CHI Research on Pegasus-III*

The spherical tokamak (ST) may require and the advanced tokamak would considerably benefit from the elimination of the central solenoid. \textsc{Pegasus}-III is a ST non-solenoidal startup development station under design and fabrication dedicated to solving the startup problem. On \textsc{Pegasus}-III, Transient and Sustained coaxial helicity injection (T- and S-CHI) will be explored, as well as possible synergies of CHI with local helicity injection and EBW heating and current drive. T-CHI has shown promising capability on the HIT-II and NSTX STs. However, in both these machines the vacuum vessel was electrically cut. For reactor applications a simpler biased electrode configuration is required in which the insulator is not part of the external vacuum vessel. To develop this capability \textsc{Pegasus}-III will use a double biased electrode configuration, which would a first of its kind for the reactor-relevant development of the CHI concept. The system is capable of generating plasma start-up currents at the levels that can be supported by the external poloidal field coils, which is estimated to be $\sim 300$ kA. The CHI design for \textsc{Pegasus}-III will be described.