A scenario for dual-frequency electron cyclotron current drive near the mid-radii region of tokamak

Abstract Tokamaks are typically equipped with dual-frequency or even multi-frequency electron cyclotron resonance heating and current drive (CD) systems, with the 105/140 GHz dual-frequency system being a typical example. The dual-frequency system expands the range of toroidal magnetic fields applicable in tokamaks, enhancing its operational flexibility. However, different frequencies of electron cyclotron waves correspond to distinct magnetic field ranges, in most cases only one frequency is utilized. To enable the dual-frequency system to simultaneously and effectively drive off-axis currents near the mid-radii region of the tokamak to achieve negative central magnetic shear, this paper proposes a new CD scenario for dual-frequency electron cyclotron waves. Higher-frequency wave is launched from the top to drive off-axis currents in the mid-radii region, while lower-frequency wave is launched from the equatorial-plane to drive off-axis currents outside the mid-radii region. Based on this scenario, numerical simulations were conducted on the HL-3-like tokamak equipped with a 105/140 GHz dual-frequency system. The 140 GHz wave launched from the top effectively drive the off-axis currents in the mid-radii region via the Fisch–Boozer mechanism, exhibiting a wide current profile and high CD efficiency. The 105 GHz wave is launched through the equatorial-plane using the Ohkawa mechanism to drive off-axis currents outside the mid-radii region. Its CD profile is localized with lower efficiency. By scanning the toroidal field B T , the B T value range for both waves to effectively drive off-axis currents near the mid-radii region was determined to be: 2.1 T ⩽ B T ⩽ 2.35 T. Based on the characteristics of the current profiles, the power injection ratio between high-frequency and low-frequency wave should be greater than or equal to 2. Finally, the electron density and electron temperature ranges affecting this CD scenario were also studied and determined.