Revisiting the Stark Width and Shift of He II Pα

Reexamining the Stark width and shift of He ii Paschen-α: Studies using laser-induced plasma, Thomson scattering, and computer simulations

We report on experimental and theoretical studies on the Stark profile of the He ii Paschen-α line over a wide range of plasma parameters. This line was emitted from a laser-induced plasma with electron densities in the range of 8.1×10^{22}-4.46×10^{24}m^{-3} and electron temperatures of 1.2-7.6eV as independently measured using the two-color Thomson scattering method. The line shapes were calculated using a computer simulation method, treating the ions and electrons on an equal footing and taking into account the full Coulomb interaction between the hydrogenlike atomic radiator and plasma perturbers penetrating the wave-function extent of the bound electron. We found a very good agreement between the experimental and theoretical Stark widths and shifts, which on average agree within 5%. In addition, practical analytical approximations for the linewidth and line shift are provided, validated against extensive calculations in the density and temperature ranges of 10^{23}-10^{25}m^{-3} and 1-16eV, respectively.

Plasma Stark effect of He ii Paschen-α: Resolution of the disagreement between experiment and theory

There is a significant disagreement between the experimental and theoretical plasma Stark shift of the hydrogenlike helium Paschen-α line (λ=468.568nm). Here, it is demonstrated that the controversy can be resolved by accounting for the plasma polarization shift and other related effects arising from the charged plasma particles penetrating the wave-function extent of the bound electron. For experimental verification, a laser-induced helium plasma with n_{e}=1.50×10^{24}m^{-3} and T_{e}=68200K, as independently determined by using the Thomson scattering method, was studied. Excellent agreement is observed between the theoretical and experimental line width and line shift, and more generally, for the entire line shape.

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.

Spectral Line Shapes in Plasmas II

The Spectral Line Shapes in Plasmas (SLSP) code comparison workshop series [...]