The next-generation magnetic recoil spectrometer for measurements of the absolute neutron spectrum at Pacific Fusion's demonstration system.

The MAgnetic Recoil Spectrometer (MARS) will be used to diagnose neutron spectra from deuterium-tritium fusion experiments at Pacific Fusion's demonstration system, providing time-integrated measurements of yield (13-15 MeV), ion temperature, fuel and liner areal density, and alpha heating. Two design advances distinguish MARS from prior magnetic recoil spectrometers. First, redesigned ion optics reduce the focal-plane length to a size compatible with a silicon strip detector, making MARS the first inertial confinement fusion magnetic recoil neutron spectrometer with an electronic readout, enabling real-time data analysis and per-channel time-gating. Second, a dual-foil geometry (half CH2, half CD2) enables simultaneous spectral measurements of recoil protons and deuterons in a single shot, expanding neutron energy coverage to ∼6-20 MeV. The conceptual design of the instrument, synthetic modeling framework, and shielding strategy are described. Monte Carlo transport and ion optics simulations show signal-to-background ≫10 across the spectrum except for the alpha knock-on tail, with per-channel electronic time-gating providing additional background suppression. The open-source simulation framework additionally serves as a synthetic diagnostic for forward fits of parametric plasma models, showing good agreement with the true parameters.