The linear Breit-Wheeler process ($\gamma\gamma\longrightarrow e^{-} e^{+}$) is one of the fundamental processes of quantum electrodynamics with many important astrophysical applications. However, almost a century has passed since it was theoretically proposed in 1934, and there is still no experimental observation of the linear Breit-Wheeler process using real photons. To produce linear Breit-Wheeler pairs, the collision of dense and energetic photons is required due to the small cross section and high photon energy threshold of this process, which is challenging to be achieved in laboratory conditions. On the other hand, the rapid development of laser technology has allowed the state-of-the-art laser facilities to produce laser pulses with intensities above $10^{22}\Wcmsqd$. Such high-intensity, multi-PW, and multi-beam laser facilities open up brand new opportunities for the experimental realization of the linear Breit-Wheeler pair creation.

This dissertation presents two schemes for producing the linear Breit-Wheeler pairs using currently available laser pulses, with the assistance of numerical tools we developed.We show that utilizing collective plasma effects beneficial for the linear Breit-Wheeler pair creation, one can achieve pair yields that was previously thought to be unattainable at current laser intensities. Compared to the pair yields reported in previous studies, the pair yields in the systems we propose are approximately two to three orders of magnitudes larger. We further show that by interacting with the strong fields in the laser-plasma system, a large proportion of the produced positrons can be accelerated to form collimated energetic positron beams in hundreds-of-MeV to GeV level. Our studies not only propose possible schemes for the first experimental observation of the linear Breit-Wheeler process in laboratory, but have also emphasized the existence of the previously overlooked linear Breit-Wheeler process in high-intensity laser-plasma interaction systems. The self-organized formation of collimated positron beams may also serve as a potential mechanism for energetic positron source.

With the advent of petawatt-class laser facilities, laser intensities reach unprecedented levels enabling novel and efficient regimes of secondary particle and radiation beams. A regime involving relativistic transparency of dense plasmas and the generation of a Megatesla-level azimuthal magnetic field is shown to be promising for generating energetic electrons and collimated gamma-ray beams. This dissertation focuses on the above regime to explore the acceleration mechanism of electrons, to optimize the gamma-ray yield, to examine the application to two-photon pair production, and to investigate the feasibility of magnetic field detection.

First, we investigate direct laser acceleration in the presence of a strong azimuthal magnetic field. Test-particle models are built to explain the enhanced acceleration. The magnetic fields mitigate electron dephasing and allow an efficient acceleration over a short distance. We then report the important role of the laser phase velocity on electron confinement in this acceleration regime.

We investigate the emission of collimated gamma-ray beams from laser-irradiated channel targets through three-dimensional kinetic simulations. We find a strong power scaling of conversion efficiency into MeV-level photons. The electron-positron pair production via two-photon collisions directly benefits from such a power scaling. We explore two schemes of generating pairs through the linear Breit-Wheeler process: colliding two gamma-ray beams and colliding one gamma-ray beam with blackbody radiation. The strong power scaling boosts the pair yield to the level of 100 000.

Our research on the hollow-channel regime corroborates the robustness of prefilled channels. Due to the influence of ion motion, electrons acceleration and photon emission degrade in hollow channels. With a broader angular spread of gamma-ray beams, the pair yield in hollow channels is shown to be less efficient.

At last, we examine the feasibility of detecting Megatesla-level magnetic fields. We choose XFEL beams to detect magnetic fields, based on the magnetic field inducing a polarization rotation via the Faraday effect. A setup of structured targets with a prefilled channel which mitigates the reduction of rotation caused by relativistic transparency is necessary to achieve rotations that exceed 0.1 mrad. A study on laser focusing configurations suggests there is flexibility regarding laser intensities.

The Direct Fusion Drive (DFD) is an advanced deep-space propulsion technology that utilises a field-reversed configuration (FRC) plasma core based on the PFRC-II laboratory test-bench located at the Princeton Plasma Physics Laboratory. A rigid-body model is derived for the translational kinematics of the FRC plasma and a numerical solver is developed in MATLAB to aid controller design and analysis for the FRC plasma centroid position. Two types of coil actuators are considered: superconducting flux-conservers and constant-current electromagnets. For constant-current coil actuators, the open-loop response of the FRC plasma centroid position to radial perturbations is shown to exhibit standing radial oscillations about the origin at constant frequency (closed trajectories neither decaying nor growing), while for axial perturbations, the FRC plasma position is shown to be axially unstable (driven away from the origin axially). Results for flux-conservers are inconclusive, requiring further work. A well centred FRC plasma is critical for interpretation of diagnostics and analysis of experimental data, the rigid-body model and numerical solver devised are attempted first steps toward this goal.

This dissertation investigates direct laser acceleration (DLA) of electrons in ultra-intense laser-plasma interactions, emphasizing how plasma-generated fields, laser frequency modulation, and radiation friction influence electron dynamics and radiation output. In the DLA regime, a laser propagating through underdense plasma can transfer energy efficiently to electrons, especially when assisted by quasi-static plasma fields. These fields induce betatron oscillations that confine electrons transversely while enabling resonant interactions with the laser field. A key mechanism in DLA is betatron resonance, which occurs when the betatron oscillation frequency matches the average frequency of the laser field experienced by the electron, resulting in net energy gain.

The first part of this work uncovers a new DLA regime driven by frequency modulation of the laser field as perceived by the oscillating electron. This modulation enables net energy gain through a third-order resonance—a specific higher-order resonance where the laser completes three oscillations per betatron cycle. In the absence of modulation, energy gain and loss cancel out; with modulation, the oscillation slows near the electron’s turning points, allowing net gain. Additionally, the study shows that superluminal laser phase velocities enhance higher-order resonances by introducing a global minimum in the frequency ratio between the laser and betatron oscillations. This effect suppresses detuning and sustains resonance over a broader energy range, providing a more robust pathway for energy transfer in relativistic DLA.

The second part of the dissertation shifts focus to x-ray emission and radiation reaction effects. Using particle-in-cell simulations, a backward x-ray emission mechanism is identified in which laser-accelerated electrons are turned around by the plasma field at the density down-ramp and re-collide with the exiting laser pulse, emitting hard x-rays. The resulting backward-directed photon source is more collimated and compact than its forward counterpart, with comparable or higher conversion efficiency. Finally, a test-particle model incorporating radiation friction reveals an attractor effect driven by plasma-induced superluminosity, where electrons converge to similar energy and radiation outputs despite varying initial conditions. These findings provide new insight into the interplay between laser dynamics, plasma fields, and radiation processes—advancing the design of compact, high-brightness electron and photon sources for future high-intensity laser facilities.

Recent advances in magnetic field generation have made experimentally accessible new regimes in magnetized high energy density physics. However, relatively little is currently known about the effects of kilotesla-level magnetic fields in relativistic laser-produced plasma. In this dissertation, I discuss a number of common plasma configurations, including solid targets and laser propagation in underdense plasma, in which applied magnetic fields can fundamentally alter plasma dynamics and impact important observables, such as ion and electron energy. In this regime, the changes induced by the magnetic field proceed mostly from the magnetization of hot electrons. Crucially, clear effects can be produced even when the magnetic field is not initially strong enough to magnetize electrons, suggesting that experimentally available applied magnetic fields may now be capable of delivering improvements for applications of relativistic short pulse laser-plasma interaction.