Controlling sources of low mode asymmetry in ignition experiments at the National Ignition Facility(NIF)

Low mode 3D drive and capsule asymmetries (l = 1 and 2) are important degradation mechanisms for high yield NIF indirect drive implosions. Sources of asymmetry include laser power, capsule ablator thickness and composition, laser/target positioning errors, drive losses from target diagnostic windows, and Cross Beam Energy Transfer (CBET). The resultant mode-1 asymmetries are seen on NIF implosions as high bulk imploded hotspot velocities (~ 100km/s), measured by comparison of neutron time of flight measurements on several lines of sight. The expected asymmetry inferred from the laser diagnostics and target characterization, has been compared with hotspot velocity and shape measurements on multiple implosions to calibrate sensitivity of the implosions to asymmetry and identify systematic trends in the facility and target performance that may be mitigated. Modifications to the configuration of the diagnostics windows has reduced their mode-1 and 2 impact on recent DT shots. Capsule thickness variations and misalignment are characterized pre shot and can be mitigated by power adjustments of order +/- 1% to specific groups of laser beams.