Speaker: Audrey DeVault (MIT)
Title: Approaches to Developing Time-Resolving Neutron Diagnostics at the NIF
Abstract: The development of a time-resolving neutron diagnostic underway for the purpose of analyzing inertial confinement fusion (ICF) implosions at the National Ignition Facility (NIF). Time-resolved neutron spectra can be analyzed to infer Ti(t), ρR(t), and Yn(t), which provide valuable information about the dynamics of an ICF implosion. In particular, they would help identify factors limiting burn propagation in order to improve gain from the igniting implosions now repeatedly achieved at the NIF. Two approaches to obtaining time-resolved neutron spectra are presented: the time-resolving Magnetic Recoil Spectrometer (MRSt) and the tomographic neutron Time of Flight (nTOF) scheme. The MRSt utilizes a front end similar to the current magnetic recoil spectrometer fielded on NIF, with a CD foil converting neutrons to deuterons and a magnet system which momentum-separates the deuterons. However, the backend is yet to be finalized. A novel, simplified backend concept is currently being investigated, utilizing a printed circuit board (PCB) to passively de-skew the incoming signal in time. The PCB takes advantage of propagation delay to slow down the signal from higher-energy deuterons before recombining with that of simultaneously produced lower-energy deuterons, producing energy spectra for discrete timesteps throughout the implosion. Design and prototyping of a PCB backend is underway, and crucial design features are identified. The tomographic nTOF scheme employs multiple nTOF detectors at varied distances from Target Chamber Center (TCC). Close-in nTOFs capture more time-of-emission information, while far-out nTOFs capture more energy information, allowing for multiple samplings of the neutron source spectra in energy and time-of-emission space, enabling tomographic reconstruction of the source spectra. Synthetic data studies are underway to probe the feasibility of this approach. Models of neutron source spectra in energy and time-of-emission are used to generate synthetic datasets of collinear nTOFs, with tunable distance, noise, time binning, and instrument response function. An adaptive-rate Monte Carlo retrieval code is then used to tomographically reconstruct the neutron source function. Effective reconstruction of the neutron source function in energy and time-of-emission is shown to be feasible with multiple collinear nTOFS, and constraints on detector precision and placement are put forth.
This work is supported by Lawrence Livermore National Laboratory under Contract B652285. A. DeVault is supported by NNSA LRGF under Grant No. DE-NA0003960