The stabilization of the tilt motion of spheromak plasmas in a conducting vessel by a rotational motion and the relaxation of its rotational velocity are investigated by means of a three-dimensional magnetohydrodynamic simulation. Two types of rotational velocity are considered; the velocity with a toroidal component only, and the velocity with toroidal and poloidal components. These components are assumed to be proportinal to the corresponding components of the initial magnetic field. In both cases, the initiation of the tilting instability can be delayed by plasma rotation, but for complete stabilization, a large rotational velocity such as Mach one is required. However, when the rotational velocity is larger than Mach one, a different kind of instability is found to take place. The analysis of the relaxation of the rotational velocity shows that the toroidal velocity component and the poloidal velocity component relax in different manners; i.e. the toroidal component is conserved, while the poloidal component decreases gradually with time.
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