New x-ray drive measurements using View Factor hohlraums on the National Ignition Facility*

To understand the radiation drive impinging on the capsule, we used the “View Factor” targets. Such target has one end of a nominal hohlraum (~1/3 in length) cut away to provide an “open-end” without the laser-entrance-hole (LEH), thus allowing an unobscured view of the hohlraum interior. It was first used about a decade ago to great success in understanding the radiation drive deficient for the high gas filled (0.96 mg/cc He4) hohlraums with “low-foot” and “high-foot” laser drive. This platform has now been revisited to investigate the hohlraum predictive capability for modern ICF designs which differ from previous designs by using low gas filled (0.3 mg/cc He4) hohlraums with different (HyE and “SQN”-like) laser pulse shapes.  The experimental results show similarities to the earlier experiments for the comparison of radiant intensity with models. Namely, when viewed through the open-end, the total radiant intensity tends to agree with simulations. However, the radiant intensity is over-predicted by simulations when viewed through the LEH. Our analysis indicates that the “M-band” (> 1.8 keV radiation) is over-predicted in simulations, which could account for much of the discrepancy when viewed through the LEH simply due to the geometry of the View Factor setup. Detailed results of the measurement and its implications to the ICF experiments will be presented.