Numerical investigation of electron cyclotron and electron Bernstein wave current drive in EXL-50U spherical torus

Non-inductive current start-up and maintenance are significant research topics for low-aspect-ratio spherical torus (ST). Electron cyclotron heating and current drive (ECH&CD) system with frequencies of 28 GHz and 50 GHz is prepared for experiments on EXL-50U ST. This paper provides a set of scheme for effective current drive using EC waves on EXL-50U. The fundamental X-mode (X-I) injection of 28 GHz EC wave is effective for current start-up in low-density conditions, reaching the maximum efficiency η ∼ 1.2 A/W at center electron density/temperature ne0 = 1 × 1017 m-3/Te0 = 0.1 keV. These results provide an important reference for the first experimental validation of the EXL-50U. When ne0 > 7 × 1017 m-3, 50 GHz EC wave can be used to drive the current at higher densities. Owing to the steep magnetic field gradients in the ST configuration, both fundamental and second-harmonic (X-I and X-II) resonance layers are present in the plasma. The X-II resonance becomes significant at higher densities because X-II absorption increases with ne0; however, the current-driven efficiency decreases rapidly because the power reaching X-I is reduced. Finally, the current driven by the O-X-EBW double-mode conversion scheme in an over-dense plasma is discussed. The estimated dimensionless current drive efficiency of electron Bernstein wave (EBW) current drive in the range of ζec = 0.2 ∼ 0.5 is expected.