Measurements of the gold bubble plasma temperature using L-shell emission spectroscopy*

Predictive modeling of indirect drive implosions on the National Ignition Facility (NIF) requires accurately simulating several processes within the hohlraum environment. The time-dependent absorption of laser energy by, and laser propagation through the hohlraum plasma depend on the conditions and motion of the hohlraum plasma. This is especially true in the gold bubble region, where the outer NIF beams deposit their energy. High quality experimental data of the gold bubble plasma temperature, T_e, can inform and constrain the models, leading to better hohlraum predictive capabilities. Several experiments have been carried out at the NIF which employed time-resolved and/or spatially resolved spectroscopy of self-emission from the gold bubble plasma in the L-shell spectral region (8-12 keV). Time resolved data observing the entire gold bubble region was obtained during ~4 ns of the experiment including the peak laser drive. The electron temperature was determined using average charge states inferred by the spectral position of the 3d_5/2-2p_3/2  feature and fitting to calculations from the atomic kinetics code SCRAM [1], [2]. Several experiments incorporated Zn dopants in the bubble region whereby line ratios of the He-alpha and Ly-alpha structures help to cross-calibrate the T_e measurements made by the Au self-emission, allowing Au self emission to be used as a T_e measurement on virtually all hohlraum experiments. Results and uncertainties of Au L-shell as a T_e diagnostic are discussed for a variety of experimental configurations.

 

[1] E. Trabert, et al. Rev. Sci. Instrum. 79, 10E313 (2008)

[2] S. Hansen, PhD dissertation, University of Nevada, Reno (2003)