Non-inductive current drive by DC helicity injection via magnetic flux rope merging in VEST

The Local Helicity Injection (LHI) technique is a non-inductive startup and current-drive method for Spherical Torus that deals with the limited-central-inductive-flux swing. The recent LHI technique begins by discharging 3D helical magnetic flux ropes. To use the LHI technique as a startup, the seed plasma should be formed by 3D magnetic flux ropes. With this seed plasma, the plasma current can be driven further via LHI and the Taylor relaxation process that convert the localized injected helicity into global helicity and macroscopic plasma current. In achieving the seed plasma from 3D helical flux ropes, it has been suggested by NIMROD simulation that the merging of flux ropes undergoing external kink mode plays an important role. However, experimental conditions for both the merging and LHI have yet to be suggested and validated, resulting in the possibility of operational failure. Here, the two magnetic field conditions are suggested for successful LHI operation. The total vacuum magnetic field strength is the key to the merging of flux ropes, resulting in a limited radial operating range in ST for a given discharge power. After forming seed plasma via the merging of flux ropes, the proper vertical field is the key to injecting helicity as it adjusts the proximity between seed plasma and flux ropes. Only when these two conditions are met, a peaked radial profile of ion temperature is observed with non-inductively driven plasma current, which means that magnetic reconnection is a crucial component of helicity injection with high proximity.