Ramp-Up of CHI-Initiated Plasmas on NSTX

Ongoing experiments on the National Spherical Torus Experiment have demonstrated ohmic transformer flux savings by initiating the discharge with transient coaxial helicity injection (CHI). The combined use of discharge cleaning of the CHI electrodes and use of lithium evaporation, along with the use of poloidal field coils, to produce a buffer flux that prevents arcs at the top of the device has been shown to reduce the radiation from low-Z impurities. Without such impurity reduction, CHI-initiated discharges could not have their plasma current increased by ohmic ramp-up; however, the CHI-initiated discharges with reduced low-Z radiation can be ramped up by induction and exhibit higher plasma current than discharges without the benefit of CHI initiation.