Well, time for speculation...
In order to create a GigaTon killer we would at least need 100 kg or so antimatter.
If one EP is around 250 MW (guess thats the normal conversion), thats quite a bit in order to do some cooling and keeping a pretty strong magnetic field or even a gravitic field (yeah, thats traveller).
If imperial scientists would be able to collect or create neutral charged antimatter, cooled down to a solid state, this could be captured the gravitic way pretty well. Say an 20 cm diameter antimatter gold sphere would be around 80 kg (=1,6 GT - shiver) is not a problem.
Without gravitic technolgy you might have to use charged antimatter, which behaves unfriendly, as it does not want to stay together. So here I really got no idea how much AM could be trapped, but I will look around....
