Off-axis neutral beam current drive in KSTAR

The second neutral beam injection (NBI) system in the Korea Superconducting Tokamak Advanced Research (KSTAR) includes two ion sources that are injected with vertical angles, expected to drive off-axis currents intrinsically. The motional Stark effect (MSE) diagnostic has been detecting off-axis current drive in several plasma discharges where the second neutral beam is injected. A well-controlled experiment has revealed clear effects of obliquely injected neutral beam sources on the current density profiles. While the horizontally injected beam source contributes to the formation of centrally peaked current profiles, the other two drives more current off-axis by almost the same amount. This is consistent with the change of the loop voltage and the evolution of the internal inductance during the injection of the beam. Decomposition of the measured (total) current profiles into inductive (Ohmic) and non-inductive parts and the results are compared with the modeling from NUBEAM.