Traveling-wave antenna for the current drive using the fast wave in the lower hybrid-frequency range in the Versatile Experiment Spherical Torus (VEST).

Highlights • The development of an efficient external-current-drive device is required. • The Lower Hybrid Fast Wave can propagate into central region in higher-density plasma. • The combline-type traveling-wave antenna (TWA) is proposed for the heating and current-drive efficiencies. • The coupling research between the antenna and simulated plasma model is carried out. • The simulation and experiment results are compared regarding the coupling efficiency. Abstract An antenna using the fast wave in the lower hybrid-resonance frequency range is proposed for the significant attainment of the heating and current-drive efficiencies. For the operational stability regarding the change of the plasma load, the proposed antenna is designed as the combline-structure traveling-wave antenna. It consists of two tapered coaxial cables, two exciting straps of the folded-monopole structure, 12 radiating straps, 11 field isolators, and the Faraday shield. Regarding the vacuum load, the simulated and measured results show the wide impedance bandwidth (45 MHz, VSWR < 2) of the proposed antenna in conjunction with the traveling-wave-antenna structure and the folded-monopole exciting strap. Regarding the plasma load, the simulated results show that the proposed antenna provides the impedance bandwidth (35 MHz, VSWR < 2), and the fast wave is coupled with an input power from 51.5 − 90%. The measured and simulated coupling efficiencies are highly compatible in the Versatile Experiment Spherical Torus (VEST). According to the frequency-range changes, the simulated-variable range of the parallel refractive index (n ||) is from 3 − 4.5 in the operational frequency band of 475–510 MHz. [ABSTRACT FROM AUTHOR]