Pressure gradient-driven plasmoid formation and ejection from magnetic island

The interplay between a magnetic island and a pressure gradient-driven plasmoid (p-plasmoid), which are nonlinearly excited by the interchange instability owing to the pressure gradient in magnetized plasmas, is investigated using two-fluid simulations. The pressure gradient drives interchange instabilities, and the instability excites a long-wavelength magnetic island and zonal E×B flows. A p-plasmoid, which is devoid of a pressure gradient, is found to form inside the magnetic island. The p-plasmoid begins to grow at the O-point of the magnetic island and then develops through reinforced pressure flattening and magnetic flux accumulation. The pressure flattening inside the magnetic island is less significant than that in the p-plasmoid, so that their propagation velocities in the poloidal direction are different. Specifically, the p-plasmoid propagates with the zonal E×B flow, whereas the island propagates with the sum of the zonal flow and the electron diamagnetic flow. As a result, the p-plasmoid is ejected from the magnetic island at the X-point. In the process of ejection, the magnetic field lines of the magnetic island are reconnected around the X-point, so that the p-plasmoid propagation slows during reconnection, and then the p-plasmoid is ejected from the island through the X-point and reenters the island. This series of events repeats intermittently and originates from the pressure gradient across the neutral magnetic surface, which is strongly flattened in the p-plasmoid compared to that in the magnetic island.