Spectroscopic Measurements of the Equatorial Electron Temperature in a NIF Hohlraum Using Mid-Z Tracer Elements*

Spectrally and temporally resolved x-ray emission measurements of highly charged mid-Z ions are used to determine the electron temperature (T_e) in the equatorial region of a laser-driven ignition-type NIF hohlraum. A mid-Z tracer “dot” of 400 µm in diameter and 0.16 µm thickness is coated onto the outside of a high-density carbon (HDC) capsule. The measured x-ray spectra are fit using SCRAM simulations and a genetic algorithm to give a peak T_e of (2.7 ± 0.4) keV at the hohlraum equator between the HDC capsule and the gold wall. The measurements show a decrease in T_e near the hohlraum equator even before the laser drive is turned off giving evidence that the equatorially-directed laser beams are losing their energy before they reach the hohlraum equator. While we find agreement with post-shot simulations when the plasma reaches its peak temperature, some disagreement between the measured and simulated T_e remains in the earlier part of the laser heating and also in the temporal signature of the cooling phase near the end of the laser drive. We present a detailed error analysis of the spectroscopic measurements and a discussion of the uncertainty in the derived T_e.