Determining fuel areal density distributions from nuclear scattering signatures

It is important to measure the magnitude and uniformity of the compressed fuel as it significantly affects the performance of inertial confinement fusion (ICF) implosions [1]. We describe a Monte-Carlo based method to determine fuel areal density distributions from measurements of the yields of 14 MeV neutrons and the ratio of down scattered neutrons to 14 MeV neutrons (dsr) along a number of lines-of-site (LOS) around the NIF target chamber. The compressed fuel is modeled as a spherical shell whose directionally dependent areal density is specified by the 9 coefficients of a spherical harmonic series summed to L = 2. The Monte-Carlo fires 14 MeV neutrons through the shell to determine the yields and dsr’s at each detector position and is repeatedly run under the supervision of a Levenberg-Marquardt minimizer that adjusts the 9 coefficients to find the areal density that produces the best match to the measurements. We present areal density distributions for selected ICF implosions at NIF and a series of 2-D HYDRA simulations.

 

[1] O. A. Hurricane, et al., Phys. Plasmas, 24, 092706 (2017)