Temporal Evolution of Toroidal Ion Flow in Field-Reversed Configurations Generated by the Field Reversed Theta Pinch

Field-reversed configurations (FRCs) exhibit toroidal ion flow immediately after formation, with velocities reaching the Alfvén speed [1]. Due to the large ion Larmor radius relative to the plasma radius, such flow can excite rotational instabilities with toroidal mode number n = 2. The ion spin-up, driven by quasi-neutrality-maintaining mechanism within the plasma, can influence electron spin-up, thereby affecting the overall current distribution and rotational structure.

In the Nihon University Compact Torus Experiment-III (NUCTE-III) [2], FRC plasmas are produced via the field-reversed theta pinch (FRTP) method. The plasma initially exhibits toroidal flow in the paramagnetic direction, followed by spin-up in the diamagnetic direction. This behavior is similar to that observed in the FAT-CM device, where FRCs are formed through collisional merging [3]. One proposed driving factor for the spin-up is the internal plasma dynamics, rapid magnetic compression during formation [4], although the detailed physical processes remain under investigation.

This study investigates the generation and temporal evolution of toroidal ion flows in FRC plasmas formed by the FRTP method, and discusses possible underlying mechanisms of FRC spin-up.