We experimentally investigate the generation and temporal evolution of toroidal ion flow in field-reversed configurations (FRCs) produced by the field-reversed theta-pinch method. Immediately after formation, the plasma exhibits rotation in the paramagnetic direction, which subsequently reverses to diamagnetic rotation within about 10 μs, reaching velocities up to 20 km/s. Complementary ion Doppler spectroscopy of the core region and Mach probe measurements in the scrape-off layer (SOL) reveal near rigid-body rotation in the core and microsecond-scale viscous transfer of angular momentum toward the SOL. The observed dynamics cannot be fully accounted for by conventional spin-up mechanisms such as particle loss, end-shorting, or flux decay. A comparison with collisional-merging FRCs demonstrates that rapid axial compression and the accompanying magnetic field reconfiguration drive an initial paramagnetic spin-up, after which the plasma self-organizes into stable diamagnetic rotation.
Kikuchi et al. (Wed,) studied this question.
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