Power flow in the negative-ion based neutral beam injection for JT-60

The negative ion based neutral beam injection system for JT-60 has operated since 1996 injecting neutral beam into JT-60 plasmas. A power flow measurement in the beam line and ion source with a water calorimeter had shown that 40%-50% of accelerated beam particles were intercepted on the two accelerator grids and the grounded grid at an ion source gas pressure of 0.2-0.3 Pa. Much of the beam loss was not caused by stripping loss of the negative ions, but rather by direct impingement of the negative ions onto the grids. After reducing the acceleration area by masking the edge area (about 13% of the extraction area) of the accelerator grid so as to minimize the edge effect of magnetic field in the arc chamber, the loss in the accelerator decreased by roughly 25%. In comparing a deuterium beam with a hydrogen beam, the neutral beam power with deuterium is lower by 30% than that of hydrogen at the same arc power, although the heat load onto the grounded grid does not change so much. The power deposition ratios along the beamline were as follows: the beam scraper in the ion source tank and the neutralizer cell received about 3% and about 7%, respectively, of the accelerated beam power, while the ion dumps for both D- and D+ received 20%-30% in total, and 30%-34% reached the neutral beam calorimeter. (C) 2000 American Institute of Physics. [S0034-6748(00)52602-1].