporal structures. At smaller scales the energy of fluctuations is dissipated and plasma is eventually heated. Despite collisions are usually ruled out from the description of these systems, it has been recently shown that plasma collisionality may be locally enhanced, owing to the presence of fine structures in velocity space. In this perspective, further insights are here given by comparing the characteristic times of collisional dissipation with the time scales of other collisionless processes, such as nonlinear coupling and linear instability onset. When taking into account fine velocity structures, these characteristic times could be comparable. A novel scenario for the description of heating in weakly-collisional plasmas, even including inter-particle collisions, is finally proposed
Inverse probability weighting to estimate causal effect of a singular phase in a multiphase randomized clinical trial for multiple myeloma.
Kelvin-Helmholtz instability at proton scales with an exact kinetic equilibrium
The Kelvin-Helmholtz instability is a ubiquitous physical process in ordinary
fluids and plasmas, frequently observed also in space environments. In this
paper, kinetic effects at proton scales in the nonlinear and turbulent stage of
the Kelvin-Helmholtz instability have been studied in magnetized collisionless
plasmas by means of Hybrid Vlasov-Maxwell simulations. The main goal of this
work is to point out the back reaction on particles triggered by the evolution
of such instability, as energy reaches kinetic scales along the turbulent
cascade. Interestingly, turbulence is inhibited when Kelvin-Helmholtz
instability develops over an initial state which is not an exact equilibrium
state. On the other hand, when an initial equilibrium condition is considered,
energy can be efficiently transferred towards short scales, reaches the typical
proton wavelengths and drives the dynamics of particles. As a consequence of
the interaction of particles with the turbulent fluctuating fields, the proton
velocity distribution deviates significantly from the local thermodynamic
equilibrium, the degree of deviation increasing with the level of turbulence in
the system and being located near regions of strong magnetic stresses. These
numerical results support recent space observations from the Magnetospheric
MultiScale mission of ion kinetic effects driven by the turbulent dynamics at
the Earth's magnetosheath (Perri et al., 2020, JPlPh, 86, 905860108) and by the
Kelvin-Helmholtz instability in the Earth's magnetosphere (Sorriso-Valvo et
al., 2019, PhRvL, 122, 035102).Comment: 14 pages, 11 figure
Fourier-Hermite decomposition of the collisional Vlasov-Maxwell system: Implications for the velocity-space cascade
Turbulence at kinetic scales is an unresolved and ubiquitous phenomenon that characterizes both space and laboratory plasmas. Recently, new theories, {\it in-situ} spacecraft observations and numerical simulations suggest a novel scenario for turbulence, characterized by a so-called phase space cascade -- the formation of fine structures, both in physical and velocity space. This new concept is here extended by directly taking into account the role of inter-particle collisions, modeled through the nonlinear Landau operator or the simplified Dougherty operator. The characteristic times, associated with inter-particle correlations, are derived in the above cases. The implications of introducing collisions on the phase space cascade are finally discussed
Local energy transfer rate and kinetic processes: the fate of turbulent energy in two-dimensional Hybrid Vlasov-Maxwell numerical simulations
The nature of the cross-scale connections between the inertial range turbulent energy cascade and the small-scale kinetic processes in collisionless plasmas is explored through the analysis of two-dimensional Hybrid Vlasov-Maxwell numerical simulation (HVM), with α particles, and through a proxy of the turbulent energy transfer rate, namely the Local Energy Transfer rate (LET). Correlations between pairs of variables, including those related to kinetic processes and to deviation from Maxwellian distributions, are first evidenced. Then, the general properties and the statistical scaling laws of the LET are described, confirming its reliability for the description of the turbulent cascade and revealing its textured topology. Finally, the connection between such proxy and the diag- nostic variables is explored using conditional averaging, showing that several quantities are enhanced in the presence of large positive energy flux, and reduced near sites of neg- ative flux. These observations can help determining which processes are involved in the dissipation of energy at small scales, as for example ion-cyclotron or mirror instabilities typically associated with perpendicular anisotropy of temperature
Casos de tuberculosis declarados a la Red Nacional de Vigilancia Epidemiológica. España, 2004
- …