Uniform Negative Ion Beam Extraction in a Cesium-Seeded Large Negative Ion Source with a Tent Magnetic Filter

For improvement of spatial uniformity of negative ion beams, the external magnetic filter was replaced to a novel tent-shaped magnetic filter in the JAEA 10 ampere negative ion source. The tent filter geometry is based on new findings in surface production of negative ions in high electron temperature plasmas. In order to link the tent filter, the original line-cusp field configuration on the source chamber was re-arranged to form a symmetric magnetic field like the one commonly employed for many of positive ion sources aiming at high proton yield. This magnetic field configuration allows fast electrons emitted from cathode filaments to rotate azimuthally in the ion source by B × grad B drift. As the result, the source plasma with the tent filter showed a flat profile and the spatial deviation of extracted negative ion beam profile from a wide area of 340 mm × 140 mm in the tent filter was decreased to a half of that in the external magnetic filter. Spatially uniform plasma, that is, uniform particle flux onto the plasma grid generated negative ions with good uniformity with a highly symmetric magnetic field configuration.