Implementation of the foil-on-hohlraum technique for the magnetic recoil spectrometer for time-resolved neutron measurements at the National Ignition Facility

The next-generation Magnetic Recoil Spectrometer, called MRSt, will provide time-resolved measurements of the DT-neutron spectrum from Inertial Connement Fusion (ICF) implosions at the National Ignition Facility (NIF). These measurements will provide critical information about the time evolution of the fuel assembly, hot-spot formation, and nuclear burn. The absolute neutron spectrum in the energy range of 12-16 MeV will be measured with high accuracy (~5%), unprecedented energy resolution (~100 keV) and, for the first time ever, time resolution (~20 ps). Crucial to the design of the system is a CD conversion foil for the production of recoil deuterons positioned as close to the implosion as possible. The foil-on-hohlraum technique has been demonstrated by placing a 1-mm-diameter, 40-um-thick CD foil on the hohlraum diagnostic band along the line-of-sight of the current time-integrated MRS system, which measured the recoil deuterons. In addition to providing validation of the foil-on-hohlraum technique for the MRSt design, substantial improvement of the MRS energy resolution has been demonstrated.