Signature change in spherical vacuum spacetimes

This thesis follows the approach of papers (1, 2) by exploring signature changes in other metrics. The metrics we chose to investigate are the Schwarzschild metric and the Tolman metric. The Schwarzschild metric was originally chosen in order to investigate whether the neighborhood of the singularity inside a black hole can be replaced with a Euclidean region, and also to see whether this Euclidean region can lead to new universes by providing "wormholes" through to other Lorentzian universes. By this we mean that, if one follows "time-like" geodesic paths from a Lorentzian region into a Euclidean region, they bounce (instead of hitting a singularity) and can then pass through a second signature change into another Lorentzian region. Consideration of how geodesics pass through a signature change naturally leads to the Tolman metric, whose vacuum cases cover the Schwarzschild/Kruskal-Szekeres manifold with all possible sets of radial geodesic coordinates. We take the opportunity to explore several cases of signature change in other Tolman models

X-Ray Generation from Metal Targets Coated with Wavelength-Scale Spheres

X-ray yield measurements from targets coated with wavelength-scale spheres are compared with measurements from polished targets. Evidence for a hotter resonant electron temperature due to field enhancements from Mie resonances in the spheres is investigated

Hot Electron and X-ray Production from Intense Laser Irradiation of Wavelength-Scale Polystyrene Spheres

Hot electron and x-ray production from solid targets coated with polystyrene-spheres which are irradiated with high-contrast, 100 fs, 400 nm light pulses at intensity up to 2×1017 W/cm2 have been studied. The peak hard x-ray signal from uncoated fused silica targets is an order of magnitude smaller than the signal from targets coated with submicron sized spheres. The temperature of the x-rays in the case of sphere-coated targets is twice as hot as that of uncoated glass. A sphere-size scan of the x-ray yield and observation of a peak in both the x-ray production and temperature at a sphere diameter of 0.26 μm, indicate that these results are consistent with Mie enhancements of the laser field at the sphere surface and multipass stochastic heating of the hot electrons in the oscillating laser field. These results also match well with particle-in-cell simulations of the interaction

Control of Strong-Laser-Field Coupling to Electrons in Solid Targets with Wavelength-Scale Spheres

Irradiation of a planar solid by an intense laser pulse leads to fast electron acceleration and hard x-ray production. We have investigated whether this high field production of fast electrons can be controlled by introducing dielectric spheres of well-defined size on the target surface. We find that the presence of spheres with a diameter slightly larger than half the laser wavelength leads to Mie enhancements of the laser field which, accompanied by multipass stochastic heating of the electrons, leads to significantly enhanced hard x-ray yield and temperature

K-shell Spectroscopy of Plasmas Created by Intense Laser Irradiation of Micron-scale Pyramid and Sphere Targets

K-shell spectra of targets with microstructured features irradiated by an intense femtosecond laser have been studied. Examination of Kα emission from laser irradiated Si targets coated with micron-scale polystyrene spheres indicates that the emission is enhanced by a factor of ∼3 over emission from planar solids. Sphere-coated targets also emit K-shell He-like Si radiation indicating the presence of a hot dense plasma beneath the microspheres. Furthermore, Kα from Ti foils coupled to micro-tipped reentrant pyramid and wedge shaped targets has been studied, however, no significant enhancement of the Kα yield is observed for these kinds of targets. These studies illustrate that, with correct tailoring of the target surface, field enhancements can be used to increase X-ray emission from intensely irradiated targets