Monte Carlo Simulations of Suprathermal Enhancement in Advanced Nuclear Fusion Fuels

Abstract

Suprathermal fusion reactions, initiated by energetic particles slowing down and scattering in dense plasmas, can modify the burn dynamics at inertial confinement fusion (ICF) regimes. A 0D time-dependent Monte-Carlo code has been developed to assess the suprathermal energy gain from fast fusions in DT, deuterium, $^{11}$BH$_3$ and $^{11}$BHDT fuels. It incorporates modified Li-Petrasso stopping powers, thermal broadening of cross-sections, anisotropic nuclear elastic and neutron elastic scattering, and a physical model for the p$^{11}$B alpha-particle spectra. Results show that earlier predictions of suprathermal criticality in pure deuterium are overestimated by more than an order of magnitude; no realistic density-temperature regime supports a self-sustaining chain reaction. Only DT demonstrates a critical regime provided there is no neutron leakage. Fast protons in $^{11}$BH$_3$ have an optimum energy of 4 MeV for maximising suprathermal enhancement. In this case the additional energy from fast fusions is unlikely to exceed 40% of the initial proton beam energy. The possibility of an alpha-particle-driven "avalanche" mechanism is ruled out since the ionic stopping is dominated by collisions involving small energy transfer. Suprathermal multiplication processes are dominated by neutron-driven ion up-scattering and likely play a limited role in purely aneutronic fuels.