Physics design of electron dumps for the beamline of CFEDR advance neutral beam equipment (CANBE)

Negative ion based neutral beam injection (NNBI) with high energy is still a potential heating and current drive method for future large-scale fusion reactors. One feature of the negative ion source is that the electrons are also accelerated with the negative ions in the accelerator. Besides the co-extracted electrons from the source plasma, the electrons are inevitable to be generated in the major acceleration stage due to the stripping loss of negative ions or the ionization of background gas. Many of those accelerated electrons can eject out of the negative ion source, which are a risk for the downstream components in the beamline of NNBI system, especially the thermo-sensitive cryopumps. In order to prevent the ejecting electrons impacting on the cryopumps, the electron dumps were designed for the beamline of the CFEDR advance neutral beam equipment (CANBE). CFEDR (China Fusion Engineering DEMO Reactor) is the update version of the CFETR (China Fusion Engineering Test Reactor). Based on a self-consistent model of the negative ion accelerator, the generations and the motions of the ejecting electrons were simulated from different negative ion sources that would be tested on the CANBE. According to the orbits of the ejecting electrons, a set of fixed and movable electron dumps were designed in front of the neutralizer inside the CANBE beamline. The effects of the electron dumps were quantitatively analysed.