Simulation of nuclear observables in inertial confinement fusion experiments

Abstract

The neutrons and gamma rays created in nuclear reactions provide a way to measure conditions in Inertial Confinement Fusion (ICF) experiments. Interpretation of nuclear measurements requires detailed theoretical models and numerical simulations. This thesis presents a computational study of the nuclear observables based on radiation hydrodynamics simulations of ICF experiments. Current measurement techniques are replicated to highlight their strengths and shortcomings. Novel analyses are developed to measure conditions in experiments which are currently unmeasured.

Novel features of the scattered and high energy neutron spectrum are investigated. The neutron backscatter edge spectral shape is shown to be determined by the scattering rate weighted ion velocity distribution. A spectrum model is developed, tested and shown to infer currently unmeasured values for fluid velocity and temperature in the dense DT fuel. For magnetised spherical implosions, secondary DT neutrons exhibit yield increases and spectral anisotropy. The alpha knock-on component of the tertiary neutron spectrum shows sensitivity to hotspot transparency.

Images of the DT primary and scattered neutrons are calculated for several radiation hydrodynamics simulations. The primary neutron image shape analysis is shown to be robust against differential attenuation effects. The use of multiple energy gates to measure areal density within different angular ranges is demonstrated. Activation diagnostics are shown to have finite angular resolution due to the extended nature of the hotspot.

The time history of fusion gamma rays is found to be correlated to the rate of mechanical work performed on the hotspot. The length of peak delays between fusion and carbon gamma rays indicate the degree of burn truncation. Images of carbon gamma rays can be used to measure large scale asymmetries in the remaining ablator.

Combining and expanding on nuclear measurements allows a more complete picture of hydrodynamic conditions. This will aid in understanding and improving ICF experiments.