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 ( jTPC) and vertical charge-separation currents ( jVTF) . Therefore, jTPC and jVTF 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 jTPC increased and jVTF decreased during start-up, indicating a dominant role played by jTPC 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.