Research on an unconfined spheromak and its current path in a magnetized coaxial plasma gun

The dynamics of plasma and ejection characteristics of spheromaks produced by a magnetized coaxial plasma gun are studied. By placing three magnetic probes at various axial positions, the distribution of current paths in the gun is found to vary in two distinct discharge modes. During the first half-period of a discharge, the plasma moves forward in the form of a current sheet, while the diffuse distribution of current paths in the second half-period indicates a deflagration mode. The evolution images and photodiode signals of the plasma show that only a single spheromak is ejected during the entire discharge. This is because the diffuse current paths reduce the $\mathit{J}\times \mathit{B}$ Lorentz force on the leading-edge plasma, which cannot be ejected from the gun. In addition, the existence of kinks in the plasma flow in two discharge modes proves that the instability is driven by ${\mathit{E}}_{\mathrm{r}}\mathbf{\times }{\mathit{B}}_{\mathrm{z}}$ drift, which causes rotation of the central column. Spheromak velocities increase linearly with discharge current amplitude but are inversely proportional to the gas puff mass. In ejected spheromaks, both toroidal and poloidal magnetic fields are axisymmetric, with field strength increasing with discharge current. During magnetic reconnection events, the toroidal electric field ${\mathit{V}}_{\mathrm{z}}\mathbf{\times }{\mathit{B}}_{\mathrm{r}}$ drives toroidal current that generates an additional poloidal field that amplifies the starting magnetic field in the spheromak plasma. This study clarifies the relationship between the formation of a single spheromak and the current distribution, and also provides a new way to optimize the spheromak's injection performance.