Search

The propagation of a shock wave in solids can stress them to ultra-high pressures of millions of atmospheres. Understanding the behavior of matter at these extreme pressures is essential to describe a wide range of physical phenomena, including the formation of planets, young stars and cores of super-Earths, as well as the behavior of advanced ceramic materials subjected to such stresses. Under megabar (Mbar) pressure, even a solid with high strength exhibits plastic properties, causing the shock wave to split in two. This phenomenon is described by theoretical models, but without direct experimental measurements to confirm them, their validity is still in doubt. Here, we present the results of an experiment in which the evolution of the coupled elastic-plastic wave structure in diamond was directly observed and studied with submicron spatial resolution, using the unique capabilities of the X-ray free-electron laser. The direct measurements allowed, for the first time, the fitting and validation of a strength model for diamond in the range of several Mbar by performing continuum mechanics simulations in 2D geometry. The presented experimental approach to the study of shock waves in solids opens up new possibilities for the direct verification and construction of the equations of state of matter in the ultra-high pressure range, which are relevant for the solution of a variety of problems in high energy density physics., Comment: 14 pages, 15 figures, submitted to Nature

A new diagnosis method for high energy ions utilizing a single CR-39 detector mounted on plastic plates is demonstrated to identify the presence of the high energy component beyond the CR-39's detection threshold limit. On irradiation of the CR-39 detector unit with a 25 MeV per nucleon He ion beam from conventional rf-accelerators, a large number of etch pits having elliptical opening shapes are observed on the rear surface of the CR-39. Detailed investigations reveal that these etch pits are created by heavy ions inelastically backscattered from the plastic plates. This ion detection method is applied to laser-driven ion acceleration experiments using cluster-gas targets, and ion signals with energies up to 50 MeV per nucleon are identified.

We demonstrate generation of 10-20 MeV/u ions with a compact 4 TW laser using a gas target mixed with submicron clusters, corresponding to tenfold increase in the ion energies compared to previous experiments with solid targets. It is inferred that the high energy ions are generated due to formation of a strong dipole vortex structure. The demonstrated method has a potential to construct compact and high repetition rate ion sources for hadron therapy and other applications., Comment: 7 pages, 4 figures

A new simple method to get soft X-ray micro-radiographs of biological material with a spatial resolution reaching 50 nm is described. A thin film of lithium fluoride (LiF) works as image detector, storing the micro-radiograph obtained exposing biological material to Extreme Ultra-Violet and soft X-ray radiations. Scanning near-field optical microscope (SNOM) is used to read the micro-radiograph at nanometric resolution up to L/12, collecting the fluorescence (L = 650 nm) excited by an Ar+ laser light (L = 458 nm)

Nowadays the possibility to obtain micro-radiographs is limited in resolution by the detection properties of the used technique. Here we show a new method to create and observe micro-radiographs. A thin film of Lithium Fluoride works as image detector, storing the micro-radiograph obtained exposing biological material to Extreme Ultra-Violet and soft X-ray radiations. To read the stored image, collecting the optically stimulated luminescence of the active color centers produced by the X-rays, a Scanning Near-Field Optical Microscope (SNOM) is used, for its peculiarity to overcome the diffraction limit, showing details smaller than the wavelength of the collected light. The samples were illuminated with 458 nm laser light and an uncoated tapered optical fiber was collecting the fluorescent signal emitted by the Lithium Fluoride color centers. The SNOM images of these micro-radiographs stored in Lithium Fluoride films showed an optical resolution of 50 nm, corresponding to lambda/12, where lambda is the wavelength of the collected light.

The study of fast electrons generated in laser-matter interactions and their transport through matter is of great interest for the fast ignition concept in the inertial confinement fusion. A typical experimental approach is the interaction of intense laser radiation with foil targets. The transport of the electrons through the foil can be studied with several techniques including X-ray emission measurements and direct measurements on electrons leaving the foil. Some laser-matter interaction features can be monitored using spectroscopy in the visible range. Here we will focus our attention on some of the experimental techniques that have been used in laser-foil experiments at ILIL laboratory in Pisa and in a VULCAN Petawatt experiment at RAL. In particular we will describe high resolution X-ray spectroscopy with a bent crystal, K-alpha flux measurements, direct measurements of the angular distribution and spectrum of the electrons, and spectroscopy of the 3/2ω and 2ω harmonics of the laser radiation.

LiF films have been grown on silicon substrate, irradiated with soft x-rays to create fluorescent regular micrometric-spaced arrays based on colour centres, and studied by Scanning Near-field Optical Microscope (SNOM). Strong variations in the local reflectivity have been observed in the infrared region between 6.1 and 9.2 mum and tentatively ascribed to a modulated variation of the refractive index of the coloured zone with respect to that of the uncoloured LiF matrix. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.