Dr Ramy Aboushelbaya

Abstract:

Accurate characterization of laser pulses used in experiments is a crucial step to the analysis of their results. In this paper, a novel single-shot frequency-resolved optical gating (FROG) device is described, one that incorporates a dispersive element which allows it to fully characterize pulses up to 25 ps in duration with a 65 fs per pixel temporal resolution. A newly developed phase retrieval routine based on memetic algorithms is implemented and shown to circumvent the stagnation problem that often occurs with traditional FROG analysis programs when they encounter a local minimum.

Abstract:

The plasma Raman instability can efficiently compress a nanosecond long high power laser pulse to sub-picosecond duration. Although many authors envisaged a converging beam geometry for Raman amplification, here we propose the exact opposite geometry; the amplification should start at the intense focus of the seed. We generalise the coupled laser envelope equations to include this non-collimated case. The new geometry completely eradicates the usual trailing secondary peaks of the output pulse, which typically lower the efficiency by half. It also reduces, by orders of magnitude, the initial seed pulse energy required for efficient operation. As in the collimated case, the evolution is self-similar, although the temporal pulse envelope is different. A two-dimensional particle-in-cell simulation demonstrates efficient amplification of a diverging seed with only 0:3mJ energy. The pulse has no secondary peaks and almost constant intensity as it amplifies and diverges.

Abstract:

Raman back-scatter from an under-dense plasma can be used to compress laser pulses, as shown by several previous experiments in the optical regime. A short seed pulse counter-propagates with a longer pump pulse and energy is transferred to the shorter pulse via stimulated Raman scattering. The robustness of the scheme to non-ideal plasma density conditions is demonstrated through particle-in-cell simulations. The scale invariance of the scheme ensures that compression of XUV pulses from a free electron laser is also possible, as demonstrated by further simulations. The output is as short as 300 as, with energy typical of fourth generation sources.

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.