New diagnostic possibilities using HT fusion gammas during the NIF THD campaign

In this article, a gas-Cherenkov detector system is designed for the NIF THD campaign. A very small fraction of DT fusion reactions produce a 16.7 MeV gamma ray instead of a 14 MeV neutron, the time history of the fusion gammas is an accurate representation of the fusion reaction history. Gas Cherenkov detectors with a gamma-to-electron converter can be tuned to be only sensitive to gamma rays above a certain energy. We will have a cluster of several Cherenkov detectors installed at NIF, just outside the target chamber wall. The multiple detectors will allow us to measure the time history for several different (integral) threshold energies, ranging from 3.5 MeV to 20 MeV. The HT fusion reaction (yielding an alpha particle and a 19.8 MeV gamma ray) is a very rare process in a typical fusion plasma, but the reaction has a 100% branching ratio to gamma rays, unlike the DT fusion reaction.