Characterization of direct-drive-implosion core conditions on OMEGA with time-resolved Ar K-shell spectroscopy

Direct-drive-implosion core conditions have been characterized on the 60-beam OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system with time-resolved Ar K-shell spectroscopy. Plastic shells with an Ar-doped deuterium fill gas were driven with a 23 kJ, 1 ns square laser pulse smoothed with 1 THz smoothing by spectral dispersion (SSD) and polarization smoothing (PS) using birefringent wedges. The targets are predicted to have a convergence ratio of ∼15. The emissivity-averaged core electron temperature (T e ) and density (n e ) were inferred from the measured time-dependent Ar K-shell spectral line shapes. As the imploding shell decelerates the observed T e and n e increase to 2.0 (±0.2) keV and 2.5 (±0.5)×10 24 cm −3 at peak neutron production, which is assumed to occur at the time of the peak emissivity-averaged T e . At peak compression the n e increases to 3.1 (±0.6)×10 24 cm −3 and the T e decreases to 1.7 (±0.17) keV. The observed core conditions are close to those predicted by a one-dimensional hydrodynamics code.