Cesium effects have been investigated in a semi‐cylindrical negative ion source whose inner dimensions are 34 cm in diameter and 129 cm in length. D−and H−ion beams were extracted and accelerated by a multiaperture extraction system, which has 240 holes of 11.3 mm in diameter within an area of 6 cm×120 cm. The cesium effects, i.e., enhancement of the negative ion current and reduction of the optimum pressure and electron current, have been observed both in deuterium and in hydrogen operations. The enhancement factor of the D−ion beam current by cesium was higher than that of the H−ion beam current. Though the D−current had a strong tendency of saturation with arc discharge power in pure volume operation, the D−ion beam current was increased linearly with arc discharge power in cesium seeded operation. The ration of the D−ion beam current to the H−ion beam current was 0.8 in cesium seeded operation while the ratio in pure volume operation was 0.5. The D−ion beams more than 2.2 A with an average current density of about 10 mA/cm2was obtained at an energy of 100 keV for 5 sec at the beam targets which were located at 4 m and 6.4 m from the source.