| Relativistic short pulse laser matter interactions are of great interest in the area of high energy density physics which includes the fields of astrophysics, inertial confinement fusion (ICF), and fast ignition (FI). Such interactions can result in the creation of hot dense matter at keV temperatures near solid density which is crucial for the studying radiation transport, understanding particle transport processes, and benchmarking of computer models in this plasma regime. The plasma by short pulse lasers, with intensity greater than 1019 W/cm2 and picosecond pulses, exists for only several tens of picoseconds, has a nonuniform temperature and density, and has a non- Maxwellian electron distribution thus making characterization a challenge. This work presents for the first time a systematic study of the temperature gradient inside of micron thick solid targets using K-shell spectroscopic techniques and demonstrates the importance of energetic electrons, temporal evolution of the plasma and opacity in the analysis of high intensity short pulse laser plasmas.;Tamped titanium foils were with a short-pulse laser with intensity greater than 1019 W/cm2. Target parameters such as size, tamper material, and tamper thickness were varied to optimize heating and uniformity of the titanium plasma. Comparison of measured titanium K-shell spectra, from a 250 x 250 x 5 mum3 titanium foil tamped with aluminum, with a collisional radiative model indicated that the front ∼0.2 mum reached a peak temperature of Te,peak= 1300 eV at solid density. The remaining bulk material had temperature Te,bulk = 100 eV. This experimental result is consistent with previous observations and the use of a tamper greatly enhances the density uniformity of the plasma. Further reduction of the lateral and longitudinal target dimensions was needed to refine the analysis and prompted a second experiment. Five targets, with dimensions of 100 x 100 x 0.4 mum3, had a copper foil were buried at different depths (i.e. 0-1.5 mum) and irradiated with a short pulse laser with intensities greater than 1020 W/cm2. The measured K-shell spectra again indicated a temperature gradient in the longitudinal direction inside the target. Analysis using a hydrodynamic code and a collisional radiative atomic code showed that the hot electron population, time dependent plasma conditions, and opacity significantly alters the calculated K-shell line ratios thus producing up to a factor of two error in plasma temperature. |
