Current amplification in SPHEX operated as a spherical tokamak

We report on further studies of current drive in the SPHEX spheromak device when operated as a spherical tokamak. Previous work has demonstrated that the current amplification factor, defined as the ratio of the toroidal plas ma current to the driving gun current, increased linearly with the applied toroidal field. Results are presented from a more detailed analysis of this scaling for a fixed gun current of 60 kA, utilizing direct internal measur ements of current density and equilibrium modelling of internal magnetic fi elds. The data indicate that the increase in the current amplification fact or is largely the result of changes in the wavelength of the helical distor tion of the open flux (identified with the global n = 1 mode) connected to the helicity injector: the increase in toroidal current on closed flux surf aces is much more modest. We discuss changes in current profiles at differi ng applied toroidal fields and demonstrate the operating regime over which helicity-injected current drive is effective in SPHEX. The implications of this work for proposed helicity injection schemes in the next generation of spherical tokamaks are discussed.