Search

A 3.4 ps duration, high-repetition rate laser is used to create an aluminium ablation plasma at intensities between 1015W/cm2and 1017W/cm2. K-shell emission is observed from the target surface and several 100 μm in to the plume. High resolution, spatial resolved measurements of the He$\beta $(1s3p– 1$s^{2})$, Heγ(1s4p– 1$s^{2})$, He$\delta $(1s5p– 1$s^{2})$and He$\varepsilon $(1s6p– 1$s^{2})$transitions are obtained using a highly dispersive toroidal crystal spectrometer coupled to a large area charged-coupled device. This data shows unusual intensity modulations on He$\beta $transitions. These oscillations are observed to 50 μm above the target and are found to be reproducible, and the intensity of the modulations is laser irradiance dependent.

The emission spectrum from a plasma is often used to extract information on the hydrodynamic, radiative and atomic processes which occur. As these processes are coupled, detailed interpretation of spectroscopic data is often complex and uncertain. We present an experiment and modelling study of a `simple' hydrogen and helium-like plasma. The target and laser geometry were designed to minimise spatial gradients transverse to the plasma flow. The plasma was spatially and temporally diagnosed using Thomson scattering, X-ray imaging and spectroscopic measurements. The combination of the optical and X-ray results constrains the numerical modelling to provide a more reliable plasma diagnostic. This is essential for the interpretation of high-resolution measurements and of more complex, non 1-dimensional plasmas.

We review the potential of x-ray scattering as a dense plasma diagnostic and present data taken from experiments in which x-ray scattering from dense plasmas is developed as a diagnostic tool. In one type of experiment the scattered photons are detected as a function of angle using direct detection onto a CCD chip. Such experiments are designed primarily to observe the static ion–ion structure factor, which is expected to dominate the scattering for moderate to high Z plasmas at a few electronvolts temperature. In a second type of experiment we have used a curved crystal to observe spectrally resolved x-ray scattering at a fixed angle. This experiment was designed to observe the dynamical structure factor of the plasma.

Laboratory astroplasma physics experiments advance both our astrophysics and plasma physics knowledge. Contemporary high-energy, high-power laser technology enables us to reproduce in the laboratory the conditions of temperature and pressure that are met in extreme stellar environments. The focus is on experiments designed to address key aspects of the plasma physics occurring in supernova remnants. In this approach, a plasma physics model of the astrophysical object is identified and then scaled, and applied to a laboratory experiment. This offers the possibility of detailed measurements, which can be repeated as the input conditions are altered. Results from a scaled experiment designed to address aspects of collisionless plasma interaction in a young supernova remnant are presented. This experimental study is based on the interaction of two millimetre-scale counter-streaming laser-produced plasmas, created from exploded thin plastic foils in an intense transverse magnetic field. The dynamics of the two plasmas and their interaction are studied with, and without, magnetic fields, through spatially and temporally resolved measurements of the electron density.

AbstractWe report results and modelling of an experiment performed at the Target Area West Vulcan laser facility, aimed at investigating laser–plasma interaction in conditions that are of interest for the shock ignition scheme in inertial confinement fusion (ICF), that is, laser intensity higher than ${10}^{16}$$\mathrm{W}/{\mathrm{cm}}^2$impinging on a hot ($T>1$keV), inhomogeneous and long scalelength pre-formed plasma. Measurements show a significant stimulated Raman scattering (SRS) backscattering ($\sim 4\%{-}20\%$of laser energy) driven at low plasma densities and no signatures of two-plasmon decay (TPD)/SRS driven at the quarter critical density region. Results are satisfactorily reproduced by an analytical model accounting for the convective SRS growth in independent laser speckles, in conditions where the reflectivity is dominated by the contribution from the most intense speckles, where SRS becomes saturated. Analytical and kinetic simulations well reproduce the onset of SRS at low plasma densities in a regime strongly affected by non-linear Landau damping and by filamentation of the most intense laser speckles. The absence of TPD/SRS at higher densities is explained by pump depletion and plasma smoothing driven by filamentation. The prevalence of laser coupling in the low-density profile justifies the low temperature measured for hot electrons ($7\!{-}\!12$keV), which is well reproduced by numerical simulations.

