Practical Interplanetary Propulsion via One-Plug Collimated Nonthermal p-¹¹B Fusion Exhaust: A Concept Paper on Proton-Boron (p-¹¹B) Propulsion for Near-Term Research Prioritization

This concept paper identifies a performance regime for fusion propulsion that has not been systematically explored: direct magnetic collimation of non-thermal proton-boron (p-¹¹B) fusion products, exploiting the asymmetric entropy structure of the three-alpha final state. We argue that p-¹¹B products occupy a region of phase space where entropy is concentrated primarily in angular distribution, while momentum magnitude remains narrowly bounded at the MeV scale. Under Liouville's theorem, magnetic nozzle geometries can trade angular dispersion for spatial dispersion without reducing phase space volume. Because propulsion depends only on axial momentum and is indifferent to the spatial spread of the exhaust, this trade is favorable in principle. We revise the reference architecture to a one-plug open magnetic nozzle: the p-¹¹B alpha source is placed just aft of a high-field magnetic plug and at the inception of a monotonic aft-diverging nozzle. Aft-born products enter the nozzle directly; forward-born products outside the plug loss cone are mirrored aft; the small loss-cone leakage is intercepted by a localized alpha direct-conversion assembly for auxiliary electrical power. This topology avoids the two-mirror trapped-particle population that arises if products are born between a forward mirror and an aft nozzle throat. Adiabatic analysis indicates that collimation is physically effective at aft mirror ratios up to approximately 10, where particles undergo non-adiabatic detachment with most perpendicular energy already converted to parallel motion. We introduce a total propulsive energy fraction ψ that includes capture, nozzle collimation, leakage, thermalization, neutralization, radiation, and recirculation losses. For a 10-day Earth-Jupiter brachistochrone transfer, ψ values of 0.7, 0.4, and 0.3 correspond to estimated propellant mass fractions of approximately 30%, 37%, and 42%, respectively. These estimates define a target design space, not a demonstrated performance level. The paper does not propose or validate a p-¹¹B ignition scheme; it asks whether, given a source of nonthermal charged fusion products, a magnetic exhaust architecture can convert them into useful directed momentum.