Measurements of dense fuel hydrodynamic conditions in burning plasma NIF experiments using backscattered neutron spectroscopy

Measurement of the dense fuel hydrodynamic conditions, such as areal density, temperature, and velocity, during fusion burn is key in understanding the performance on inertial confinement fusion (ICF) experiments. Previous theoretical studies and experiments on the OMEGA 60 laser have shown that backscattered neutron spectroscopy can be used to directly measure dense fuel conditions during stagnation. In particular, the nT backscatter edge was measured and analysed, this spectral feature is produced by the 180 degree scattering of 14 MeV DT neutrons from tritons in the fuel. As recent ICF experiments on NIF enter the burning plasma regime, evidence for changing dense fuel conditions is looked for in the nT backscatter edge. In simulation, hotspot self-heating leads to the dense fuel being heated by thermal conduction and alpha deposition. In addition, burn will continue into the expansion phase such that the dense fuel will be exploding outwards at the time of peak neutron production. The NIF neutron time-of-flight spectroscopic measurements of the nT edge show evidence of these phenomena. A simple model is used to extract velocities and apparent temperatures of the dense fuel.