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.

The Monte Carlo method has been used to simulate the interaction of incident electrons of 1–10 keV with Si, Cu, Ag, and Au solids. In the simulation, the trajectories of secondary electrons inside the solids have also been included. The distributions of secondary-electron-emission sites on the surfaces of Si, Cu, Ag, and Au show multifractal behavior. For the same solids, the information dimension ${\mathit{D}}_1$, the correlation dimension ${\mathit{D}}_2$, and the minimum singularity ${\mathrm{\alpha }}_{\min }$ decrease with increasing energy of the incident electrons while the maximum singularity ${\mathrm{\alpha }}_{\max }$ increases. For the same primary-electron energy, ${\mathit{D}}_1$, ${\mathit{D}}_2$, and ${\mathrm{\alpha }}_{\min }$ increase with increasing atomic number of the solid, while ${\mathrm{\alpha }}_{\max }$ decreases. The Hausdorff dimension ${\mathit{D}}_0$ is almost unaltered in any case if the number of emitted secondary electrons is large enough. The results show that the higher the primary-electron energy and the lower the atomic number of the solid, the more dispersive is the distributed probability of secondary-electron-emission sites on the solid surface.

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 quench protection system in a large-scale superconductor test facility (LSTF) aims to protect the superconducting magnets from being damaged by long time and severe conducting current. A super-high pulse current from oscillation between a pulse inductor and a storage capacitor flows reversely into the main circuit to force the current in the main circuit cross zero, which ensures the reliable turn-off of the mechanical switch to protect the superconducting magnets. Furthermore, the pulse inductor also plays an essential role in adjusting the pulsewidth and the limiting pulse peak value. For the inductor, the pulse current is up to 130 kA and the rated inductance is 30 μH. To release the electromagnetic (EM) force largely and reinforce the structure and insulation properties, a circular multi-winding structure with epoxy resin-casting technique is presented. Based on the 2-D/3-D model built in finite element analysis software, the structure resist, deformation, heat, and EM environment of the design are simulated. The terminals are also optimized to minimize the damage from the EM force and extend the operation life of the inductor. In addition, a fatigue analysis is carried out to estimate the working life of the inductor and verify the reliability of the structure under the 130-kA pulse current working condition.