Investigation of relaxation dynamics of magnetic plasma bubble expansion*

The PBEX experiment (Plasma Bubble Expansion Experiment) investigates the dynamics of the interaction between a hot, dense and magnetized plasma launched by a compact coaxial gun and a low-density background magnetized plasma generated by a linear plasma device. The study of relaxation dynamics resulting from this interaction is of significant interest as it offers a model for understanding the phenomena such as extragalactic radio lobes solar coronal mass ejections. Under specific operating regime, the bubble plasmas generated by the coaxial gun exhibit Spheromak-like equilibrium. However, observations made by a fast-framing camera reveal that when the bubble plasma is injected into a background magnetized plasma instead of a vacuum, it undergoes asymmetrical deformation caused by magnetic tension forces. Subsequently, the bubble expansion into the background magnetized plasma leads to the growth of the magneto-Rayleigh-Taylor instability at the interface between two plasmas and finger-like structures are observed at the upper edge of the bubble. To characterize the density, temperature, and magnetic field configuration at the positions where the MRT instability is detected, a triple Langmuir probe and a B-dot magnetic probe array are utilized. And the estimated growth rate of these "fingers" derived from the experimental data aligns with the characteristics of the MRT instability.