Measurements of Hbeta Stark central asymmetry and its analysis through standard theory and computer simulations

Full radiator-perturber interaction in computer simulations of hydrogenic spectral line broadening by plasmas

Spectral line broadening by plasmas can be computed by solving the equation of motion for the dipole of the radiating system perturbed by a fluctuating potential obtained from computer simulations. Such calculations have relied on the multipole expansion of the radiator-plasma interaction often keeping only the dipole term. With increasing density, however, higher multipoles as well as plasma perturbers overlapping the bound electron wave functions are expected to become important. For hydrogenic systems, the atomic matrix elements of the full Coulomb and screened Coulomb interactions are given by analytical formulas. Using these results, a computer simulation approach that accounts for the full radiator-plasma interaction is developed. One benefit is the removal of inherent strong collision divergences in the multipole expansion approximation. Furthermore, it yields the plasma polarization shift produced by perturbers penetrating the wave function of the radiator bound electrons. The model was applied to hydrogenic argon Ly-α, Ly-β, and Ly-γ spectral lines in a dense argon plasma at free electron densities of 10^{24} or 10^{25}cm^{-3} and temperature of 800eV relevant to plasma diagnostic techniques for inertial confinement fusion implosions.

Studies of spectral line merging in a laser-induced hydrogen plasma diagnosed with two-color Thomson scattering

Laser-induced hydrogen plasma in the density and temperature range of (0.1-5)×10^{23}m^{-3} and (6000-20000)K, respectively, was precisely diagnosed using two-color Thomson scattering technique, inferring the electron number density, electron temperature as well as ion temperature. Simultaneously, spectra of the Balmer series of spectral lines from H-β to H-ζ were measured and plasma emission coefficient calculated within the quasicontiguous frequency-fluctuation model. The theoretical spectra are found to be in good agreement with experimental ones, including higher-density data where discrete lines were observed to merge forming a continuum.

Stark-Broadening of Ar K-Shell Lines: A Comparison between Molecular Dynamics Simulations and MERL Results

Analysis of Stark-broadened spectral line profiles is a powerful, non-intrusive diagnostic technique to extract the electron density of high-energy-density plasmas. The increasing number of applications and availability of spectroscopic measurements have stimulated new research on line broadening theory calculations and computer simulations, and their comparison. Here, we discuss a comparative study of Stark-broadened line shapes calculated with computer simulations using non-interacting and interacting particles, and with the multi-electron radiator line shape MERL code. In particular, we focus on Ar K-shell X-ray line transitions in He- and H-like ions, i.e., Heα, Heβ and Heγ in He-like Ar and Lyα, Lyβ and Lyγ in H-like Ar. These lines have been extensively used for X-ray spectroscopy of Ar-doped implosion cores in indirect- and direct-drive inertial confinement fusion (ICF) experiments. The calculations were done for electron densities ranging from 1023 to 3×1024 cm−3 and a representative electron temperature of 1 keV. Comparisons of electron broadening only and complete line profiles including electron and ion broadening effects, as well as Doppler, are presented. Overall, MERL line shapes are narrower than those from independent and interacting particles computer simulations performed at the same conditions. Differences come from the distinctive treatments of electron broadening and are more pronounced in α line transitions. We also discuss the recombination broadening mechanism that naturally emerges from molecular dynamics simulations and its influence on the line shapes. Furthermore, we assess the impact of employing either molecular dynamics or MERL line profiles on the diagnosis of core conditions in implosion experiments performed on the OMEGA laser facility.

Effect of the Ions on the Electron Collision Operator through Electronic Trajectory Modification

We investigate the ion effect on the broadening of the spectral line profile by the free electrons collisions with the emitters in plasmas. We only considered the weak collisions’ contribution. This effect has a consequence on the trajectories of the free electrons through the electric microfield created by the ions of the plasma. Thanks to the Meijer’s functions, the calculation of the electronic Stark broadening is precisely established.

Mini-Review of Intra-Stark X-ray Spectroscopy of Relativistic Laser–Plasma Interactions

Intra-Stark spectroscopy (ISS) is the spectroscopy within the quasi-static Stark profile of a spectral line. The present paper reviews the X-ray ISS-based studies recently advanced for the diagnostics of the relativistic laser–plasma interactions. By improving experiments performed on the Vulcan Petawatt (PW) laser facility at the Rutherford Appleton Laboratory (RAL), the simultaneous production of the Langmuir waves and of the ion acoustic turbulence at the surface of the relativistic critical density gave the first probe by ISS of the parametric decay instability (PDI) predicted by PIC simulations. The reliable reproducibility of the experimental signatures of PDI—i.e., the Langmuir-wave-induced dips—allowed measurements of the fields of the Langmuir and ion acoustic waves. The parallel theoretical study based on a rigorous condition of the dynamic resonance depending on the relative values of the ion acoustic and the Langmuir fields could explain the disappearance of the Langmuir dips as the Langmuir wave field increases. The ISS used for the diagnostic of the PDI process in relativistic laser–plasma interactions has reinforced the reliability of the spectral line shape while allowing for all broadening mechanisms. The results can be used for a better understanding of intense laser–plasma interactions and for laboratory modelling of physical processes in astrophysical objects.

Revealing the second harmonic generation in a femtosecond laser-driven cluster-based plasma by analyzing shapes of Ar XVII spectral lines

We present experiments dealing with a femtosecond laser-driven cluster-based plasma, where by analyzing the nonlinear phenomenon of satellites of spectral lines of Ar XVII, we revealed the nonlinear phenomenon of the generation of the second harmonic of the laser frequency. For performing this analysis we developed new results in the theory of satellites of spectral lines. From such lineshape analysis we found, in particular, that the efficiency of converting the short (40 fs) intense (3x10¹⁸ W/cm²) incident laser light into the second harmonic was 2%. This result is in the excellent agreement with the 2-Dimensional Particle-In-Cell (2D PIC) simulation that we also performed. There is also an order of magnitude agreement between the thresholds for the SHG found from the line shape analysis and from the 2D PIC simulations.

Analysis of time-resolved superposed atomic hydrogen Balmer lines and molecular diatomic carbon spectra

We present analysis of superposition spectra following laser-induced breakdown (LIB) of methane. Both hydrogen-beta and hydrogen-gamma lines contain discernible contributions from diatomic carbon emissions for time delays of 1 to 2 μs from pulsed, 8 ns, infrared Nd:YAG laser radiation LIB. Analysis of the atomic lines and molecular C(2) spectra reveal electron and molecular excitation temperatures of typically 13,000 and 5000 K, respectively.