A Scalable H-Bridge Current-Drive System for Poloidal Field Coils of a Tokamak

Che-Men Chu^1*, Jean Nelson¹, Bing-Huang He², Keng-Yu Lin³, Zhi-Hen Ke³, Yung-Wei Pi¹, Chun-Yi Chen², Jie-Ling Liou⁴, Yu-Ting Hsieh¹, Yu-Chi Lin³, Wei-Fong Shen², Po-Yu Chang¹

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

* Presenter:Che-Men Chu, email:chemen.chu@gmail.com

Nuclear fusion offers a nearly limitless, carbon-free energy source by fusing light nuclei at extreme temperatures and pressures. Among various approaches, magnetic confinement fusion (MCF) in the tokamak configuration has achieved the most progress, using strong magnetic fields to confine plasma within a torus. In a tokamak, poloidal field coils (PFCs) generate magnetic fields that initiate, position, and stabilize the plasma. They require rapid, bidirectional, and precisely controlled current drive. We developed a scalable current-drive system for the Formosa Integrated Research Spherical Tokamak (FIRST), the first tokamak being built in Taiwan, designed to deliver kiloampere-level PFC currents with high reliability. The system employs a modular H-bridge built with Infineon half-bridge IGBT modules, each powered by two 2.33 F supercapacitors connected in parallel and charged to 360V as the energy source. A 1-kHz pulse-width modulation (PWM) enables arbitrary current profiles for plasma startup, ramp-up, and equilibrium control. To avoid simultaneous conduction of series-connected IGBTs, a short “dead time”, the interval when all switches are off, is applied before complementary switching. The Miller plateau defines the minimum required dead time, which is 10 μs. In addition, during turn-off, rapid collector–emitter voltage transitions generate electromagnetic-pulse (EMP) spikes that can induce circuit ringing, highlighting the need for proper snubber and shielding design. A transient voltage suppressor (TVS) diode is used. A single module provides a current up to 1.5 kA, while two modules connected in parallel are expected to deliver ≈3 kA. This capacitor-bank-fed H-bridge thus serves as a prototype architecture for the FIRST tokamak, combining scalability, EMP resilience, and precise current controllability for advanced plasma operation.

Keywords: magnetic confinement fusion, poloidal field coil, H-bridge