Hohlraum energetics and implosion symmetry with elliptical phase plates using a multi-cone beam geometry on OMEGA

Hohlraum energetics and implosion-symmetry experiments were conducted on the OMEGA Laser System using laser beams arranged in three cones and smoothed with elliptical phase plates. The peak radiation temperature (Tr) increased by 17 eV, with phase plates for gas-filled halfraums irradiated with 20 beams using a ∼7-kJ shaped laser pulse (PS26), corresponding to a 44% increase in the peak x-ray flux. The improved coupling correlates with reduced, cone-dependent losses from stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). Phase plates reduce SRS and SBS by controlling the on-target laser-intensity envelope and the speckle modal power spectrum. An implosion symmetry scan was performed by varying the length and beam pointing of vacuum and gas-filled, thin-walled (3-μm) Au hohlraums irradiated with 40 beams using a ∼14-kJ PS26. Gated-x-ray (hv > 3 keV) images taken along radial and axial views of the self-emission from Ar-doped, D2-filled, plastic-shell implosions quantified the indirect-drive-implosion symmetry. A shift in symmetry was observed between vacuum and gas-filled hohlraums having identical beam pointing. The ratio of x-ray drive at the poles of the capsule relative to the waist increased for the gas-filled hohlraum. Levels of hard-x-ray production (hv > 20 keV) and SRS were reduced with trace amounts of high-Z dopants (i.e., Ne, Kr) in the hohlraum plasma, while the peak Tr increased ∼5 eV