Gamma-ray Imaging Diagnostic for Inertial Confinement Fusion

Abstract

In Livermore California at the Lawrence Livermore National Laboratory exists the National Ignition Facility (NIF). The NIF’s goal is to study high-density high-temperature plasmas and try and reach thermalnuclear fusion by a method known as Inertial Confinement Fusion (ICF). This occurs when 192 UV lasers interact with the inner walls of a gold cylinder, known as a hohlraum, and produce X-rays. Inside the hohlraum consists a capsule filled with deuterium and tritium. The X-rays ablate the outer plastic shell causing an inward shockwave to compress the D-T fuel to high density and undergo nuclear fusion. The fusion reaction creates 14.1 MeV neutrons that can interact with the carbon 12 of the plastic outer shell producing 4.44 MeV gammas from 12C(n,n’γ)12C reactions. This dissertation examines and produces a prototype system to image these 4.44 MeV gammas. This is a valuable diagnostic system that can characterize the symmetry of the ablated surface, which gives information as to how symmetrically the capsule is imploding. A prototype Gamma-Ray Imaging system (GRI) was built and tested at two large facilities. The first is another ICF facility that implodes capsules but at a much lower yield. These capsules produced X-rays in the ~300-400 keV range that were successfully imaged using the GRI system. The second facility is the High Intensity Gamma Source (HIGS) at Duke University that uses a free electron laser Compton scattered off a relativistic electron packet to produce a 4.7 MeV gamma beam. A number of radiograph shadow targets were placed in the 32 mm wide gamma beam and imaged by the GRI system. The results of these two test are discussed in this dissertation. The GRI system along with the Neutron Imaging System, and other diagnostics systems, are intended to compliment each other and provide useful information that can be used to design future capsules or diagnose poor implosions. This will help NIF achieve its goal of thermonuclear fusion.