Equilibrium and stability studies of oblate field-reversed configurations in the Magnetic Reconnection Experiment

The equilibrium and stability of oblate field-reversed configurations (FRCs) have been studied in the Magnetic Reconnection Experiment [M. Yamada, Phys. Plasmas 4, 1936 (1997)]. In the absence of a passive stabilization, tilt and shift instabilities often become unstable, with the tilt in particular limiting the plasma lifetime. The tilt instability can be mitigated by either including a passive stabilizing conductor, or by forming very oblate plasmas. Large perturbations (n=2 and 3) may still remain after passive stabilization is applied. These perturbations have the characteristics of co-interchange modes, which have never been observed, and can lead to the early termination of the plasma. The co-interchange modes can be minimized through the formation of plasmas with a very oblate shape, leading to the maximum FRC lifetime. A code has been developed to calculate equilibria for these plasmas. A rigid-body model explains the improved stability of oblate plasmas to n=1 tilt modes. Numerical calculations indicate improved stability to n2 co-interchange modes for the very oblate plasma shapes.