Electrons in the negative-ion-based NBI on JT-60 U

The stripped electron trajectories in a large negative ion accelerator with multi‐apertures and three acceleration stages, where non‐uniform stray magnetic field is horizontally created, are calculated in the JT‐60 negative ion source by a 3‐D numerical code. The horizontal non‐uniform stray field results in a significant power loss of the stripped electrons in the outmost acceleration channel on the grounded grid (GRG). The power loss in the outmost acceleration channel is more than twice higher than that in the central channel due to the weaker stray field although the total power loading on the GRG is by 25% larger than that by assuming a uniform stray field.