An ion source with a hollow cathode set at a position where no backstreaming electrons can arrive has been proposed and fabricated. The arc chamber is a magnetically line‐cusped semicylinder (i.e., semicircular cross section) whose axis is perpendicular to the ion beam axis and the hollow cathode is positioned on the extended line of the cylinder axis. Preliminary experiments were made on this source. The highest arc and gas efficiencies obtained were 0.64 A/kW and about 70%, respectively, and a proton ratio of 85% was attained. In addition, the fraction of metal impurities in the beam was measured to be negligibly small. These source performances are comparable to those of the conventional tungsten‐filament magnetic multipole line‐cusp sources for neutral beam injectors.