Synthetic nuclear diagnostics for inferring plasma properties of inertial confinement fusion implosions

Abstract

A suite of synthetic nuclear diagnostics has been developed to post-process radiation hydrodynamics simulations performed with the code Chimera. These provide experimental observables based on simulated capsule properties and are used to assess alternative experimental and data analysis techniques. These diagnostics include neutron spectroscopy, primary and scattered neutron imaging, neutron activation, γ-ray time histories and carbon γ-ray imaging. Novel features of the neutron spectrum have been analysed to infer plasma parameters. The nT and nD backscatter edges have been shown to provide a shell velocity measurement. Areal density asymmetries created by low mode perturbations have been inferred from the slope of the downscatter spectrum down to 10 MeV. Neutron activation diagnostics showed significant aliasing of high mode areal density asymmetries when observing a capsule implosion with 3D multimode perturbations applied. Carbon γ-ray imaging could be used to image the ablator at high convergence ratio. Time histories of both the fusion and carbon γ signals showed a greater time difference between peak intensities for the perturbed case when compared to a symmetric simulation.

Density determination of the thermonuclear fuel region in inertial confinement fusion implosions

“…Chimera is a 3-D Eulerian radiation magnetohydrodynamics code developed at Imperial College, London. Synthetic nuclear diagnostic data derived from Chimera include neutron images, time-of-flight spectra, as well as synthetic FNAD data 28 Using ray-tracing methods, synthetic burn-averaged primary and downscattered neutron images were produced for all three neutron lines of sight currently operating at NIF.…”

The effect of areal density asymmetries on scattered neutron spectra in ICF implosions

“…Working with higher energy scattered neutrons can also be used to reduce the impact of multiple scattering. 17 At these higher energies, both the scattering cross section and the number of multiply scattered neutrons are lower. At 200 mg/cm 2 , the ratio of doubly to singly scattered neutrons is just $2% within 10-12 MeV compared to $10% below the nT edge, and these rise linearly with increased areal density.…”

“…Additionally, due to the extended nature of the hotspot neutron source, hqLi is likely to be dominated by low modes. 17 For mode l ¼ 1 areal density asymmetries, this is particularly simple,…”

“…However, scattering from the remaining ablator can prove a non-negligible contribution to the scattered neutron spectrum. 17 A minimal extension to the spectral model, described above, could include the cross sections for ablator scattering interactions in a similar manner to those of the fuel. Carbon c-ray measurements [29][30][31] could be used to infer the carbon areal density as an input or constraint to this extended model.…”

“…III will be compared to results from a more complete neutron transport description of an ICF implosion. The inverse ray trace method described in Crilly et al 17 allows the calculation of neutron spectra given 3D grids of hydrodynamic conditions. As a suitable test problem, we consider an ice block model for a mode 1 asymmetry where a stationary spherical hotspot is displaced within a dense cold fuel layer, as in Fig.…”