Stabilization of the Resistive Wall Mode and Error Field Modification by a Rotating Conducting Wall

The hypothesis that the Resistive Wall Mode (RWM) can be stabilized by high-speed differentially rotating conducting walls is tested in the laboratory. A solid rotating wall capable of routine operation at speeds of 300 km/h, equivalent to a magnetic Reynolds number (Rm) of 5, was designed, assembled, and fielded. Fast wall rotation is found to decrease the RWM growth rate and increase the RWM stable operation window to higher plasma current (Ip), thus demonstrating the stabilizing effect of the wall. The interaction of the rotating wall with on-axisymmetric fields (error fields) is found to lead to asymmetries in wall rotation direction. Analytic theory is used to demonstrate that as wall rotation increases the error field is not necessarily shielded but can instead destabilize the RWM. Error fields are also found to mediate MHD mode-locking bifurcations, which are observed for the first time in a linear plasma column. A torque balance model which includes the effect of the error field, plasma rotation, and wall rotation is developed and applied to the experiment. Asymmetry in wall rotation is also found in the torque balance, with one wall rotation direction eliminating the mode-locking bifurcation. Insertable probes are used to characterize the plasma and show that the column is diamagnetic at low Ip. This diamagnetic equilibrium enables the line-tying boundary condition at the device anode to be verified. At high Ip a persistent helical state is found and reconstructed using correlation techniques. Probes also illustrate that individual flux ropes from the device's discretized plasma gun array merge to form an axisymmetric profile within a short axial distance.