Professor Justin Wark

Abstract:

We present a detailed analysis of K-shell emission from laser-produced rapidly-expanding Al plasmas. This work forms part of a series of experiments performed at the Vulcan laser facility of the Rutherford Appleton Laboratory, UK. 1-D planar expansion was obtained by over-illuminating A1-microdot targets supported on CH plastic foils. The small size of the A1-plasma ensured high spatial and frequency resolution of the spectra, obtained with a single crystal spectrometer, two vertical dispersion variant double crystal spectrometers, and a vertical dispersion variant Johann Spectrometer. The hydrodynamic properties of the plasma were measured independently by spatially and temporally resolved Thomson scattering, utilizing a 4ω probe beam. This enabled sub- and super-critical densities to be probed relative to the 1ω heater beams. The deduced plasma hydrodynamic conditions are compared with those generated from the 1-D hydro-code Medusa, and the significant differences found in the electron temperature discussed. Synthetic spectra generated from the detailed term collisional radiative non-LTE atomic physics code Fly are compared with the experimental spectra for the measured hydrodynamic parameters, and for those taken from Medusa. Excellent agreement is only found for both the H- and He-like A1 series when careful account is taken of the temporal evolution of the electron temperature. © 2001 Published by Elsevier Science Ltd.

Abstract:

The extension of the atomic kinetics code suite FLY to a multi-cell postprocessor for hydrodynamic plasma simulation codes, called SWARM is presented. First, the collisional radiative model on which FLY is based is reviewed as far as it corresponds to the model in SWARM, then more attention is paid to the differences. Multiple cell simulations require a more complicated algorithm to solve the radiative transfer problem, taking into account geometry and point of observation. The feedback of the radiation on the atomic state populations is taken into account in a more sophisticated escape factor model. Finally, the limitations of the model are discussed. © 2001 Elsevier Science Ltd. All rights reserved.

Abstract:

We present detailed simulations of the optical spectra emitted from an Argon plasma whose conditions correspond to those thought to prevail within sonoluminescing bubbles. The model incorporates detailed atomic physics based on atomic data from the Opacity Project database, and includes bound-bound, bound-free, and free-free transitions. Line broadening is treated by use of the modified semi-empirical method. The spectral model is used as a postprocessor of hydrodynamic simulations. We find good agreement with experimental spectra and accurately reproduce experimental pulse widths. We also predict that whilst the majority of the optical emission corresponds to bound-free transitions, there remains the possibility of observing line emission in both the UV and IR regions of the spectrum. © 2001 Elsevier Science Ltd. All rights reserved.

Abstract:

The increase in the XUV mass absorption coefficient of liquid aluminium, produced by high-power-laser shock-compression, is measured using XUV laser radiography. At a photon energy of 63 eV a change in the mass absorption coefficient by up to a factor of ∼2.2 is determined at densities close to twice that of solid and electron temperatures of the order of 1 eV. Comparison with hydrodynamic simulations indicate that the absorption coefficient scales with density as ρ1.3±0.2.

Abstract:

We present detailed simulations of the optical spectra emitted from an argon plasma whose conditions correspond to those thought to prevail within sonoluminescing bubbles. The model incorporates detailed atomic physics based on atomic data from the Opacity Project database, and includes bound-bound, bound-free and free-free transitions. Line broadening is treated using the modified semi-empirical method. The spectral model is used as a postprocessor of hydrodynamic simulations. While finding excellent agreement with the shape of experimental spectra, we calculate an intensity that is a factor of 100 greater than that in experiment. We also predict that whilst the majority of the optical emission corresponds to bound-free transitions, there remains the possibility of observing broad line emission in both the UV and IR regions of the spectrum.