Magnetic field generation in plasmas driven by Weibel-like instabilities

Chiappetta, Cinzia and Boella, Elisabetta (2024) Magnetic field generation in plasmas driven by Weibel-like instabilities. PhD thesis, Lancaster University.

Text (2024CinziaPhD)
2024CinziaPhD.pdf - Published Version
Available under License None.
Download (5MB)

Abstract

The origin of magnetic fields in astrophysics remains one of the most crucial scientific questions, given that their temporal and spatial scales are fundamental for explaining observations of energetic events such as gamma-ray bursts (GRBs) and supernova remnants (SNRs). It is now widely recognised that the creation and intensification of magnetic fields depend significantly on plasma instabilities. For instance, the Weibel instability (WI), also known as the current filamentation instability (CFI), can amplify magnetic fields and lead to the formation of electromagnetic shock waves, where particles can be accelerated to high energies while emitting strong bursts of radiation. Significant efforts have been made to reproduce these fundamental mechanisms in the laboratory through experiments that preserve the scale differences of astrophysical scenarios. Due to advancements in sophisticated simulation tools, it is now possible to understand a wide range of astrophysical problems and develop laboratory experiments to replicate these events. In this thesis, I examine the onset and long-term development of the CFI in counterstreaming electron-ion flows. Using two-dimensional kinetic simulations performed with the semi-implicit energy-conserving code ECsim, I investigate the evolution of the instability on ion timescales. The numerical results indicate that the magnetic field driven by the instability survives for hundreds of ion plasma periods. The instability produces magnetic field filaments that evolve from sub-electron scales to beyond the ion inertial length, depending on the flow velocity and the plasma anisotropy. The ion anisotropy, which remains substantial throughout the simulations, sustains the coalescence process of magnetic filaments. In the second phase of the numerical investigation into CFI, I studied the interaction between a neutral ultra-relativistic electron-positron beam and a magnetised plasma using the PIC code OSIRIS. I investigated how a pre-existing magnetic field, oriented parallel to the beam propagation, modifies the growth and saturation of kinetic instabilities. Without a magnetic field, the dominant instability is the CFI, which causes modes perpendicular to the direction of the beam. By increasing the strength of the magnetic field, it is possible to observe a transition toward progressively more oblique modes. The growth rate of these modes is smaller than that of the CFI. In all cases, these instabilities generate a magnetic field perpendicular to both the beam velocity and the wavenumber. These simulations indicate that the instability-driven field reaches higher values at saturation in the presence of higher degrees of magnetisation. The effect of longitudinal beam variation was also explored, demonstrating that even less dense, longer longitudinal beams can still cause kinetic instabilities, although the magnetic field grows at a slower rate. Finally, to probe the physics underpinning the interaction in the laboratory, I examined the propagation of the electron beam into the plasma, considering the CLARA laboratory parameters. The preliminary results show the growth of the electric field components and the transverse magnetic field