Electrostatic analysis and preliminary design of transmission line for the CRAFT NNBI test platform

•The -400 kV transmission line is the link between the ion source and the power supply system.•The design elements of -400 kV transmission line are reported.•A 2-D simplified electrostatic analysis model of transmission line is established.•The effect of the layout of transmission line on the distribution of electric field is reported.•The optimal layout of -400 kV transmission line is determined. Based on the development of Experimental Advanced Superconducting Tokamak (EAST), Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) has launched the project of China Fusion Engineering Test Reactor (CFETR). In order to explore and master fusion DEMO level key technologies, build key prototype systems for CFETR, a Comprehensive Research Facility for Fusion Technology(CRAFT)is being established. Negative-ion based Neutral Beam Injectors (NNBI), as one of the important parts of CRAFT, can provide neutral beam with 200∼400 keV beam energy and 2 MW beam power. Stable high voltage transmission is needed for the establishment and safe operation of NNBI. As one of the key components of high voltage transmission, transmission line (TL) plays an extremely important role. In this paper, four indexes, including the maximum electric field intensity (Emax), the inner surface electric field intensity (Esur) of the electromagnetism interference screening (EMIS), the equivalent distributed capacitance (Ceq) and the electric field non-uniformity coefficient (f), are used to evaluate the electric field performance of TL. And the relationship between these indexes and the position of acceleration grid (AG) conductor, the position of extraction grid (EG) conductor and the radius of EMIS is obtained by finite element (FE) analysis. Under the requirements of Esur and f, in order to reduce the maximum electric field intensity in the TL and equivalent distributed capacitance of the TL, the layout position of the TL is determined, which lays some theoretical foundations for the engineering design of the TL.