A model that solves simultaneously both the electron and atomic kinetics was used to generate synthetic X-ray
spectra to characterize high intensity ultrashort-laser-driven target experiments. A particle-in-cell simulation
was used to model the laser interaction for both cluster and foil targets and provided the initial electron energy
distribution function (EEDF) for a Boltzmann solver. Previously reported success in the spectroscopic characterization
of an irradiated Ar cluster target has motivated the authors to apply this technique in a feasibility study
to assess the possibility of recording time resolved spectra of a 10 micron Ti foil target irradiated by a 500 fs,
I= 1.0 × 1018W/cm2 short-pulse laser. Though this model suggests that both Ar cluster and Ti foil plasmas are
held in a highly non-equilibrium state for both the EEDF and the ion level populations for several picoseconds,
the spectral line features of the foil experiment was shown to evolve too quickly to be seen by current ultrafast
time resolved spectrometers.
Atomic kinetics and spectral modeling have revealed that level populations of plasma atoms in laser-ablated plumes may behave in a time-dependent manner, i.e. far from Local Thermodynamic Equilibrium, and that cascading population mechanisms can lead to long-lived atomic line emission. The time-scales associated to this phenomena and the interpretation of spectral data critically depend on the details of the atomic kinetics model and the quality of the rate coefficients. In order to generate accurate atomic data for neural atoms and low-charge ions present in plasma plumes, a semi-empirical techniques has been implemented in the Los Alamos atomic structure and electron scattering codes. This procedure has allowed neutrals with complex atomic structure--such as those atoms from elements often used in industrial applications--to be calculated with spectroscopic quality. Details of the atomic kinetics model for the case of a Li-Ag plasma plume and the rates generated with this new procedure are presented and discussed.
Time- and spatially-resolved spectroscopy in conjunction with detailed modeling constitutes a powerful technique for the understanding of plasma plume dynamics. To this end, in a series of experiments performed at Sandia National Laboratories, laser-generated LiAg plasma plumes were produced by irradiation of solid targets using a Nd laser. Time- and spatially-resolved (along a direction normal to the target's surface) optical spectra were recorded with a framing spectrograph. In order to limit gradients along a direction perpendicular to the target normal, targets with strips of LiAg coated on top of Pt were used. The PT plume collisionally confines the LiAg, thus reducing the LiAg lateral expansion. This technique allows a better characterization of the LiAg plasma. The spectra displays line transitions in Li and Ag atoms, and evidence of ions in the plume is suggested by the presence of forbidden lines and Stark-broadened line shapes. A spectroscopic model based on collisional-radiative atomic kinetics, detailed line shapes, and radiation transport calculations is used to interpret the data. From this analysis temperature, density and ionization in the plume as a function of time and position along the normal to the target surface can be extracted.