Trending low mode asymmetries in NIF capsule drive using a simple viewfactor metric

The mode-1 x-ray drive asymmetry of indirect-drive Inertial Confinement Fusion(ICF) implosions at the National Ignition Facility(NIF) has been estimated using a simple static ViewFactor model. The model takes as input measured laser performance data in the foot and peak, the hohlraum configuration, and laser to hohlraum pointing. These estimates are compared with neutron time-of-flight measurements of directionality and magnitude of the resultant hotspot bulk velocity (similar to 20-100 mu m/ns) for 39 NIF shots using High Density Carbon (HDC) ablators and show strong correlation on a statistically significant number of shots. The most important factors identified so far are random quad-to-quad peak power laser imbalances, the presence of lossy diagnostic windows and gaps on the hohlraum waist, capsule sag and capsule thickness mode 1. Typical mode-1 asymmetry in drive is currently similar to 0.5% for many of these sources on their own and, when summed, can lead to a neutron hotspot velocity of up to 100 mu m/ns and a reduction in yield of 35% for current NIF DT layered implosions. Our goal is to identify, quantify and mitigate all potential sources of mode-1 asymmetry (which also include target and laser alignment imperfections, foot power and Cross Beam Energy Transfer imbalances) to enable higher quality implosions on NIF.

Automated summary

The study estimates the mode-1 x-ray drive asymmetry in indirect-drive ICF implosions at NIF using a static ViewFactor model and compares it with neutron time-of-flight measurements, revealing factors such as laser imbalances and hohlraum configuration contributing to the asymmetry. Efforts are being made to identify, quantify, and mitigate these sources of asymmetry to achieve higher quality implosions.