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

摘要

Summary form only given. Magnetic multicusp ion sources, alsoknown as bucket-type sources, are suitable for producing large-area ionbeams and they have been developed as high-current ion sources inneutral beam injector (NBI) systems for nuclear fusion plasma heating.Neutral beams are produced by neutralization of ion beams in thesystems. Recently, development of a negative-ion-based NBI system hasbecome desirable for higher energy neutral beam production. Theneutralization efficiency of a positive ion beam decreases as beamenergy rises (20% at 100 keV, <5% at 200 keV). However, theneutralization efficiency of a negative ion beam is more than 60% athigh energy (<100 keV). The Large Helical Device (LHD) is underconstruction at National Institute for Fusion Science (NIFS). Anegative-ion-based NBI system is planned to provide plasma heating ofthe LHD. In this research, we developed a large-area negative ionsource, which is 1/3 the scale of the ion source planned for the LHDNBI. High-current H^- beam production (>15 A: 1/3 currentof LHD ion source) was a condition of the design. The bucket plasmasource has a 35 cm×62 cm area and a 20.6 cm depth. A large-areahigh-density plasma (>10¹²/cm³) is produced bya high power are discharge (>100 kW) with tungsten filaments. For H^- production, a magnetic filter is used to lower the electrontemperature of the negative ion production region near the beamextraction grid