Recent Results from the HIST Spherical Torus Experiment

In the HIST (R=0.30 m, a =0.24 m, A=1.25) spherical torus (ST) experiments, the internal magnetic configurations and MHD activity of helicity-driven ST plasmas with peak toroidal plasma currents of about 100 kA have been investigated in the tokamak/spheromak hybrid operation regime. As the magnetic configuration approaches from a spheromak to a high q ST, the flux amplification rate and the ratio of closed poloidal flux to total flux decrease, although the total toroidal current increases substantially, because the n=1 fluctuations concerning current drive mechanism of the spheromak are stabilized by the external toroidal field. The intermittent increase in the toroidal current has been observed at the magnetic axis in the tokamak operation. The toroidal current may be driven through axisymmetric merging processes by the repetitive injection of tokamak-like plasmoid accelerated from the gun. The other interesting result is that the current density changes from a hollow profile to a peaked profile during the sustainment. The generation of this peaked profile can be explained by the OH transformer effect of the central open flux. In order to test the current profile control by combination of inductive current drive and helicity injection, we are installing an OH coil inside the center conductor, removing the flux conserver.