Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Calculations of the patterns of x-ray diffraction from shocked crystals derived from the results of non-equilibrium molecular dynamics (NEMD) simulations are presented. The atomic coordinates predicted from the NEMD simulations combined with atomic form factors are used to generate a discrete distribution of electron density. A fast Fourier transform (FFT) of this distribution provides an image of the crystal in reciprocal space, which can be further processed to produce quantitative simulated data for direct comparison with experiments that employ picosecond x-ray diffraction from laser-irradiated crystalline targets. © 2008 IOP Publishing Ltd.

Abstract:

We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at 13.5 nm wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of 2× 1014/cm2. The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about 38 eV, supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy. © 2008 The American Physical Society.

Abstract:

The study of the basic physical processes underlying the generation of fast electrons during the interaction of high-intensity short laser pulses with solid materials and the transport of these fast electrons through the target material are of great importance for the fast ignition concept for inertial confinement fusion and for the development of ultra-short X-ray sources. We report on the experimental investigation of fast electron transport phenomena by means of the spatial and spectral characterization of the X-ray emission from layered targets using bent crystal spectrometers and a new diagnostic technique based on a pinhole-camera equipped with a CCD detector working in single-photon regime for multi-spectral X-ray imaging [1]. In particular, differences of fast electron transport features depending on the atomic number and/or the resistivity of the target material have been studied. The experiments were carried out at relativistic laser intensities, both in the longer (≃ps) pulse interaction regime relevant for fast ignition studies [2] and in the short-pulse (≃100 fs) interaction conditions related to basic physics studies as well as to the development of ultrashort Kα X-ray sources.

Abstract:

A comprehensive scientific program is being pursued at LLE to explore the physics of fast ignition. The OMEGA EP Laser was completed in April 2008, adjacent to the 60 beam, 30 kJ OMEGA Laser Facility. OMEGA EP consists of four beamlines with a NIF-like architecture, each delivering up to 6.5 kJ of UV laser energy in long pulse (ns) mode into the OMEGA EP target chamber. Two of the beamlines can operate as high-energy petawatt lasers, with up to 2.6 kJ each with 10 ps pulse duration. These beams can either be injected into the OMEGA EP target chamber or combined collinearly into the existing OMEGA target chamber for integrated fast-ignitor experiments. Fuel-assembly experiments on OMEGA have achieved high fuel areal densities, and the effects of a cone on the fuel assembly are being studied. Experiments on short-pulse laser systems in collaboration with other institutions are being pursued to investigate the conversion efficiency from laser energy to fast electrons. A coherent transition radiation diagnostic to study the transport of the electrons in high-density material is being developed. Integrated experiments with room-temperature targets on OMEGA will be performed in 2008. Simulations of these integrated experiments show significant heating of up to 1 keV due to the hot electrons from the short-pulse laser. © 2008 IOP Publishing Ltd.