NEGATIVE-ION PRODUCTION IN A LARGE SEMICYLINDRICAL ION-SOURCE

A large semicylindrical negative ion source, whose dimensions are 34 cm in diameter and 104 cm in length, has been developed. By optimizing a chamber depth and a filter strength, a H- current of 650 mA (7.7 mA/cm2) was obtained at an arc condition of 1.3 Pa, 70 V, and 1200 A. In order to increase the H- current and to reduce an operating pressure, a small amount of Cs was injected. As a result, the H- current was increased from 550 mA (6.5 mA/cm2) to 850 mA (10 mA/cm2) under an arc power of 70 V x 800 A. The most significant feature of the cesium effect was the reduction of the operating pressure. With maintaining the sufficient current of 670 mA (8 mA/cm2), the operating pressure could be lowered to 0.03 Pa. At this pressure, the highest gas efficiency of 20% was achieved.