Experimental study on spatial uniformity of H− ion beam in a large negative ion source

The origin of the beam non-uniformity, that is one of the key issues in JT-60 negative ion sources, was experimentally examined by measuring correlations between the intensity of the H− ion beam and plasma parameters, such as an electron temperature and plasma density in the JAERI 10 A negative ion source. The ion source was operated without cesium. Although a uniform filter field was applied, the electron temperature near plasma grid (PG) was higher than 1 eV in the upper half region of the longitudinal direction, where the spatial beam intensity decreased rapidly. This indicates that the beam non-uniformity is caused by a higher electron temperature than 1 eV, namely, by a leakage of the high-energy electrons toward the PG. To investigate the path of high-energy electrons leaking into the PG, a metal plate of 50 mm × 50 mm was placed in the plasma. By placing the plate in a weak magnetic field of <30 G leading to a driver region near the upper wall, the electron temperature near the PG was sufficiently reduced to 1 eV in the upper half region, where the beam intensity was simultaneously increased. As a result, a 20% gain of the beam current was obtained. Further, the source of the high-energy electrons leaking toward the PG was examined by changing the relative filament position to the upper wall. It is found that the filaments closest to the upper wall, i.e., at 80 mm from the upper wall is one of the sources of the high-energy electrons leaking into the PG.