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 plasma 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 measurements of current density and equilibrium modelling of internal magnetic fields. The data indicate that the increase in the current amplification factor is largely the result of changes in the wavelength of the helical distortion of the open flux (identified with the global n = 1 mode) connected to the helicity injector: the increase in toroidal current on closed flux surfaces is much more modest. We discuss changes in current profiles at differing 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.