From what I can find, presently the only known means of producing antimatter consist of directing particle accelerators at various targets, and only infrequently getting positrons or anti-protons as byproducts of particle interactions.
Assuming a sufficiently large source of energy: Have more efficient means of producing antimatter been conceived?
And based on these is there a known upper bound on antimatter production efficiency? For example:
If all known theory requires that an anti-particle be produced with its particle pair, then efficiency will always be under 50%.
Are there known processes for "transmuting" matter into anti-matter? If so, do these allow for a theoretical upper-bound on production greater than 50% of input energy?
Magnetic field containers can be used for the storage of antimatter. Also, increasing the size of the particle accelerator will increase the volume output of antimatter, the key fuel in warp drives. The size of particle accelerator can be 100 miles around by diameter and 30 stories high.