NIF-scale re-emission sphere measurements of early-time Tr = 100 eV hohlraum symmetry (invited)

The indirect-drive National Ignition Campaign (NIC) proposes to set the first 2 ns of hohlraum radiation symmetry by observing the instantaneous soft x-ray re-emission pattern from a high-Z sphere in place of the ignition capsule. To assess this technique under NIC conditions, we used the Omega Laser Facility to image the re-emission of Bi coated spheres with 200 ps temporal, 50-100 microm spatial, and 30% spectral resolution. The sphere is driven by 70% NIC-scale vacuum Au hohlraums heated to Tr=100 eV using two cones per side laser beam illumination. The experiments have demonstrated the required accuracies of <3%P(2)/P(0) and <3%P(4)/P(0) Legendre mode flux asymmetry at both 900 and 1200 eV re-emission photon energies. The re-emission patterns at 900 and 1200 eV are also consistent with each other and their relative dependence on radiation temperature. We measured the P(2)/P(0) and P(4)/P(0) dependence to laser cone power ratio. View factor calculations are in agreement with the experimentally measured radiation flux and re-emit images when assuming 55% inner beam and 100 % outer beam coupling into x rays at the hohlraum wall.