Magnetic Relaxation in Coaxial Helicity Injection Discharges in the HIT-II Spherical Torus

Abstract

Recent Coaxial Helicity Injection (CHI) studies using the Helicity Injected Torus device (HIT-II) have produced discharges with toroidal plasma currents up to 350 kA. Direct measurements using an internal magnetic probe array show a total poloidal flux in the confined plasma significantly greater than the vacuum injector flux, confirming both the unambiguous presence of a closed-flux core region and the generation of poloidal flux through magnetic relaxation. The key innovation for producing these discharges is a sufficiently high ratio of CHI injector current to toroidal field current, quantified as the dimensionless product λINJd greater than 0.3, where d is the effective distance between the electrodes and λINJ is the inverse magnetic scale length associated with the CHI injector (defined as μ0IINJ/ψINJ, where IINJ is the injector current and ψINJ is the poloidal injector flux connecting the electrodes). This critical value of λINJ d is understood as a balance between two competing processes: reconnection in the injector generating closed poloidal flux, and resistive decay of the closed flux and current. A value of λINJd > 0.3 corresponds to a rate of relaxation greater than the rate of resistive decay, allowing the build-up of poloidal flux and toroidal current.