Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We present new experiments to study the formation of radiative shocks and the interaction between two counterpropagating radiative shocks. The experiments are performed at the Orion laser facility, which is used to drive shocks in xenon inside large aspect ratio gas cells. The collision between the two shocks and their respective radiative precursors, combined with the formation of inherently three-dimensional shocks, provides a novel platform particularly suited for the benchmarking of numerical codes. The dynamics of the shocks before and after the collision are investigated using point-projection x-ray backlighting while, simultaneously, the electron density in the radiative precursor was measured via optical laser interferometry. Modeling of the experiments using the 2D radiation hydrodynamic codes nym and petra shows very good agreement with the experimental results.

Abstract:

We demonstrate experimentally the resonant excitation of plasma waves by trains of laser pulses. We also take an important first step to achieving an energy recovery plasma accelerator by showing that a plasma wave can be damped by an out-of-resonance trailing laser pulse. The measured laser wakefields are found to be in excellent agreement with analytical and numerical models of wakefield excitation in the linear regime. Our results indicate a promising direction for achieving highly controlled, GeV-scale laser-plasma accelerators operating at multikilohertz repetition rates.

Abstract:

We report experiments in which powder-diffraction data were recorded from polycrystalline vanadium foils, shock-compressed to pressures in the range 10 – 70 GPa. Anisotropic strain in the compressed material is inferred from the asymmetry of Debye-Scherrer diffraction images, and used to infer residual strain and yield strength (residual von Mises stress) of the vanadium sample material. We find residual anisotropic strain corresponding to yield strength in the range 1.2 GPa – 1.8 GPa for shock pressures below 30 GPa, but significantly less anisotropy of strain in the range of shock pressures above this. This is in contrast to our simulations of the experimental data using a multi-scale crystal plasticity strength model, where significant yield strength persists up to the highest pressures we access in the experiment. Possible mechanisms that could contribute to the dynamic response of vanadium that we observe for shock pressures ≥ 30 GPa are discussed.

Abstract:

Particle accelerators have made an enormous impact in all fields of natural sciences, from elementary particle physics, to the imaging of proteins and the development of new pharmaceuticals. Modern light sources have advanced many fields by providing extraordinarily bright, short X-ray pulses. Here we present a novel numerical study, demonstrating that existing third generation light sources can significantly enhance the brightness and photon energy of their X-ray pulses by undulating their beams within plasma wakefields. This study shows that a three order of magnitude increase in X-ray brightness and over an order of magnitude increase in X-ray photon energy is achieved by passing a 3 GeV electron beam through a two-stage plasma insertion device. The production mechanism micro-bunches the electron beam and ensures the pulses are radially polarised on creation. We also demonstrate that the micro-bunched electron beam is itself an effective wakefield driver that can potentially accelerate a witness electron beam up to 6 GeV.