A high power neutral injection heating system based on negative ion beam technology is under construction for the Large Helical Device (LHD). An injection power of 15 MW is expected for 10s, with 180 keV for hydrogen beam. High energy is required to inject the beam tangentially into the LHD so that the use of negative ions is inevitable. The system consists of two beam lins with four negative ion sources. A large negative ion beam (160 A) is required, with a high current density (> 35 mA/cm~2). The design of the ion source is carried out on the basis of the authors' R&D during the past seven years, where highly efficient negative ions and a high quality beam could be produced. The other components of the beam line and the power supplies are designed on the basis of conventional nmeutral beam technology. Although an NBI system is designed in pulse operation, its duty cycle is rather high (10s every 5 min) because the magnetic field of the LHD is generated in a steady state. This is a difficult situation for heat removal systems such as beam dumps.
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