Distributed Capacitance Optimizing and Core Snubber Design for CRAFT NNBI
Bangxin Liu, Z. Liu, Caichao Jiang, S. Liu, Junjun Pan, Sheng Chen, Yuchen Qu, Yuanlai Xie · 2024
The NNBI (Negative Ion Source based Neutral Beam Injector) is an important part of CRAFT (Comprehensive Research fAcility for Fusion Technology, CRAFT), the goal is to achieve technical validation of a $2 \mathrm{MW} /$ @100 s neutral beam injector with the acceleration voltage of DC -400 kV. The core snubber is the key equipment to protect surface damage of accelerator grides from the breakdown arc between accelerator grides when the energy stored in the distributed capacitance release through breakdown arc by providing a non-contact resistive and inductive devices in the circuit. Distributed capacitance parasitizes various parts of the NNBI power supply system, but are mainly concentrated on TL (Transmission Lines, TL) that are tens of meters long, as well as large HVD (High-Voltage Deck, HVD) and isolation transformers. The finite element method was used to optimize the structure of the TL and HVD with the purpose of distributed capacitance and stored energy reducing. The core snubber was designed according to the distributed capacitance, acceleration voltage and ferromagnetic material characteristics. The amorphous was selected as the ferromagnetic material with the advantages of high saturation magnetic flux, high magnetic permeability and lower price per kilogram. Amorphous was cut into a strip with a width of 25 mm and a thickness of 20 um, and wound into rings with a duty ratio of 0.7. Then 88 amorphous rings with 1400 layers of winding are connected in series to form a core snubber to satisfy the situation of 8-10 nF distributed capacitance and 400 kV. The core snubber was tested at a specialized bench and achieved about 40% reduction in peak breakdown current and significant oscillation reduction.