Flux amplification in Helicity Injected Torus (HIT–II) coaxial helicity injection discharges

Abstract

Recent coaxial helicity injection (CHI) studies using the Helicity Injected Torus device [Redd et al., Phys. Plasmas 9, 2006 (2002)] have produced discharges with measured toroidal plasma currents up to 350kA and direct evidence of both poloidal flux amplification and toroidal current buildup, resulting from a steady process on millisecond time scales. Internal magnetic probes directly measure the poloidal flux amplification, and also measure a strong paramagnetism. Equilibrium reconstructions of these flux amplification discharges, using only surface magnetics, match the internal probes and multipoint Thomson scattering, and show current-profile relaxation during toroidal current ramp up. The criteria for producing flux amplification include both a sufficiently thin electrode-driven edge region and a large magnetic shear in the CHI injector region, which allows injector reconnection activity to overcome resistive decay and build up a closed plasma core. If the interelectrode distance d is small, then both criteria can be easily met. If d is comparable to the device minor radius, then the injector must be overdriven to produce significant flux amplification. The physics basis for generating CHI discharges with the theoretically maximum toroidal current is now understood, and this basis can be used to guide CHI experiments in any axisymmetric device.