> Following Fig. 9, beam core and plasma core configurations can produce direct thrust by directing the charged particles produced into an exhaust beam using a magnetic nozzle. Gas core systems use the energy released from the reaction to heat a gas that is exhausted for thrust. Finally, solid core configuration heats a metal core like Tungsten that acts as a heat exchanger to a propellant that is then exhausted from a regular nozzle.
Not the same paper, but goes into more detail.
https://www.sciencedirect.com/science/article/pii/S266620272...