China Fusion Engineering Test Reactor (CFETR) is a new tokamak device, which is one design option under the consideration of the China National Integration Design Group employs superconducting magnets. The main target of this project is to build a self-sufficiency fusion engineering tokamak reactor with its fusion power is 50-200 MW. Toroidal Field (TF) coils, Center Solenoid (CS) coils and Poloidal Field (PF) coils, whose rated current range from 52.9 kA to 90 kA, produce the magnetic field for plasma generation and confinement. The core of Quench Protection Unit (QPU) is to divert the coil current into Discharge Resistor (DR) by using commutation technology, which should be determined by the rated current and voltage requirement of magnets. Different kinds of technologies for QPU are considered, and an advanced commutation technology composed of a mechanical Bypass Switch (BPS), Vacuum Circuit Breaker (VCB) and LC commutation circuit based on artificial current zero technology has been worked out to meet the current and voltage requirement (10 kV/100 kA). To provide a faster and more reliable removal of the energy stored in those superconducting coils, this paper detailed analyzes the feasibility of a commutation scheme based on artificial current zero technology by analyzing and simulating each stage of the commutation process under such large rated current and high voltage conditions. Both analysis and simulation results verified that the operating reliability of QPU can be guaranteed by this 100 kA DC commutation scheme.

In the quench protection (QP) process of superconducting fusion devices, the operating reliability and efficiency of power components are affected by stray parameters of the fast discharge resistor (FDR) system, especially the system stray inductance. In this paper, the fundamental condition of the QP operating process and the large power FDR system structure layout for the Large-scale Superconductor Test Facility (LSTF) are presented. The negative affected VCB, CPC circuit action and the energy discharging process are further analyzed. The stray inductance optimization method of the resistor module and structure connections are proposed to reduce the stray inductance value. Finally, the optimization results are presented by the Q3D module of the FEA software.

The Large-scale Superconductor Test Facility (LSTF) is an important part of comprehensive research facilities in support of China Fusion Engineering Test Reactor (CFETR). In order to protect the superconducting magnets from being damaged in quench accident, the Quench Protection System (QPS) should be designed to commutate the energy stored in the coils. The snubber circuit is used to reduce the reverse recovery current and voltage of thyristor in QPS, and it can impact the reliability and stability of the QPS. In general, the studies of snubber circuit are focused on the DC turn-off process of thyristor used in HVDC, which has small turn-off current. However, the turn-off current can reach to 100 kA in the QPS of LSTF. In this paper, a thyristor reverse recovery model is described, and the key parameters of reverse recovery current are obtained from the testing experiment. Based on the transient analysis of snubber circuit, a systematic approach is used to minimizing the resistance and capacitance of snubber circuit of QPS working condition is proposed. By the MATLAB simulation and comparison, minimum parameters of snubber circuit were obtained, and proved that the approach can improve the reliability and stability of system, while reduce the cost of QPS.

When a quench occurs, a fast discharge resistor (FDR) system is required to dissipate the huge energy stored inside a superconducting magnet during the quench protection (QP) process. The FDR system design is a key and hard factor in the QP process due to the necessity of protecting the superconducting magnet, especially under the condition of large current and high energy. In this paper, a novel conceptual design of the FDR system with parameter variance is put forward. The design can meet the stable operating requirement under extremely large currents up to 100kA. The system structure can also be altered to provide specific power characteristics to meet the QP energy transfer demand. In this paper the working condition of the QP process is introduced first and the structure design is illustrated. Then the layout, structure, material and stability of a single resistor module is described. Finally, the simulation process with FEA is presented to verify the feasibility of the FDR system design. By analyzing the conceptual design of the FDR system, it is shown that the conceptual design can be acceptable and economical for the energy transfer and consumption for superconducting magnets in fusion devices.

The quench protection (QP) system is crucial to transfer the quench energy out within specific time when the quench phenomenon occurs in the superconducting magnet. The fast discharge resistor (FDR) system in the QP system is responsible for transferring and consumption of the quench energy. In large-scale superconductor test facility (LSTF), with extreme quench parameters concerning about the large current up to 90 kA, high inductance up to 2 H, and huge energy up to 8.1 GJ, the structure stability, heat controlling, and power feasibility of the FDR system are hard to be guaranteed. Therefore, a novel FDR system based on forced-air cooling method is put forward and discussed. The FDR system is with the characteristic of effectiveness protection, safety operating, and economic efficiency when compared with previous systems. In addition, based on the designed concept, the analysis and test process of the FDR system are carried out to testify the effectiveness and veracity. In particular, the structure stability of the shape deformation and cooling method of the temperature rise are analyzed. With the proof of simulation and testing results, the FDR system can serve for the QP system development of the new fusion device, the LSTF.