Experimental demonstration of early time, hohlraum radiation symmetry tuning for indirect drive ignition experiments

Early time radiation symmetry at the capsule for indirect drive ignition on the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)] will be inferred from the instantaneous soft x-ray re-emission pattern of a high-Z sphere replacing the ignition capsule. This technique was tested on the OMEGA laser facility [J. M. Soures, R. L. McCrory, T. Boehly et al., Laser Part. Beams 11, 317 (1991)] in near full ignition scale vacuum hohlraums using an equivalent experimental setup to the one planned for NIF. Two laser cones entering each laser entrance hole heat the hohlraums to radiation temperatures of 100 eV, mimicking the NIF ignition pulse foot drive. The experiments have demonstrated accuracies of +/- 1.5% (+/- 2%) in inferred P-2/P-0 (P-4/P-0) Legendre mode incident flux asymmetry and consistency between 900 eV and 1200 eV re-emission patterns. We have also demonstrated the expected tuning capability of P-2/P-0, from positive (pole hot) to negative (waist hot), decreasing linearly with the inner/ outer beams power fraction. P-4/P-0 on the other hand shows very little variation with power fraction. We developed a simple analytical viewfactor model that is in good agreement with both measured P-2/P-0 and P-4/P-0 and their dependence on inner beam power fraction. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624497]