Magnetic containment field

From Memory Alpha, the free Star Trek reference

The magnetic containment field is a field that prevents the contact of anti-matter with normal matter in a warp core. The magnetic containment field is produced by the magnetic constrictor coils in the core and concentrates the anti-matter to the center of the reaction chamber. It is also produced by the power transfer conduits and near the antimatter pods. Also, the field is needed to propel anti-matter up to the reaction chamber.

If the field were to collapse or fall below 15% of its maximum integrity, the starship would be destroyed. Additionally, the magnetic field is vulnerable to quantum filaments, as they charge the vessel like a live electric wire, thus weakening the field by the inherent electromagnetic field electricity produces. (TNG: "Disaster")