Investigating Energy Transport in High Density Plasmas Using Buried Layer Targets

The work presented in this thesis investigates energy transport in laser irradiated solid targets containing a diagnostic buried iron layer. Energy transport in laser-plasmas is important to inertial confinement fusion and other applications, for example laser ablation, particle acceleration and x-ray production.
The steep temperature and density gradients between the critical density (maximum penetration density for the laser) and ablation surface, plus the role of fast electron and radiation make energy transport in laser-plasmas a complex, non-linear issue. Laser energy can be transported into a solid target by thermal conduction, hot electron heating and radiation transport. To understand the in- terplay between these non-linear heating processes it is important to accurately characterise plasma conditions as the energy transport occurs. An experiment conducted at the Lawrence Livermore National Laboratory, USA irradiated buried iron layer targets using a 2 ps, 1017 Wcm−2 laser with the subsequent L-shell iron emission recorded using a high resolution (resolving power ≃ 500) grating spectrometer. The HYADES 1D hydrodynamic fluid code and the PrismSPECT collisional-radiative code were used to simulate the plasma conditions and the L-shell iron emission. A comparison between the simulated spectra and experi- mentally recorded L-shell emission suggests that the iron layer is heated instanta- neously by hot electrons and radiation transport and that this modifies thermal electron conduction. The thermal flux limiter and laser energy-hot electron conversion efficiency have been determined by comparing experimentally recorded L-shell emission to simulated synthetic spectra. As the iron layer expands and cools, the population of lower ionisation states increases. A novel technique has been developed to characterise the electron temperature and density from L-shell emission spectra using the Saha-Boltzmann equation and multiple line ratios of adjacent ionisation states.
An experiment at the LASERIX facility, France used an extreme ultraviolet (EUV) laser as a back-lighter, to probe high density laser irradiated buried iron layer targets. The transmission through the iron layer was simulated using TOPS, PROPACEOS, IMP and HYADES opacity models. This investigation has found that higher opacities are required for plasmas at 20 eV and 0.3 gcm−3 in order to account for the drop in transmission at 20 ps after laser irradiation. Radiation transport dominates the heating of the buried iron layer when irradiated by a well defined prepulse. The expanding coronal preplasma efficiently produces hot electrons, however because of the larger stopping distance associated with ’superthermal’ electrons, the heating due to hot electrons is negligible compared to the radiation heating effect.