Modulated deuteron spectra observed with the magnetic recoil neutron spectrometer at the National Ignition Facility

The Magnetic Recoil Spectrometer (MRS) at the National Ignition Facility is used to measure the neutron spectrum from deuterium–tritium-fuelled inertial confinement fusion implosions via neutron–deuteron elastic scattering and magnetic dispersion of recoil deuterons. From the MRS-determined neutron spectrum, the yield ( upper Y Subscript n Y n Yₙ ), apparent ion temperature ( upper T Subscript i T i Tᵢ ) and areal density ( rho upper R ρ R ρ R ) are determined. However, anomalous energy modulations in recoil deuterons have been observed in several high-yield indirect drive experiments ( upper Y Subscript n Baseline tilde 10 Superscript 16 Baseline minus 10 Superscript 18 Y n ∼ 10 16 − 10 18 Yₙ~ 10¹⁶-10¹⁸ ). 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 a plausible mechanism behind energy modulations. Based on a statistical analysis of synthetic deuteron spectra, the modulation-induced errors are found to be within the errors of the determined upper Y Subscript n Y n Yₙ , upper T Subscript i T i Tᵢ and rho upper R ρ R ρ R values and thus do not have a significant impact on the MRS measurement.