[hobscoop]

Say you shoot the deuterium and tritium ions at each other in beams. Making two ions fuse isn't quite the same as hitting a fixed bullseye. Typically, physicists consider a 'cross section' for a reaction between two particles as a function of their relative velocity: a notional area that allows one to compute the probability of a reaction occurring, which may be thousands of times larger or smaller than what we typically think of as the physical size of the particles.

The cross sections for fusion reactions are significantly less than 1000th those of elastically bouncing off, even at the optimal velocities. Fusion reactions release about 1000 times[2] the typical energy needed by the reactants in order to get close enough for there to be any significant probability of reaction. So a one-pass beam system is always going to lose out to elastic scattering.

[1] https://en.wikipedia.org/wiki/Nuclear_cross_section [2] D + T releases 17.6 MeV of energy; typical fuel temperatures are 15 keV.