The deuterium-deuterium (D-D) fusion reaction rate in a spherically co
nvergent ion focus is observed to significantly exceed the rate predic
ted by a collisionless flow model. However, a careful consideration of
ion-neutral collisions and the trapped neutral density in the cathode
account for the extra reactivity without invoking anomalous ion trapp
ing in the converged core region. This conclusion is supported by prot
on collimation measurements, which indicate that the bulk of the obser
ved reactivity originates outside the core region. In addition, a clas
sical flow model, where charge exchange collisional effects on the ion
and fast neutral distributions are included, provides fusion rate est
imates that we quantitatively consistent with the observed D-D fusion
neutron production rate.