Sustainment of elongated field reversed configurations with localized rotating magnetic field current drive

The effects of rotating magnetic field (RMF) antenna length on the sustainment of RMF driven field reversed configurations (FRC) have been investigated in the Translation, Confinement, and Sustainment (TCS) facility. FRCs could be sustained with axial lengths well beyond the RMF antenna, presumably due to induced axial flow. The FRC length, l s , in TCS was weakly dependent on the antenna length, l a , with l s ∼2 l a being achieved for the shortest antenna employed. Reducing antenna length led to more stable operation, with the FRCs less prone to rotational instabilities. Shorter antenna lengths also resulted in increases in the plasma temperature. At larger ratios of l s / l a the RMF had to supply a greater torque per unit length, and it did this by increasing its penetration into the FRC, brought about automatically by a slight decrease in plasma density. Optimal overall current drive efficiency occurred at values of l a /2r s of about 1.3, where r s is the separatrix radius, and values of l s / l a of about 1.5. In contrast, operation with the RMF antenna covering the full TCS confinement chamber, l a /2r s ∼3.5, led to non-steady behavior with the FRC length decreasing during the discharge.