Transient coaxial helicity injection plasma start-up in NSTX and CHI program plans on NSTX-U

Transient Coaxial Helicity Injection (CHI) in NSTX has generated toroidal plasma currents up to 300 kA. When induction from the central solenoid is then added to these discharges they maintain up to 300 kA additional current compared to discharges initiated by induction only. Standard inductive discharges in NSTX require 50% more solenoid flux than a CHI started discharge to reach 1 MA. The CHI-initiated discharges have low plasma density and normalized internal plasma inductance of 0.35 through the inductive ramp, typical of advanced scenarios planned for future STs. The Tokamak Simulation Code (TSC) has been used to understand the scaling of CHI generated toroidal current with variations in the external toroidal field and injector flux. These simulations show favorable scaling of the CHI start-up process with increasing machine size, consistent with theory. Scaling based on the analysis of experimental results and TSC simulations indicates the possibility for substantial current generation by CHI in the planned upgrade to NSTX. These results demonstrate that CHI is a viable solenoid-free plasma startup method for future STs.