Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We have measured the absolute response and detective quantum efficiency of image plates (IPs) for 5.9 keV x-rays using a calibrated iron-55 source. The types of IPs considered in this study are now commonly used as x-ray detectors in high-intensity laser-plasma interaction experiments, where conventional CCD fails because of the intense electromagnetic pulse that follows a high-intensity shot. Since the plates are not read out immediately after each laser shot, a detailed fading analysis of the plates is also presented. This work is important for future implementation of IPs as absolute x-ray photon detectors in large-scale laser facilities. © 2008 IOP Publishing Ltd.

Abstract:

Laser-driven, quasimonoenergetic electron beams of up to ∼200MeV in energy have been observed from steady-state-flow gas cells. These beams emitted within a low-divergence cone of 2.1±0.5mrad FWHM display unprecedented shot-to-shot stability in energy (2.5% rms), pointing (1.4 mrad rms), and charge (16% rms) owing to a highly reproducible gas-density profile within the interaction volume. Laser-wakefield acceleration in gas cells of this type provides a simple and reliable source of relativistic electrons suitable for applications such as the production of extreme-ultraviolet undulator radiation. © 2008 The American Physical Society.

Abstract:

Collective Thomson scattering with extreme ultraviolet light or x rays is shown to allow for a robust measurement of the free electron density in dense plasmas. Collective excitations like plasmons appear as maxima in the scattering signal. Their frequency position can directly be related to the free electron density. The range of applicability of the standard Gross-Bohm dispersion relation and of an improved dispersion relation in comparison to calculations based on the dielectric function in random phase approximation is investigated. More important, this well-established treatment of Thomson scattering on free electrons is generalized in the Born-Mermin approximation by including collisions. We show that, in the transition region from collective to noncollective scattering, the consideration of collisions is important. © 2008 The American Physical Society.

Abstract:

Significant reduction of inherent large divergence of the laser driven MeV proton beams is achieved by strong (of the order of 109V/m) electrostatic focussing field generated in the confined region of a suitably shaped structure attached to the proton generating foil. The scheme exploits the positively charging of the target following an intense laser interaction. Reduction in the proton beam divergence, and commensurate increase in proton flux is observed while preserving the beam laminarity. The underlying mechanism has been established by the help of particle tracing simulations. Dynamic focussing power of the lens, mainly due to the target discharging, can also be exploited in order to bring up the desired chromaticity of the lens for the proton beams of broad energy range. © 2008 American Institute of Physics.