Magnetohydrodynamic Modelling of Supersonic Jets and Colliding Blast Waves for Laboratory Astrophysics Investigation

The thesis is related to laboratory astrophysics, and investigates with this technique, the launching mechanism for young stellar object jets and the interaction of two supernovae remnant in the Sedov-Taylor regime. Recent experiments performed at Imperial College on the pulsed-power magpie facility have successfully shown the formation of magnetically driven radiatively cooled plasmas jets formed from radial wire arrays, which are relevant to studying the launching mechanisms of astrophysical jets [A. Ciardi, et al. Phys. Plasmas 14, p056501 (2007)]. The experiments have been now extended to study episodic mass ejection ( 25 ns [F. A. Suzuki-Vidal, et al. 49th Annual Meeting of the Division of Plasma Physics, UO4.00007 (2007)]) and the interaction of jets and magnetic bubbles with an ambient gas. The dynamics of the interaction is investigated through three-dimensional resistive magneto-hydrodynamic simulations using the code gorgon [A. Ciardi, et al. Phys. Plasmas 14, p056501 (2007) – J.P. Chittenden, et al. Plasma Phys. Control. Fusion 46 B457 (2004)]. In particular ablation of the cathode is investigated numerically to explain the periodicity and subsequent formation of multiple bubbles. Comparison with experiments is offered to validate the results. The complex structure of the magnetic field is investigated, the conservation of the magnetic flux is explained and the consequent confinement offered to the central jet. Furthermore the interaction of the plasma outflows with an ambient gas is investigated. The formation of shocks in the ambient gas, as well as the formation of three-dimensional Mach stems is analyzed. In addition, recent experiment at Imperial College performed by the QOLS group, by laser-heating a medium of atomic clusters [R. A. Smith, et al. 2007 Plasma Phys. Control. Fusion 49 B117-B124 (2007)], shows the capability to create plasmas with sufficiently high energy densities to launch strong shocks. Interactions between high-Mach number shock waves are believed to be responsible for many of the complex, turbulent structures seen in astrophysical objects including supernova remnants. The experiment of two colliding Sedov-Taylor regime blast-waves is modelled. Detailed 3D numerical modeling is performed in order to study the importance of thermal conduction, rarefaction waves, refractive shock waves and complex three-dimensional mach stem formation. The simulated data are benchmark against a three-dimensional tomography image (newly developed experimental technique). The collision of two blast-waves should reproduce the non uniform interstellar medium where supernovas normally expand

Magnetic field generation and diffusion by a laser-produced blast wave propagating in non-homogenous plasma

In this paper we discuss the magnetic field self generation, via the so-called Biermann battery effect, and its diffusion for a blast wave (BW) expanding in a perturbed background medium. A series of simulations verify the bi-linear behavior of the Biermann battery source term both in amplitude and in wavenumber. Such a behavior is valid in the limit of no diffusivity. When diffusivity is also considered, we observe an inverse proportionality with the wavenumber: for large wavenumber perturbation magnetic diffusivity plays a key role. Writing the induction equation in a dimensionless form we discuss how, in terms of magnetic properties, the BW can be subdivided into three main regions: the remnant where the frozen-in-flow approximation holds, the thin shell where the magnetic field is in fact generated but at the same time begins to diffuse, and the shock front where the magnetic field diffuses away. A possible experimental scenario that could induce magnetic fields of about 100 gauss is finally investigated. Simulations have been performed with the code DUED

Improved robustness study of a shock ignited target, with DUED code including non-local electron transport and 3D laser ray-tracing

Accurate descriptions of laser power coupling to the plasma and electron energy transport are crucial for designing shock-ignition targets and assessing their robustness (in particular with regard to laser and positioning errors). To this purpose, the 2D DUED laser fusion code has been improved with the inclusion of a 3D laser ray-tracing scheme and a model for non-local electron transport. 2D simulations with the upgraded code are presented; the dependence of the fusion yield vs target displacement is studied. Two different irradiation configurations are considered

Guiding of charged particle beams in curved capillary-discharge waveguides

A new method able to transport charged particle beams along curved paths is presented. It is based on curved capillary-discharge waveguides in which the induced azimuthal magnetic field is used both to focus the beam and keep it close to the capillary axis. We show that such a solution is highly tunable, it allows to develop compact structures providing large deflecting angles and, unlike conventional solutions based on bending magnets, preserves the beam longitudinal phase space. The latter feature, in particular, is very promising when dealing with ultra-short bunches for which non-trivial manipulations on the longitudinal phase spaces are usually required when employing conventional devices

Study of plasma wakefield acceleration mechanism for emittance dominated regimes via hybrid and pic simulations

Electron plasma wakefield acceleration (PWFA) mechanism is a promising non conventional acceleration scheme. Nonetheless further investigation is still needed to fully uncover the instability mechanisms so to mitigate them and make PWFA an effective tool. This work focuses in this direction, we discuss the necessity to use well matched driver bunches to further mitigate witness instabilities. Specifically we propose to inject driver bunches with larger emittance than the matched one (overcompressed bunch) so to let the system reach the matching condition by itself. This preliminary results lead us to the following consideration: while a limited number of cases can be studied with a particle-in-cell code, we understand the necessity for fast systematic analysis: we briefly introduce the hybrid code Architect

Characterisation of beam driven ionisation injection in the blowout regime of Plasma Acceleration

Beam driven ionisation injection is characterised for a variety of high-Z dopant. We discuss the region of extraction and why the position where electrons are captured influences the final quality of the internally-injected bunch. The beam driven ionisation injection relies on the capability to produce a high gradient fields at the bubble closure, with magnitudes high enough to ionise by tunnelling effect the still bounded electrons (of a high-Z dopant). The ionised electrons are captured by the nonlinear plasma wave at the accelerating and focusing wake phase leading to high-brightness trailing bunches. The high transformer ratio guarantees that the ionisation only occurs at the bubble closure. The quality of the ionisation-injected trailing bunches strongly and non-linearly depends on the properties of the dopant gas (density and initial ionisation state). We use the full 3D PIC code ${\tt ALaDyn}$ to consider the highly three-dimensional nature of the effect. By means of a systematic approach we have investigated the emittance and energy spread formation and the evolution for different dopant gases and configurations

Efficient plasma wakefield acceleration simulations via kinetic-hydro code

Start-to-end simulations are needed for sensitivity stud- ies and online analysis of experimental data of the Plasma Wakefield Acceleration experiment COMB at SPARC_LAB facility, Frascati (Italy). Ad hoc tools are needed for the plasma section modeling. Particle in cell codes are the most widely used tools for this purpose, but they suffer from the considerable amount of computational resources they re- quire. We seek for a simple, portable, quick-to-run approach. For this purpose we introduce a time-explicit cylindrical hybrid fluid-kinetic code: Architect. The beam particles are treated with PIC-like kinetic approach, while the plasma wake is treated as a fluid. Since the number of computational particles used by the hybrid model is significantly reduced with respect of full PIC codes with the same number of di- mensions, the time required for a simulation is reduced as well