Stability of Magnetic Bubbles

We use electromagnetic hybrid (fluid electrons, kinetic ions) simulations to investigate the expansion of a dipolar magnetic field through plasma injection and the stability of the inflated bubble in the presence of plasma loss and other processes. The simulations consist of a stationary, magnetized, background plasma and a magnetic dipole. Another population of plasma is then injected at the center of the dipole and along its axis. The initial expansion of the bubble is described in the adjoining poster. When the dipole is weak, plasma injection results in the formation of a diamagnetic cavity without any appreciable change in the dipolar field. On the other hand, when the dipole field is strong enough to trap the injected plasma, the initial field is expanded and a magnetic bubble is formed. By stopping plasma injection after the bubble formation, we show that the bubble is quite stable and can survive for times much longer than the initial expansion phase. This stability is tied to the trapping of the injected plasma which provides the necessary currents to keep the bubble expanded. It is also found that plasma loss through magnetic reconnection results in depletion of the trapped population requiring periodic injection of plasma to keep the bubble inflated. Relevance of these results to radiation shielding of energetic ions will be discussed.