AbstractThe development of high current negative ion sources has been intensively continued for application to fusion research, especially neutral beam injection. Recently, the research and development of hydrogen negative ion sources has made rapid progress. Negative ion beam current has been remarkably enhanced by seeding cesium in a magnetically filtered multicusp plasma source. As a result, a negative ion beam current of more than 16 A is obtained at an energy of 120 keV. According to studies of negative ion sources during the last decade a design principle for volume production multi-cusp negative sources has been mostly approved. The present status of the development of high current negative ion sources is briefly reviewed. The performance and the characteristics of a negative ion source are discussed. Emphasis is placed on the important issues and optimized condition: low operating pressure, the relation between the plasma parameters and the extracted beam current, the optimum Cs seeding conditions, the beam extraction characteristics of the negative ion source and the suppression of electron current extracted with the negative ions. The optimum condition is described by considering the physical principle of the negative ion sources and the physical and technological consideration of the design of the negative ion sources are given.