Long-lifetime current-driven rotating kink modes in a non-line-tied plasma column with a free end

We show the first experimental evidence for a magnetohydrodynamic kink instability in a current rope with one end that is free to move. This free end is partially insulated by sheath resistivity, so that the usual frozen in flux assumption for magnetohydrodynamics is violated in this region. The free end is therefore not line-tied to the axial boundary. We find the instability threshold is well below the classical Kruskal-Shafranov threshold, culminating in a long-lifetime saturated state. The presence of an axial flow further lowers the kink threshold and gives rise to a doppler-shifted frequency from rotation of the kink, where the eigenfunction is axially pushed in the flow direction. This lowered threshold may give rise to kink instabilities in situations with small magnetic twist that would otherwise be considered stable. Striking agreement with a theoretical analysis is demonstrated. The existence of a free end may be important for open flux tubes attached to the Sun, galactic jets associated with accretion discs, and spheromak startup.status: publishe