The role of fast-electron preheating in low-adiabat cryogenic implosions on OMEGA

The compression in direct-drive, low-adiabat spherical implosions was studied using cryogenic D2 targets on the 60-beam, 351-nm OMEGA laser in the range of laser intensities from ∼3 x 1014 to ∼1.5 x 1015 W/cm2. The neutron-burn-averaged areal densities decreased as the laser intensity increased. This decrease in areal density is highly correlated with the increased hard-x-ray signals, caused by hot electrons generated by the two-plasmon-decay instability. The areal-density reduction up to ∼2, observed in the experiments, requires a preheat energy of ∼60 J (∼0.3% of total laser energy), which is consistent with the estimated preheat levels inferred from the hard-x-ray signal levels. Mitigating the generation of fast-electrons results in high areal densities which are close to the 1D predictions