Violent events such as supernova explosions or gradual processes such as mass loss from hot supergiant stars produce diamagnetic cavities: the interstellar magnetic field, frozen into ambient plasma, is excluded from a region around the source of expanding injected plasma. Particle-in-cell simulations illustrate the formation of such diamagnetic cavities in both the steady and impulsive cases. Plasma loss along the field means that the cavity always tends toward a cylindrical or barrel shape, even for very weak magnetic fields. Ion acceleration is expected out the ends of the barrel for all parameters of injection and initial field.