Analysis and Design of Particle Trap in Transmission Line for the CRAFT NNBI System

Based on the development of Experimental Advanced Superconducting Tokamak (EAST) neutral beam injectors system (NBI), Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) decided to establish a Negative-ion based Neutral Beam Injectors (NNBI) test-bed to meet the requirements of the Comprehensive Research fAcility for Fusion Technology (CRAFT), which is a largescale of R&D project for China Fusion Engineering Test Reactor (CFETR). The High Voltage Transmission Line (HVTL) is an important part of the NNBI system, which connects the ion source system and the power supply system. However, some metal particles will be produced in the production and operation of HVTL. The charged particles will move in HVTL or attach to the surface of insulators and electrodes, resulting in flashover or breakdown between internal and external electrodes, thus reducing the insulation performance of HVTL. Therefore, some restraining measures need to be taken to reduce the effect of metal particles on insulation performance, and particle trap is one of them. In this paper, the electric charge of particles and the force acting on them are analyzed, and the mechanism of the decrease of gap insulation strength of HVTL caused by the movement of metal particles is simply explained. Then the simulation model of particle trap is established, and the electric field distribution of particle trap and the working principle of trapping particles are analyzed. Finally, the design parameters of particle trap are optimized, which provides a reference basis for the engineering manufacture and experimental research of particle trap and also provides some guidance for the follow-up engineering design of HVTL.