**3.2 MHD behavior and stability of oblate FRCs**

In the MRX device, the stability of oblate FRC plasma was investigated. The plasma parameters are electron density of 0.5 - 2 x 1020 m-3, ion temperature of ~18 eV, *B*z of 300 - 200 G, *E* of 0.65 - 0.35, *s* of 1-3, and *s E* of 2-5 (Gerhardt et al., 2006). Without a passive stabilizer (a hollow conducting center conductor), tilt and shift instability (*n* = 1 mode of Fig. 10 (a) and (b)) often appeared. The tilt mode limits the plasma lifetime. The tilt instability can be mitigated by either including a passive stabilizing conductor or forming highly oblate plasmas with a strong mirror field. Without the center column, the growth of the shift mode is reduced, apparently by the large magnetic fields on the outboard side of device. Large perturbations (*n* = 2 and 3) may still remain after passive stabilization is applied ((b) and (c) of Fig. 10 (a)). These perturbations have the characteristics of co-interchange modes, which have never been observed in conventional oblate FRCs. Such modes cause the early termination of the oblate FRC shape. These co-interchange modes can be stabilized in oblate plasma with a high mirror ratio, and this produces an FRC with maximum configuration lifetime.
