Future ICF burn-history measurements using fusion gamma rays

Abstract

Summary form only given, as follows. At the Nova Laser Facility, a high-speed neutron detector records the fusion reaction rate for inertial-confinement fusion (ICF) experiments with a resolution of 30 ps. It is unlikely that the current measurement technique can provide similar resolution at the future National Ignition Facility (NIF). The larger target-to-detector distance and higher plasma ion temperature will limit temporal resolution between 125 and 600 ps for direct neutron measurements. We are investigating a measurement technique based on the 16.7-MeV gamma rays that are released in deuterium-tritium fusion. Temporal resolution is independent of the target-to-detector distance since no temporal dispersion occurs for gamma rays. Detector design must, however, account for the small branching ratio of <10/sup -4/ for gamma ray production and a large burst of 14-MeV neutrons. Our concept is to convert gamma-ray energy into a fast burst of Cerenkov light that can be recorded with a high-speed optical detector. Fusion gamma rays interact with a thin, high-z converter primarily by pair production to produce electron-positron pairs with energies of 8 MeV. Cerenkov light is produced when the electrons and positrons pass through a thin layer of material whose index of refraction is slightly greater than 1. An optical telescope relays the Cerenkov light to the detector. We have detected 16.7-MeV fusion gamma rays in preliminary experiments conducted at Nova. We used a tungsten/aerogel converter to generate Cerenkov light and recorded the signal with an optical streak camera.