arXiv subjects

A. Alexandrova

Publications and source records attributed to A. Alexandrova.

4 recordsLinked to original sources

arXiv2026

Laboratory evidence of electron pressure anisotropy driving plasmoid mediated magnetic reconnection

Plasmoid-driven magnetic reconnection in elongated current sheets is suspected to be an ubiquitous phenomenon in space and astrophysical plasmas, but the mechanisms driving its onset and dynamics are still debated. Deciphering the physical mechanisms dominating the destabilization and fragmentation of the current sheet, as well as its evolution, would have a wide impact into our understanding of the induced plasma turbulence and particle acceleration. Here, by coupling 3D hybrid simulations with laser-driven experiments that involve counterflowing high-energy-density magnetized plasmas with a long aspect ratio of their contact layer, we show that electron pressure anisotropy is the driving factor of the growth rate of the tearing instability, and will sustain the reconnection process even without classical resistivity. Dissipative mechanisms, such as resistivity and isotropization, are further found to stabilize the sheet to varying degrees, thus modifying plasmoid formation. By identifying the roles of pressure anisotropy, dissipation, and large-scale geometry, our work lays the groundwork for the evaluation of plasmoid-driven reconnection impact on the dynamics of laboratory and astrophysical plasmas.

physics.plasm-ph↗

arXiv2025

Adiabatic Anisotropic Gravitational Collapse in Painlev\'e-Gullstrand Coordinates: A Geometric Analysis

We present a detailed geometric analysis of adiabatic, anisotropic gravitational collapse formulated in a single Painlev\'e-Gullstrand coordinate system that covers both the interior and exterior, thereby eliminating cross-chart matching artifacts. Building on the Oppenheimer-Snyder framework with a phenomenologically motivated energy-density profile, we enforce the Israel junction conditions and obtain closed-form surface evolution. Within this unified chart we derive exact solutions for the complete collapse process, characterize the causal structure, and track horizon formation and evolution. In particular, we identify and analyse a double apparent-horizon phase inside the matter and show that the event horizon stabilizes at the Schwarzschild radius. We further obtain critical parameter relations that govern the dynamics, including a threshold linking initial compactness to immediate horizon formation. The model is geometrically self-consistent within Einstein's equations but exhibits violations of the standard point-wise energy conditions, highlighting known limitations of idealized anisotropic matter models and delineating the boundary where classical descriptions become inadequate. Together, these results provide geometric insights, compact analytic benchmarks and a didactic, coordinate-uniform perspective on collapse and horizon dynamics.

gr-qc↗

arXiv2021

The structure of a perturbed magnetic reconnection electron diffusion region

We report in situ observations of an electron diffusion region (EDR) and adjacent separatrix region. We observe significant magnetic field oscillations near the lower hybrid frequency which propagate perpendicularly to the reconnection plane. We also find that the strong electron-scale gradients close to the EDR exhibit significant oscillations at a similar frequency. Such oscillations are not expected for a crossing of a steady 2D EDR, and can be explained by a complex motion of the reconnection plane induced by current sheet kinking propagating in the out-of-reconnection-plane direction. Thus all three spatial dimensions have to be taken into account to explain the observed perturbed EDR crossing.

physics.plasm-ph↗

arXiv2018

Study of ultra-high gradient acceleration in carbon nanotube arrays

Solid-state based wakefield acceleration of charged particles was previously proposed to obtain extremely high gradients on the order of 1-10 TeV/m. In recent years the possibility of using either metallic or carbon nanotube structures is attracting new attention. The use of carbon nanotubes would allow us to accelerate and channel particles overcoming many of the limitations of using natural crystals, e.g. channeling aperture restrictions and thermal-mechanical robustness issues. In this paper, we propose a potential proof of concept experiment using carbon nanotube arrays, assuming the beam parameters and conditions of accelerator facilities already available, such as CLEAR at CERN and CLARA at Daresbury. The acceleration performance of carbon nanotube arrays is investigated by using a 2D Particle-In-Cell (PIC) model based on a multi-hollow plasma. Optimum experimental beam parameters and system layout are discussed.

physics.acc-ph↗