Estimating Neutron Down-Scattered Ratios Through Neutron Time-of-Flight Data on OMEGA Direct-Drive Implosions

Presently, areal densities of the deuterium–tritium (DT)-layered implosions on OMEGA are inferred from the neutron down-scattered ratio (DSR)—the ratio of ≈10- to 12-MeV scattered neutrons to the DT primary neutrons (≈13 to 15 MeV)—measured using a magnetic-recoil spectrometer (MRS)¹ and from the backscattered neutrons (≈3 to 4 MeV), which is measured using time-gated neutron time-of-flight (nTOF) detectors². DSR can also be measured using nTOF, which is routinely done at the National Ignition Facility (NIF). Carrying out nTOF measurements of the DSR on OMEGA, however, is significantly more challenging than on the NIF because of the very low areal densities found at the OMEGA scale; therefore, such a measurement has not been utilized until now. The presence of low-mode asymmetries in an implosion generally leads to asymmetries in the areal density^3,4.Recent advancements⁵ have been made in the 3D reconstruction of implosions on OMEGA5 utilizing areal densities, ion temperatures, and x-ray images obtained from various lines of sight. To further enhance this 3D reconstruction effort, additional areal-density measurements based on the DSR would provide significant augmentation. Here, we present a novel analysis of the nTOF OMEGA data to estimate the neutron DSR and benchmark it against the DSR inferred using the MRS detector.

1 J. A. Frenje et al., Rev. Sci. Instrum. 79, 10E502 (2008).

2 C. J. Forrest et al., Rev. Sci. Instrum. 83, 10D919 (2012).

3 C. J. Forrest et al., Rev. Sci. Instrum. 93, 103505 (2022).

4 V. Gopalaswamy et al., Phys. Plasmas 28, 122705 (2021).

5 K. M. Woo et al., Phys. Plasmas 29, 082705 (2022).