Development of the magnetic-field system of Formosa Integrated Research Spherical Tokamak (FIRST)

Po-Yu Chang^1*, Chun-Yi Chen², Che-Men Chu¹, Bing-Huang He², Yu-Ting Hsieh¹, Zhi-Hen Ke³, Keng-Yu Lin³, Yu-Chi Lin³, Jie-Ling Liou⁴, Jean Nelson¹, Yung-Wei Pi¹, Wei-Fong Shen²

¹Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan city, Taiwan
²International Bachelor Degree Program on Energy Engineering, National Cheng Kung University, Tainan city, Taiwan
³Department of Physics, National Cheng Kung University, Tainan city, Taiwan
⁴Department of Photonics, National Cheng Kung University, Tainan city, Taiwan

* Presenter:Po-Yu Chang, email:pchang@mail.ncku.edu.tw

Magnetic-field system, including coils, current-driver modules, magnetic field measurements, magnetic flux measurements, and plasma current measurement, of FIRST is under development. Formosa Integrated Research Spherical Tokamak is the first tokamak that is being built in Taiwan. A Tokamak, a kind of system that achieves magnetic confinement fusion as the energy source, uses a magnetic field to confine hot plasma. Therefore, generating and measuring the magnetic field in the system is essential. Three different kinds of coils, toroidal-field coils (TFCs), poloidal-field coils (PFCs), and central solenoids (CS), are used to generate the magnetic field. In FIRST, there will be 16 TFCs providing a magnetic field of 0.1 T at r = 45 cm in the initial phase of the development. Further, 12 PFCs will be used to provide shaping magnetic fields such that FIRST's major and minor radii are 45 cm and 32 cm, respectively. Finally, CS provides a quick varying magnetic field for generating a loop voltage for generating plasma, driving the plasma current for confinement and for ohmically heating the plasma potentially to 100 eV as the first milestone. To drive those coils, currents up to 10s of kiloamperes with controllable and reversible current profiles must be provided. Therefore, current-driver modules, consisting of supercapacitors and switching modules combining the H-bridge configuration and pulse-width modulation, are being developed. To date, unidirectional current up to 7.5 kA and bidirectional current up to ±3 kA can be provided by the current-driver modules. In addition, three-dimensional (3D) B-dot probes were developed for measuring magnetic fields in FIRST. The 3D B-dot probe consists of three orthogonal coils so that the field's direction and amplitude are measured. The cross section and the number of turns of each coil of the 3D B-dot probe are 10 mm x 10 mm and 59, respectively. It is a small 3D B-dot probe that can be placed in many locations in FIRST to construct the magnetic profile. In addition, as the flux tube, big loop coils will be used to measure the magnetic flux in FIRST. A Rogowski coil will also be used to measure the plasma current, potentially up to 100 kA. By combining the field, flux, plasma-current, and density and temperature measurements from other groups, plasma profiles can be retrieved during experiments. The suite of magnetic field generators and diagnostics has been developed and will be shown in this talk.

Keywords: Tokamak, Magnetic confinement fusion, Current-driver module, B-dot probe, Rogowski coil