Time Dependent Gamma Measurement of Ablator Areal Density Velocity Near Bangtime on Inertial Confinement Capsules*

The Gamma Reaction History (GRH) diagnostic at the National Ignition Facility (NIF) measures the neutron induced inelastic scattered gammas as a function of time. By taking different gamma thresholds, the carbon signal from the ablator of the capsule can be isolated from the gammas created by the surrounding hohlraum and thermal mechanical package. The areal density of the carbon near bangtime gives insight into trajectory of the shell as well its compression and stagnation due to the hot spot outward pressure. Measured values of the major NIF campaigns are compared with expected shell areal density scaling from simple hydrodynamic simulation (Helios) as well as an analytical toy rocket model. Furthermore, the time difference of the carbon gamma peak compared to the fusion burn gives information about the movement of the shell throughout the burn time. Shell velocity at bangtime reveals insights to shot performance and gives a lower limit for residual kinetic energy. The measured carbon rhoR for the different major NIF capsule campaigns will be reported, contrasted and discussed.