Physical design of electron dumps for the beamline of CFETR advance neutral beam equipment (CANBE)

Abstract Negative-ion-based neutral beam injection (NNBI) at 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. In addition to the electrons co-extracted from the source plasma, electrons are inevitably generated in the main acceleration stage due to the stripping loss of negative ions or the ionization of background gas. Many of these accelerated electrons can be ejected by the negative ion source, which poses a risk for the downstream components in the beamline of the NNBI system, especially the thermosensitive cryopumps. To prevent the ejected electrons from impacting the cryopumps, electron dumps have been designed for the beamline of the China Fusion Engineering Test Reactor (CFETR) advance neutral beam equipment (CANBE). Using a self-consistent model of the negative ion accelerator, the generation and motion of the ejected electrons were simulated for different negative ion sources to be tested on the CANBE. Based on the orbits of the ejected electrons, a set of fixed and movable electron dumps was designed to be placed in front of the neutralizer inside the CANBE beamline. The effects of the electron dumps were quantitatively analyzed.