Hi Lee

Positrons were the first antimatter ever made in particle accelerators (old name was "betatron") and they can be made more efficiently, and stored more easily, than anti-protons. But if they can be harvested directly from the radiation belts even better - lower power costs and they're already in space.

Another study at NIAC describes a system to make and use positrons for rocket propulsion - the gamma-rays produced by positron annihilation can be converted to useful heat (thermalised) more easily than anti-proton annihilations, which spit pions and neutrinos before they make gamma-rays.

Michio Kaku claims he made a betatron as a kid - but he's an anti-nuke lunatic, so he's not liked around here.

Adam