Characterization of electron density and ionization of a uranium laser produced plasma using laser absorption spectroscopy

Abstract

High-resolution tunable laser spectroscopy is used to measure time-resolved absorption spectra for ten neutral uranium transitions and six singly-ionized transitions in a laser produced plasma. Spectral lineshapes are analyzed to determine temporal variations in ion and neutral total column densities, excitation temperatures, kinetic temperatures, and collisional broadening effects as the plasma cools. Comparison of ion to neutral column densities shows a ratio greater than 10 at times $<$15 μs after plasma onset, with the ratio not reaching unity until 50 μs. Spectral lineshapes are analyzed to separate Stark and van der Waals contributions to collisional broadening, from which electron densities are determined and found to decrease from $\sim$10¹⁵–10¹³ cm⁻³ over times from 4–25 μs. Using absorption spectroscopy to determine charge properties and electron density over these time scales and at low magnitudes provides valuable insight into plasma properties not obtainable using conventional emission spectroscopy. Comparisons between ion and neutral densities, excitation temperatures, kinetic temperatures, and electron densities could indicate potential deviations from local thermodynamic equilibrium and Saha ionization predictions.