Single Flux Rope Dynamics in the Reconnection Scaling Experiment

A magnetic flux tube threaded by current is a flux rope with helically twisted field lines. In the Reconnection Scaling Experiment (RSX) we use a plasma gun to generate a single flux rope with a choice of axial boundary conditions. If this flux rope is driven hard enough, i.e., when $J\cdot B/B^{2}$ is larger than the kink instability threshold, we measure a helically distorted kinked structure. Rather than exploding in an Alfv\'{e}n time, this kink appears to saturate to a steady amplitude, helical, gyrating flux rope, which persists as long as the plasma gun sources the current. To understand it, we have experimentally measured three-dimensional profiles of various quantities of this flux rope. These quantities include magnetic field $B$, plasma density $n$ and potential $\varphi $, ion flow velocity $v_{i}$, so that current density $J$, electron flow velocity $v_{e}$ and electron pressure $P_{e}$ can also be derived. The full set of these quantities allows us to analyze the single flux rope dynamics systematically. Besides gyrating, we also find the flux rope has a spin center, around which the $J \times B-\nabla P_{e} \ne 0$ suggesting that there should be other forces for the radial balancing. We also find that within about 30 cm distance from the plasma gun, there is a reverse current moving around with the flux rope.