Space and time resolved spectroscopy of laser produced plasmas: A study of density sensitive X ray transitions in helium like and neon like ions.

摘要

The determination of level populations and detailed population mechanisms in dense plasmas has become an increasingly important problem in atomic physics. In the present work, the density variation of line intensities and level populations in aluminum K-shell and molybdenum and silver L-shell emission spectra have been measured from high-powered, laser-produced plasmas. For each case, the density dependence of the observed line emission is due to the effect of high frequency electron-ion collisions on metastable levels. The density dependent line intensities vary greatly in laser-produced plasmas and can be used to extract detailed information concerning the population kinetics and level populations of the ions.; The laser-plasmas had to be fully characterized in order to clearly compare the observed density dependence with atomic theory predictions. This has been achieved through the combined use of new diagnostic instruments and microdot targets with provided simultaneously space, time, and spectrally resolved data. The plasma temperatures were determined from the slope of the hydrogen-like recombination continuum. The time resolved electron density profiles were measured using multiple frame holographic interferometry. Thus, the density dependence of K-shell and L-shell spectral lines could be clearly examined, independent of assumptions concerning the dynamics of the plasma.; In aluminum, the electron density dependence of various helium-like line intensity ratios were measured. Standard collisional radiative equilibrium models fail to account for the observed density dependence measured for the "He{dollar}sb{lcub}alpha{rcub}{dollar}/IC" ratio. Instead, a quasi-steady state atomic model based on a purely recombining plasma is shown to accurately predict the measured density dependence. This same recombining plasma calculation successfully models the density dependence of the high-n "He{dollar}sb{lcub}gamma{rcub}{dollar}/He{dollar}sb{lcub}beta{rcub}{dollar}" and "He{dollar}sbdelta{dollar}/He{dollar}sbbeta{dollar}" helium-like resonance line intensity ratios. The density dependence of metastable levels of neon-like ions is also measured. This is done by studying weak electric quadrupole transitions in neon-like molybdenum and silver ions. Detailed neon-like theoretical atomic model calculations are shown to correctly predict the observed density sensitivity of the electric quadrupole to dipole-allowed resonance line "x/3A" ratio. The measured density dependence of the "3A/3G" resonance line ratio does not agree with theory. Possible reasons for this discrepancy and suggestions for further experiments are reviewed.