On rapid plasma rotation

The conditions under which rapid plasma rotation may occur in a general three-dimensional magnetic field with flux surfaces, such as that of a stellarator, are investigated. Rotation velocities comparable to the ion thermal speed are found to be attainable only in magnetic fields whose strength B depends on the arc length l along the field in approximately the same way for all field lines on each flux surface ψ, i.e., B≃f(ψ,l). Moreover, it is shown that the rotation must be in the direction of the vector ∇ψ×∇B.