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Short pulse laser heated buried layers experiments have been performed on the Orion laser facility to study the time-resolved emission characteristics of plasmas with mass densities ≥ 1 g/cc and electron temperatures > 500 eV. Our streak camera measurements focused on the K-shell emission lines of He-like and H-like aluminum from a buried aluminum layer. The data were analyzed by comparison to synthetic spectra generated with the non-local thermodynamic equilibrium (NLTE) radiation transfer code Cretin, which yielded maximum temperatures of nearly 800 eV at near solid density. The time precise history of the temperature evolution was reproduced with a 1-D radiation hydrodynamic code; however, the known effect of lateral transport of energy out of the focal spot made exacting agree with theory difficult. Thus, we have observed densities of ≥ 1 g/cc and temperatures of > 500 eV using the 1-D analysis, which supports the idea that the aluminum plasma is locally hotter than inferred from our spatially integrating measurements and that modeling requires the inclusion of 2-dimensional effects.

Ultra-high-intensity lasers have opened up a new avenue for the creation and detailed spectral measurements of dense plasmas in extreme thermodynamic conditions. In this paper, we demonstrate the possibility of heating a dense plasma (ρ > 1 gcm−3) to a maximum temperature of 560 ± 40 eV using a few-Joule, relativistic-intensity laser pulse. Particle-in-cell, radiation-hydrodynamic and atomic physics simulation tools are used together for a full description of the plasma dynamics, from laser interaction to late-time expansion and x-ray emission, yielding overall good agreement with the spectral measurements. We discuss the sensitivity of our analysis to space-time gradients, non-equilibrium ionization processes and hot electron effects.

K shell X-ray emission measurements have been used to diagnose plasma conditions in short-pulse heated buried microdot targets on the Orion high power laser. These experiments have been used to validate simulations of short pulse laser-solid interaction that combine hybrid PIC modelling of the laser absorption with radiation-hydrodynamics simulations including an electron transport model. Comparison of these simulations with streaked K shell spectroscopy show the importance of including radial gradients in fitting the spectra. An example is presented of the emission of sulphur from a 50 µm diameter microdot sample buried in a plastic foil. Previously agreement between simulation and experiment was obtained only by treating the absorbed energy, electron temperature and beam divergence as fitting parameters. The good agreement obtained in this work used the measured laser energy and laser pulse length and calculated the laser-solid target interaction from first principles.

The newly commissioned Orion laser system has been used to study dense plasmas created by a combination of short pulse laser heating and compression by laser driven shocks. Thus the plasma density was systematically varied between 1 and 10 g/cc by using aluminium samples buried in plastic foils or diamond sheets. The aluminium was heated to electron temperatures between 500 eV and 700 eV allowing the plasma conditions to be diagnosed by K-shell emission spectroscopy. The K-shell spectra show the effect of the ionization potential depression as a function of density via the delocalization of n = 3 levels and disappearance of n = 3 transitions in He-like and H-like aluminium. The data are compared to simulated spectra, which account for the change in the ionization potential by the commonly used Stewart and Pyatt prescription; a simple ion sphere model and an alternative due to Ecker and Kroll suggested by recent X-ray free-electron laser experiments. The experimental data are in reasonable agreement with the model of Stewart and Pyatt, but are in better agreement with a simple ion sphere model. The data indicate that the Ecker and Kroll model overestimates substantially the ionization potential depression in this regime.