Gas-filled hohlraum experiments at the National Ignition Facility

A joint team from the National Laboratories at Los Alamos (LANL) and Lawrence Livermore (LLNL) has fielded the first gas-filled hohlraum experiments at the National Ignition Facility (NIF) laser, with the available four beams arranged as a single {\it f/8} beam. The gas-fill in this LANL design provides plasma pressure to tamp the hohlraum gold wall to avoid filling, the same technique used in ignition designs. A shaped laser pulse 8~ns in duration was used, with a low-power foot and a late peak of 7~TW, a contrast ratio exceeding 100 (the highest on NIF so far), and a total energy of 14~kJ. Deployed measurements include laser energy and power; back-scattered light spectrum, power and energy directly into the focusing lenses; back-scattered laser light energy outside the lenses; soft x-ray drive spectrum and power, and; gated framing-camera images of the hohlraum self-emission with x-ray energy $>$~10~keV. Our main results and conclusions are: (1)~This is the first experimental demonstration that a low-Z fill can keep the interior of a laser-driven hohlraum open long enough to ensure efficient coupling of ignition-relevant laser pulses. (2)~When backscattering losses are accounted ($\approx$~25\% reflectivity due to stimulated Brillouin scattering [SBS]), we have the radiation-hydrodynamics predictive capability necessary to understand the energy balance in such hohlraums quantitatively, as well as other details of the hohlraum-plasma evolution. (3)~Laser-plasma instabilities (LPI) can lead to considerable laser reflectivity levels, with a significant and measurable deleterious impact on hohlraum energetics. Thus, continued development of LPI predictive capability and understanding is needed. (4)~These experiments provide evidence that Stimulated Raman back-scattering losses (SRS) may be minimized with a proper choice of plasma conditions.