Experimental results of radiation driven, layered deuterium tritium implosions with adiabat shaped drives at the National Ignition Facility
Radiation driven, layered deuterium tritium (DT) implosions were carried out using 3 shock and 4 shock "adiabat shaped" drives and plastic ablators on the National Ignition Facility (NIF) [E. M. Campbell et al. AIP Conf. Proc. 429, 3 (1998) The purpose of these shots was to gain further understanding on the relative performance of the low foot implosions of the National Ignition Campaign [M. J. Edwards et al. Phys. Plasmas 20, 070501 (2013) versus the subsequent high foot implosions [T. Doppner et al. Phys. Rev. Lett. 115, 055001 (2015) The neutron yield performance in the experiment with the 4 shock adiabat shaped drive was improved by factors ~3 to ~10, compared to five companion low foot shots despite large low mode asymmetries of DT fuel, while measured compression was similar to its low foot companions. This indicated that the dominant degradation source for low foot implosions was ablation front instability growth, since adiabat shaping significantly stabilized this growth. For the experiment with the low power 3 shock adiabat shaped drive, the DT fuel compression was significantly increased, by ~25% to ~36% compared to its companion high foot implosions. The neutron yield increased by ~20% lower than the increase of ~50% estimated from one dimensional scaling, suggesting the importance of residual instabilities and asymmetries. For the experiment with the high power, 3 shock adiabat shaped drive, the DT fuel compression was slightly increased by ~14% compared to its companion high foot experiments. However, the compression was reduced compared to the lower power 3 shock adiabat shaped drive, correlated with the increase of hot electrons that hypothetically can be responsible for reduced compression in high power adiabat shaped experiments as well as in high foot experiments. The total neutron yield in the high power 3 shock adiabat shaped shot N150416 was 8.5 x 1015 0.2 x 1015, with the fuel areal density of 0.90 0.07 g/cm2, corresponding to the ignition threshold factor parameter IFTX (calculated without alpha heating) of 0.34 0.03 and the yield amplification due to the alpha heating of 2.4 0.2. The performance parameters were among the highest of all shots on NIF and the closest to ignition at this time, based on the IFTX metric. The follow up experiments were proposed to continue testing physics hypotheses, to measure implosion reproducibility, and to improve quantitative understanding on present implosion results.