Design study of a neutral beam injector for fusion DEMO plant at JAERI

Requirement and technical issues of the neutral beam (NB) system are discussed for a fusion DEMO plant. The NB system for DEMO should be characterized by high efficiency, high energy, high reliability in year-long steady operation, and low maintenance frequency. From the viewpoint of high efficiency, use of conventional electrostatic accelerator is realistic. Due to operation under radiation environment, vacuum insulation is essential in the accelerator. According to the insulation design guideline, it was clarified that a beam energy of 1.5-2 MeV is possible in the accelerator. Development of filamentless, and cesium free ion source is required to achieve low maintenance frequency, based on the existing high current/high current density negative ion production technology. The gas neutralization is not applicable due to its low efficiency (60%). R and D on an advanced neutralization scheme such as plasma neutralization (efficiency: >80%) is required. Recently, development of cw high power semiconductor laser is in progress. The paper shows a conceptual design of a high efficiency laser neutralizer utilizing the new semiconductor laser array