Solvent-refined lignite (SRL) can be produced by treating lignite (not dried) with CO-H2, donor solvent and high temperature. This reactive black solid softens at about 150°C, is soluble in many organic solvents, is very low in ash and sulfur, and appears to be a good feedstock for further upgrading. Thus a wide-ranging study was undertaken to determine the best reducing conditions for converting SRL to light distillable liquid fuels and/or chemical feedstocks. Batch autoclave studies were carried out in the temperature range of 375-450°C, hydrogen pressure range of 1500-4500 psi, with catalysts Ni-Mo-Al2O3, Co-Mo-Al2O3, Ni-W-Al2O3, Ni-W-SiO2-Al2O3, SiO2-Al2O3, Al2 O3, SnCl2, and presulfided catalysts Ni-Mo-Al2O3, Co-Mo-Al2O3, NiW-Al2O3.Varying amounts of the solvents tetrahydrofuran, tetralin, nap-thalene, and FS-120 petroleum fraction were also studied. Reductions without any solvent were studied too and were quite successful. The results were evaluated in terms of the amount of light liquids produced [Table 3 distillation fractions (1) initial boiling point to 89°C/1 torr, (2) 89-139°C, (3) 139-200°C, (4) 200-260°C, (5) above 260°C], deoxygenation, denitrification, hydrogen-carbon ratios, aromatic-aliphatic hydrogen ratios, and benzene solubility of unconverted material. Best results were obtained with a presulfided Ni-Mo-Al2O3 catalyst at 450°C, operating pressure of about 3500 psi with a 1:1 SRL-tetralin solvent ratio [90% overall conversion, ∼ 20% light liquid (1), 15% light oil (2), 20% heavy oil (3 and 4), 10% unconverted].

A developing application of laser-driven currents is the generation of magnetic fields of picosecond–nanosecond duration with magnitudes exceeding $B=10~\text{T}$. Single-loop and helical coil targets can direct laser-driven discharge currents along wires to generate spatially uniform, quasi-static magnetic fields on the millimetre scale. Here, we present proton deflectometry across two axes of a single-loop coil ranging from 1 to 2 mm in diameter. Comparison with proton tracking simulations shows that measured magnetic fields are the result of kiloampere currents in the coil and electric charges distributed around the coil target. Using this dual-axis platform for proton deflectometry, robust measurements can be made of the evolution of magnetic fields in a capacitor coil target.

The emission spectrum from a plasma is often used to extract information on the hydrodynamic, radiative and atomic processes which occur. As these processes are coupled, detailed interpretation of spectroscopic data is often complex and uncertain. We present an experiment and modelling study of a `simple' hydrogen and helium-like plasma. The target and laser geometry were designed to minimise spatial gradients transverse to the plasma flow. The plasma was spatially and temporally diagnosed using Thomson scattering, X-ray imaging and spectroscopic measurements. The combination of the optical and X-ray results constrains the numerical modelling to provide a more reliable plasma diagnostic. This is essential for the interpretation of high-resolution measurements and of more complex, non 1-dimensional plasmas.

A 3.4 ps duration, high-repetition rate laser is used to create an aluminium ablation plasma at intensities between 1015?W/cm2 and 1017?W/cm2. K-shell emission is observed from the target surface and several 100??m in to the plume. High resolution, spatial resolved measurements of the He? (1s3p ? 1s2), He? (1s4p ? 1s2), He? (1s5p ? 1s2) and He? (1s6p ? 1s2) transitions are obtained using a highly dispersive toroidal crystal spectrometer coupled to a large area charged-coupled device. This data shows unusual intensity modulations on He? transitions. These oscillations are observed to 50 ?m above the target and are found to be reproducible, and the intensity of the modulations is laser irradiance dependent.

A multichannel calorimeter system is designed and constructed which is capable of delivering single-shot and broad-band spectral measurement of terahertz (THz) radiation generated in intense laser–plasma interactions. The generation mechanism of backward THz radiation (BTR) is studied by using the multichannel calorimeter system in an intense picosecond laser–solid interaction experiment. The dependence of the BTR energy and spectrum on laser energy, target thickness and pre-plasma scale length is obtained. These results indicate that coherent transition radiation is responsible for the low-frequency component (${<}$1 THz) of BTR. It is also observed that a large-scale pre-plasma primarily enhances the high-frequency component (${>}$3 THz) of BTR.

The spatial-intensity profile of light reflected during the interaction of an intense laser pulse with a microstructured target is investigated experimentally and the potential to apply this as a diagnostic of the interaction physics is explored numerically. Diffraction and speckle patterns are measured in the specularly reflected light in the cases of targets with regular groove and needle-like structures, respectively, highlighting the potential to use this as a diagnostic of the evolving plasma surface. It is shown, via ray-tracing and numerical modelling, that for a laser focal spot diameter smaller than the periodicity of the target structure, the reflected light patterns can potentially be used to diagnose the degree of plasma expansion, and by extension the local plasma temperature, at the focus of the intense laser light. The reflected patterns could also be used to diagnose the size of the laser focal spot during a high-intensity interaction when using a regular structure with known spacing.

The reaction pathways of the electrochemical and alkali‐metal reductions of the phenoxynaphthalenes (I) and (II) are investigated.

The radical anion (II) decomposes with a first order rate constant of 4·105/s to give (III) and (IV) quantitatively with current efficiency considerably less than 100%.