Modulated deuteron spectra observed with the Magnetic Recoil neutron Spectrometer at the National Ignition Facility

Abstract: The Magnetic Recoil Spectrometer (MRS) on the National Ignition Facility is used to measure the neutron spectrum from deuterium-tritium fueled inertial confinement fusion implosions via n-d elastic scattering and magnetic dispersion of recoil deuterons. From the MRS-determined neutron spectrum, the yield ($Y_n$), apparent ion temperature ($T_i$) and areal density ($\rho R$) are determined. However, anomalous energy modulations in recoil deuterons have been observed in several high-yield indirect drive experiments ($Y_n\sim10^{16}-10^{18}$). These observations raise concerns about their potential impact on the MRS-inferred performance metrics. Analytic calculations and particle-in-cell simulations are used to examine the possible beam-plasma instabilities, which indicate the two-stream instability as the driving mechanism behind energy modulations. Based on a statistical analysis of synthetic deuteron spectra, the modulations-induced errors are found to be within the errors of the determined $Y_n$, $T_i$ and $\rho R$ values and thus do not have a significant impact on the MRS measurement.