Power Loading of Electrons Ejected From the JT-60 Negative Ion Source

The power loading of electrons ejected from the negative ion accelerator to the beamline was first measured in the negative-ion-based neutral beam injector on JT-60U. At 0.3 Pa of the operating pressure in the arc chamber, the heat flux and the total power load for the single segment were ~8 W/cm² and 27 kW for the D ion beam of 300 keV and 3.4 A, respectively. The normalized total power loading on the electron dump was no more than 2.6% of the electric power in the acceleration power supply. About 70% of the total power is originated by the electrons stripped from D ions due to collisions with residual gas molecules in the accelerator. The calculation of the stripped electron trajectories shows that the electrons stripped in the second acceleration gap are the main origin of the power loading in the beamline.