Magnetic field generation in plasmas driven by Weibel-like instabilities
Creators: name: Chiappetta, Cinzia
Publisher: name: Lancaster University
Year: 2024

description: 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