Plasma stability of a nearly spherical field-reversed configuration

A nearly spherical field‐reversed configuration (FRC) is created by employing a combination of a double‐cusp bias magnetic field and laser‐produced plasma, and its plasma stability is studied. The FRC equilibrium configuration is found to last for 20–25 μsec, which is not determined by any plasma instability in the present experiment and is estimated to be longer than the Alfven time by a factor of more than 40. That is, the occurrence of neither the n=2 rotational instability nor the active magnetohydrodynamic (MHD) (tilting) mode is observed during this time period. The stabilization of the rotational mode may be attributed to the effect of magnetic field line bending. Although the exact reason for stabilizing the tilting mode is not known, it has become clear that the geometry of an equilibrium FRC has no significant effect on its stability, based upon comparison with the other experimental results obtained thus far from theta‐pinch‐based FRCs with a very elongated cross section. It is suggested that the ion kinetic, nonadiabatic effect may be largely responsible for this stability.