Pre-thermal mechanical energy partition in fusion events: A falsifiable XFEL diagnostic for primary damage state modeling

Standard radiation damage models for fusion materials assume that reaction energy manifests exclusively as the kinetic energy of reaction products, thermalizing via Coulomb cascades. However, persistent anomalies—such as non-classical fast-ion distributions and anomalous ⟨100⟩ dislocation loops in tungsten—suggest that these models do not fully capture the primary damage state. Extending a mass-defect framework recently applied to fission environments, we propose that D+D fusion releases a pre-thermal transient pressure pulse (10–50 GPa, <1 ps) preceding kinetic thermalization. This athermal mechanical component would fundamentally reshape atomic cascade topologies, thereby systematically biasing displacement per atom (dpa) and defect recombination estimates. We propose a directly falsifiable XFEL experiment utilizing a mass-equivalent differential pair: ⁴⁸TiD₂ (fusion-active) versus ⁵⁰TiH₂ (inert control) at ≈52 amu. This selection renders baseline acoustic impedance and hydrodynamic responses indistinguishable, cleanly isolating fusion-specific signals. Laser-accelerated 3–5 MeV deuterons drive volumetric D+D fusion in freestanding single-crystal membranes, while time-resolved 50-fs XFEL diffraction monitors the sub-picosecond lattice response. The hypothesis predicts anomalous compressive Bragg broadening (Δd/d ≈ 3×10⁻² to 1.5×10⁻¹) exclusively in ⁴⁸TiD₂ within a <1 ps window—a regime preceding electron-phonon thermalization. Indistinguishable diffraction profiles at XFEL sensitivity (≈10⁻⁴) will cleanly falsify the hypothesis. Conversely, reproducible excess strain will mandate an upward revision of mechanical energy deposition in primary damage models. If confirmed, this pre-thermal mechanical partition would reframe primary damage calculations and fundamentally alter predictive frameworks for radiation-tolerant fusion material design.