Experimental investigation of the source of mode one asymmetries in indirect-drive ICF implosions*

While recent indirect-drive ICF implosions on the NIF have demonstrated record performance such as a yield amplification of 3 and hot spot pressures of ~360 Gbar¹, 3D asymmetry of the hot spot and assembled cold fuel shell continues to be a key performance degradation mechanism.  In particular, measurements of the bulk hot-spot velocity from neutron time-of-flight measurements and spatial variations in the shell areal density from neutron activation diagnostics indicate a systematic mode-1 asymmetry. Diagnostic windows are cut out of the high-Z gold or depleted uranium hohlraum in order to provide a diagnostic line-of-sight for hot spot self-emission x-rays and to allow imaging of the capsule fuel pre-shot. One hypothesis for the low-mode asymmetry is that these windows are reducing the x-ray drive in the window directions. We will report on a series of experiments systematically varying the size of the hohlraum windows and the effect on implosion shape, hot-spot velocity, rhoR variations, and overall performance.

¹S. Le Pape, et al., Phys. Rev. Lett. 120, 245003 (2018)