Laser fusion and extreme field physics

Abstract:

Laser peak powers rise inexorably higher, enabling the study of increasingly exotic high-energy-density plasmas. This thesis explores one such phenomenon, that of the interaction between a relativistically intense laser pulse and a solid-density plasma. The laser pulse is reflected. Both the reflected radiation and the electron bunches that induce the interaction have fascinating properties. Through the application of theory, simulation and experiment, this thesis strives to extend our understanding of this mechanism and thus direct the community towards potential applications for these sources. Of primary interest is the development of novel diagnostic tools. Theories have been developed and tested to describe the production of low emittance nano-Coulomb charge electron bunches. Such properties are comparable to forefront synchrotron sources but on a considerably more compact scale. These results have wide-reaching implications for future particle accelerator science and associated technologies. Furthermore, these electron bunches will initiate QED processes on next-generation laser facilities. The radiation they produce is composed of high harmonics of the incident laser pulse. This radiation can be coherently focused to unprecedented intensities and is of ultra-short duration, possibly even entering the zeptosecond regime. The intensity of X-ray harmonics has been measured on the ORION laser facility producing results consistent with theory and enabling the benchmarking of peak intensity simulations with real data. The work of this thesis has amassed interest within the community and in June 2024 its ideas will be tested on the GEMINI PW laser facility.

Abstract:

Inertial confinement fusion fuel suffers increased X-ray radiation losses when carbon from the capsule ablator mixes into the hot-spot. Here we present one and two-dimensional ion VlasovFokker-Planck simulations that resolve hot-spot self heating in the presence a localised spike of carbon mix, totalling 1.9 % of the hot-spot mass. The mix region cools and contracts over tens of picoseconds, increasing its alpha particle stopping power and radiative losses. This makes a localised mix region more severe than an equal amount of uniformly distributed mix. There is also a purely kinetic effect that reduces fusion reactivity by several percent, since faster ions in the tail of the distribution are absorbed by the mix region. Radiative cooling and contraction of the spike induces fluid motion, causing neutron spectrum broadening. This artificially increases the inferred experimental ion temperatures and gives line of sight variations.

Abstract:

The orbital angular momentum (OAM) of light is one of the most intriguing properties of electromagnetic radiation. Although OAM is more commonly associated with the mechanical movement of massive particles, researchers have shown that, under certain conditions, laser beams can carry it. This is not merely a theoretical proposition, this idea was almost immediately experimentally proven by showing that OAM can be transferred between light and matter. This has, in turn, spurred an ever-increasing interest in leveraging the interesting proprieties of the OAM of light for various technological applications. This work focuses on the effect that OAM has on high-intensity laser interactions.

High-intensity lasers have been a boon to scientific investigations in their own right. They have allowed us to experimentally research astrophysical phenomena inside of laboratories, opened the possibility to tabletop particle accelerators and gotten us closer to useful fusion energy sources. More recently, we have been able to reach extreme intensities that allow us to probe the most fundamental interactions in the universe. Predictions that had been theorized decades ago by the pioneers of the quantum theory of matter are now close to being experimentally verifiable.

In the coming chapters, I look at the fundamental nature of the OAM of light and the many discussions it has spurred. I then show that it modifies an interaction known as vacuum photon-photon scattering where beams of light can interact with each other in the absence of any mediating matter violating the constraints established by the classical theory of electromagnetism. OAM provides an extra signal that makes this light-light interaction more identifiable in an experiment. On a more practical note, I continue to look specifically at high-intensity lasers and how they can be manipulated to produce high-intensity OAM-carrying beams and how said beams can be characterized.