Solenoid-free plasma start-up in HIT-II and NSTX using transient CHI

A new method of non-inductive startup, referred to as transient coaxial helicity injection (CHI), has been successfully developed on the HIT-II experiment. In this method, a plasma current is rapidly produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields. The initial poloidal field configuration is chosen such that the plasma rapidly expands into the chamber. When the injected current is rapidly decreased, magnetic reconnection occurs near the injection electrodes, with the toroidal plasma current forming closed flux surfaces. On HIT-II, CHI-started plasmas outperform discharges initiated by induction alone and consume fewer volt-seconds. The method has now been successfully applied on NSTX for an unambiguous proof-of-principle demonstration of closed-flux current generation without the use of the central solenoid.