Get ahold of Robert Forward's book Indistinguishable From Magic, which has a nice long chapter on antimatter. Basically, the magnetic field would keep the atoms away from the walls of the container generating the field. The interior is a vacuum.
You should be able to find it at a used bookstore (it's paperback).
Available from www.barnesandnoble.com
Robert L. Forward coauthored another book "Mirror Matter: Pioneering Antimatter Physics", which is not available on B&N but they do list it.
Another book they list and carry is "Antimatter: The Ultimate Mirror" by Gordon Fraser