Solenoid-free Plasma Startup in NSTX using Coaxial Helicity Injection

The favorable properties of the Spherical Torus (ST) arise from its very small aspect ratio. However, small aspect ratio devices have very restricted space for a substantial central solenoid. Thus methods for initiating the plasma current without relying on induction from a central solenoid are essential for the viability of the ST concept. Coaxial Helicity Injection (CHI) is a promising candidate for solenoid-free plasma startup in a ST. Recent experiments on the HIT-II ST at the University of Washington, have demonstrated the capability of a new method, referred to as transient CHI, to produce a high quality, closed-flux equilibrium that has then been coupled to induction, with a reduced requirement for transformer flux [R. Raman, T.R. Jarboe, B.A. Nelson, et al., Phys. Rev. Lett., 075005-1 (2003)]. An initial test of this method on NSTX has produced about 140 kA of toroidal current. Modifications are now underway to improve capability for transient CHI in NSTX.