A field-reversed configuration (FRC) is a confinement system of plasma consisting of poloidal field only. The FRC plasma is normally produced by a negative-biased theta-pinch method. In order to confine stably the plasma in the center of the theta-pinch coil, a set of mirror coils is installed at both ends of the coil. However, non-uniformity of the plasma along the coil emerges at the formation phase because of earlier pinch of the plasma at the mirror regions than that at the center region. The non-uniformity causes a strong axial contraction starting from the mirror region towards the center region. After the contraction, the plasma rebounds in the center region and expands to the coil ends again. Since this motion is repeated few times, a quick attainment of an equilibrium of the FRC is retarded. Moreover, it is known that a wobble motion of the FRC around the geometrical axis of the coil is triggered due to asymmetrical magnetic reconnection between the negative-bias field and the main field at the mirror region. The wobble motion will disturb an on-axis translation of the FRC from the formation region to the burning region in a reactor.
In order to mitigate the axial contraction and control the wobble motion in a low level, a set of resistive metal liners is installed around the vacuum vessel made of a quartz tube in the mirror regions. Since the resistive metal liners have a comparable skin time (3~4 micro-second) with the rise time of the main field (3.5 micro-second), the strength of the main field is weakened under the resistive metal liners at the formation phase. For example, the mirror ratio can be controlled from 1.3 to 0.9~1.0 by the skin effect of the metal liners. However, the mirror ratio returns to 1.3 at the equilibrium phase because the skin time is much less than the e-holding time (120 micro-second) of the crowbarred main field.
It is found that the axial contraction speed is reduced and the rebound motion becomes ambiguous when the resistive metal liners are installed. Then, the equilibrium phase starts just after reaching the first axial contraction to the midplane of the coil. The wobble motion also becomes weak and the maximum deviation of the FRC axis from the geometrical axis of the coil is controlled less than one-third of the value without any liner.