Transient coaxial helicity injection for solenoid-free plasma startup in HIT-II

The favorable properties of the spherical torus (ST) arise from its very small aspect ratio. Methods for initiating the plasma current without relying on induction from a central solenoid are essential for the viability of the ST concept. In steady state tokamaks, the central solenoid can be dispensed with if suitable methods for initiating the plasma current are on hand. Coaxial helicity injection (CHI) is a promising candidate for solenoid-free plasma current startup in STs and tokamaks. Experiments on the Helicity Injected Torus (HIT-II) machine at the University of Washington [T. R. Jarboe, Fusion Technol. 15, 7 (1989)] have demonstrated the capability of a new method, referred to as transient CHI, to produce a high quality closed-flux equilibrium that has been successfully coupled to induction demonstrating that this new plasma current startup method is compatible with the conventional inductive method. This paper presents physics requirements for implementing this method in STs and tokamaks and supporting experimental results from the HIT-II device.