First demonstration of rotational transform control by electron cyclotron current drive in large helical device

An active current drive is a promising technique for improving plasma performances by controlling rotational transform and/or magnetic shear profiles in helical devices. A current drive based on electron cyclotron resonance heating is the most appropriate scheme for this purpose in terms of locality of driven current. Optimum conditions for an efficient electron cyclotron current drive (ECCD) in Large Helical Device (LHD) are being investigated using three-dimensional ray-tracing code, which can simulate propagation and power dissipation of electron cyclotron waves with large parallel refractive index. In the present experiment, inversion of directions of driven plasma current corresponding to injected ECCD modes was demonstrated successfully, and the results could be elucidated by the Fisch-Boozer theory. In addition, clear shifts of rotational transform were observed by motional Stark effect polarimetry. Our findings verified that the ECCD can be used as an effective actuator for controlling the rotational transform and magnetic shear profile in LHD.