Design of a finline antenna for current drive in TST-2

• Published in: Fusion engineering and design 2022-05, Vol.178 (C), p.113094, Article 113094
• Main Authors: Watanabe, O., Ko, Y., Tsujii, N., Takase, Y., Ejiri, A., Shinohara, K., Peng, Y., Iwasaki, K., Yamada, I., Yatomi, G., Moeller, C.P., Peng, Y.-K.M.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Antenna arrays; Antenna design; Antennas; Electromagnetic surface wave; Electromagnetic surface waves; Finlines; Lower-hybrid current drive; Microwave transmission; Nuclear Science & Technology; Periodic slow wave structure; Plasma currents; Refractivity; Spherical tokamak; Surface waves; Transmission lines; Wave propagation; Waveguides
• ISSN: 0920-3796; 1873-7196
• DOI: 10.1016/j.fusengdes.2022.113094

•A novel antenna is designed for plasma current drive using lower hybrid wave.•The antenna consists of a periodic fin array and was named finline antenna.•The wave propagation on fin array is used the electromagnetic surface wave.•The propagation wave on the fin array and radiation wave in the plasm is simulated by the COMSOL simulation code.•At perform the antenna operation test to the TST-2 plasma, increase of the X-ray was confirmed. A novel “finline” antenna was designed to excite lower-hybrid waves (LHWs) for plasma current drive at 2.45 GHz in the TST-2 spherical tokamak. A periodic fin array acts as a transmission line for electromagnetic surface waves. Cold tests confirmed propagation of the electromagnetic surface wave on the fin array which was dominantly Transverse Magnetic (TM). Unlike the conventional waveguide array “grill” antenna, the finline antenna requires only two ports to feed all the elements, and calculations show that there is little reflection regardless of the plasma conditions in front of the antenna. Characteristics of the microwave transmission on the fin array were investigated using a three-dimensional full-wave simulation and the fin geometry was tuned to achieve the desired refractive index. Coupling to LHWs under typical TST-2 scrape-off layer conditions was confirmed by the simulation. To demonstrate LHW excitation, the finline antenna was energized in the presence of a plasma generated by another 200 MHz LH antenna. The x-ray radiation showed strong enhancement during the finline antenna pulse which indicated successful generation of fast electrons by LHWs.