Tri-Alpha Energy are working towards the fusion reactor system that forms the heart of this Propulsion System design from 2004: Colliding Beam Fusion Reactor Space Propulsion System
Naturally, being commercially minded, TAE want to target the terrestrial energy market, but once low-neutronicity fusion power is on offer, the application to space needs is practically unlimited. Living and working in space needs power-dense power supplies – the more watts a source packs in per kilogram, the better. So while nuclear fission reactors are energy dense, their specific power (W/kg) is limited by the amount of shielding required and how hot the solid core can get while still being solid. Fusion reactors have plasma cores and are only limited by shielding issues. High neutronicity reactions require a LOT of shielding or a lot of distance. The 14 MeV neutrons from deuterium-tritium fusion, for example, are unhealthy over very large distances and will be a heating issue for any conceivable shielding apparatus.
TAE’s preferred reaction, the Proton-Boron 11 fusion/fission reaction, has very low neutronicity. The main reaction fuses a proton and a Boron 11 nucleus to make an excited Carbon 12, which fissions into three helium nucleii (Alpha particles, thus the name of the company.) Occasionally the Carbon 12 de-excites by emitting a powerful gamma-ray. Less occasionally an alpha particle reacts with a Boron to produce Nitrogen 14 and a neutron, or the proton and Boron reaction can make Carbon 11 and a neutron. These reactions represent less than 1% of the total reactions, but produce a significant source of neutrons and gamma-radiation, which need to be handled carefully.
Assuming our TAE fusion reactor is shielded sufficiently, what sort of performance does it offer? The paper offers one potential design, with 100 MW total power and 50.1 MW jet-power. Assuming a perfectly collimated jet, that means a thrust of 28.1 newtons is achieved for an effective exhaust velocity of a bit over 3,600 km/s. That’s perfect for interplanetary operations, even if it’s a bit low for interstellar missions.