Experimental study of non-inductive current start-up using electron cyclotron wave on EXL-50 spherical torus

Although experiments have demonstrated a feasibility of the electron-cyclotron-wave (ECW) non-inductive current start-up, the physical mechanism of the current start-up remains unclear, particularly the role of the preferential confinement of collisionless tail electron currents (j(TPC)) and vertical charge-separation currents (j(VTF)). Therefore, j(TPC) and j(VTF) were observed using limiter probes during the ECW non-inductive current start-up on an ENN XunLong-50 (EXL-50) spherical torus to understand the significance of these values. It was found that j(TPC) increased and j(VTF) decreased during start-up, indicating a dominant role played by j(TPC) in this process. Furthermore, it was found that the product of the gas pressure and vertical magnetic field has a limit for the ECW non-inductive current start-up. Finally, we found that the ECW non-inductive current start-up can be realized only for sufficiently low vertical fields and gas pressures in EXL-50.