Production of high-current large-area negative hydrogen ion beams by a bucket-type ion source equipped with a magnetic filter

Summary form only given. Magnetic multicusp ion sources, also known as bucket-type sources, are suitable for producing large-area ion beams and they have been developed as high-current ion sources in neutral beam injector (NBI) systems for nuclear fusion plasma heating. Neutral beams are produced by neutralization of ion beams in the systems. Recently, development of a negative-ion-based NBI system has become desirable for higher energy neutral beam production. The neutralization efficiency of a positive ion beam decreases as beam energy rises (20% at 100 keV, <5% at 200 keV). However, the neutralization efficiency of a negative ion beam is more than 60% at high energy (<100 keV). The Large Helical Device (LHD) is under construction at National Institute for Fusion Science (NIFS). A negative-ion-based NBI system is planned to provide plasma heating of the LHD. In this research, we developed a large-area negative ion source, which is 1/3 the scale of the ion source planned for the LHD NBI. High-current H/sup -/ beam production (>15 A: 1/3 current of LHD ion source) was a condition of the design. The bucket plasma source has a 35 cm/spl times/62 cm area and a 20.6 cm depth. A large-area high-density plasma (>10/sup 12//cm/sup 3/) is produced by a high power are discharge (>100 kW) with tungsten filaments. For H/sup -/ production, a magnetic filter is used to lower the electron temperature of the negative ion production region near the beam extraction grid.