Investigating Instabilities of Plasma Bubbles In A Background Magnetized Plasma^*

The plasma bubble expansion experiment (PBEX) at the University of New Mexico exploits a compact coaxial plasma gun to launch a hot and dense plasma into a background low temperature and density in linear Helicon plasma in order to investigate the relaxation dynamics of the interaction of two such magnetized plasmas. Due to the frozen-in property of ideal MHD theory, the gun plasma carries magnetic field lines with it. These field lines eventually reconnect behind the plasma forming a self-closed spheromak. When the dense gun plasma drags the frozen-in magnetic field lines into the background magnetized plasma, the self-organized spheromak is deformed due to the magnetic tension force. This results in dynamics that are significantly more complex than spheromak relaxation in vacuum. For example, Magneto Rayleigh-Taylor (MRT) instability is observed, which appears to develop into finger like structures which then experience Kevin-Helmholtz instability. Magnetic B-dot probe arrays, triple Langmuir probes, visible spectroscopy, and fast multiframe imaging are used to characterize density, temperature, and B-field dynamics of the plasma bubble relaxation.. In addition, the preliminary simulations of PBEX using an extended MHD (XMHD) code, and comparisons with experimental data, will be presented .