Formation of Transient High-$\beta$ Plasmas in a Magnetized, Weakly Collisional Regime

Abstract

We present experimental data providing evidence for the formation of transient ($\sim 20~\mu$s) plasmas that are simultaneously weakly magnetized (i.e., Hall magnetization parameter $\omega \tau > 1$) and dominated by thermal pressure (i.e., ratio of thermal-to-magnetic pressure $\beta > 1$). Particle collisional mean free paths are an appreciable fraction of the overall system size. These plasmas are formed via the head-on merging of two plasmas launched by magnetized coaxial guns. The ratio $\lambda_{gun}=\mu_0 I_{gun}/\psi_{gun}$ of gun current $I_{gun}$ to applied magnetic flux $\psi_{gun}$ is an experimental knob for exploring the parameter space of $\beta$ and $\omega \tau$. These experiments were conducted on the Big Red Ball at the Wisconsin Plasma Physics Laboratory. The transient formation of such plasmas can potentially open up new regimes for the laboratory study of weakly collisional, magnetized, high-$\beta$ plasma physics; processes relevant to astrophysical objects and phenomena; and novel magnetized plasma targets for magneto-inertial fusion.