Studies of Modulated Knock-on Ion Spectra from the Charged Particle Mode Magnetic Recoil Spectrometer in Direct-Drive ICF Implosions at NIF

Modulation in charged-particle mode magnetic recoil spectrometer (MRS) energy spectra has been observed for various direct-drive targets at NIF with both CH and SiO₂ shells. A leading hypothesis for these modulations is the ion beam self-modulation, which is studied through full and hybrid particle-in-cell (PIC) simulations. Three beam-plasma instabilities under consideration include two-stream, beam-Weibel, and filamentation instability. Realistic implosion conditions informed by SAGE simulations and hot electron scaling set the simulation parameters. Full PIC simulations at millimeters from the critical surface indicate modulations characterized by discrete peaks rather than the finer, high-frequency oscillations observed in experimental data. Current work aims to bridge this difference through large-scale hybrid PIC simulations near the critical surface, which treat electrons as a massless fluid with the EM field evolved through Ohm’s law. The fidelity of the hybrid simulation will be verified through comparison with full PIC simulations at distances close to the critical surface.