51ÁÔÆæÈë¿Ú

Abstract:

This paper presents the goals and some of the results of experiments conducted within the Working Package 10 (Fusion Experimental Programme) of the HiPER Project. These experiments concern the study of the physics connected to "Advanced Ignition Schemes", i.e. the Fast Ignition and the Shock Ignition Approaches to Inertial Fusion. Such schemes are aimed at achieving a higher gain, as compared to the classical approach which is used in NIF, as required for future reactors, and making fusion possible with smaller facilities. In particular, a series of experiments related to Fast Ignition were performed at the RAL (UK) and LULI (France) Laboratories and were addressed to study the propagation of fast electrons (created by a short-pulse ultra-high-intensity beam) in compressed matter, created either by cylindrical implosions or by compression of planar targets by (planar) laser-driven shock waves. A more recent experiment was performed at PALS and investigated the laser-plasma coupling in the 1016 W/cm2 intensity regime of interest for Shock Ignition. © 2011 SPIE.

Abstract:

In the past few years, the development of light sources of the 4^th generation, namely XUV/X-ray Free Electron Lasers provides to the scientific community outstanding tools to investigate matter under extreme conditions never obtained in laboratories so far. As theory is at its infancy, the analysis of matter via the self-emission of the target is of central importance. The characterization of such dense matter is possible if photons can escape the medium. As the absorption of K-shell X-ray transitions is minimal, it plays a key role in this study. We report here the first successful observation of K-shell emission of Nitrogen at 430 eV using an XUV-Free Electron Laser to irradiate solid Boron Nitride targets under exceptional conditions: photon energy of 92 eV, pulse duration of ∼20 fs, micro focusing leading to intensities larger than 10¹⁶ W/cm². Using a Bragg crystal of THM coupled to a CCD, we resolved K-shell line emission from different charge states. We demonstrate that the spectroscopic data allow characterization of electron heating processes when X-ray radiation is interacting with solid matter. As energy transport is non-trivial because the light source is monochromatic, these results have an important impact on the theory. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

Contemporary high-power laser systems make use of solid-state laser technology to reach petawatt pulse powers. The breakdown threshold for optical components in these systems, however, demands metre-scale beams. Raman amplification of laser beams promises a breakthrough by the use of much smaller amplifying media, that is, millimetre-diameter plasmas, but so far only 60 GW peak powers have been obtained in the laboratory, far short of the desired multipetawatt regime. Here we show, through the first large-scale multidimensional particle-in-cell simulations of this process, that multipetawatt peak powers can be reached, but only in a narrow parameter window dictated by the growth of plasma instabilities. Raman amplification promises reduced cost and complexity of intense lasers, enabling much greater access to higher-intensity regimes for scientific and industrial applications. Furthermore, we show that this process scales to short wavelengths, enabling compression of X-ray free-electron laser pulses to attosecond duration. © 2011 Macmillan Publishers Limited. All rights reserved.

Abstract:

We show that the hydrodynamic description can be applied to modeling the ionic response in dense plasmas for a wide range of length scales that are experimentally accessible. Using numerical simulations for the Yukawa model, we find that the maximum wave number k(max) at which the hydrodynamic description applies is independent of the coupling strength, given by k(max)λ(s)≃0.43, where λ(s) is the ionic screening length. Our results show that the hydrodynamic description can be used for interpreting x-ray scattering data from fourth generation light sources and high power lasers. In addition, our investigation sheds new light on how the domain of validity of the hydrodynamic description depends on both the microscopic properties and the thermodynamic state of fluids in general.