Abstract Vacuum insulation of -1 MV is a common issue for the HV bushing and the accel- erator for the ITER neutral beam injector （NBI）. The HV bushing as an insulating feedthrough has a five-stage structure and eac...Abstract Vacuum insulation of -1 MV is a common issue for the HV bushing and the accel- erator for the ITER neutral beam injector （NBI）. The HV bushing as an insulating feedthrough has a five-stage structure and each stage consists of double-layered insulators. To sustain -1 MV in vacuum, reduction of electric field at several triple points existing around the double-layered insulators is a critical issue. To reduce electric field simultaneously at these points, three types of stress ring have been developed. In a voltage holding test of a full-scale mockup equipped with these stress rings, 120% of rated voltage was sustained and the voltage holding capability required in ITER was verified. In the MeV accelerator, whose target is the acceleration of a H- ion beam of 1 MeV, 200 A/mS, the gap between the grid support was extended to suppress breakdowns triggered by electric field concentration at the edge and corner of the grid support. This modi- fication improved the voltage holding capability in vacuum, and the MeV accelerator succeeded in sustaining -1 MV stably, b-~rthermore, it appeared that the H- ions beam was deflected and a part of the beam was intercepted at the acceleration grid, This causes high heat load on the grids and breakdowns during beam acceleration. To suppress the direct interception, a new grid was designed with proper aperture displacement based on a three dimensional beam trajectory analysis, As a result, 980 keV, 185 A/ms H- ion beam acceleration has been demonstrated, which is close to the ITER requirement.展开更多
文摘Abstract Vacuum insulation of -1 MV is a common issue for the HV bushing and the accel- erator for the ITER neutral beam injector （NBI）. The HV bushing as an insulating feedthrough has a five-stage structure and each stage consists of double-layered insulators. To sustain -1 MV in vacuum, reduction of electric field at several triple points existing around the double-layered insulators is a critical issue. To reduce electric field simultaneously at these points, three types of stress ring have been developed. In a voltage holding test of a full-scale mockup equipped with these stress rings, 120% of rated voltage was sustained and the voltage holding capability required in ITER was verified. In the MeV accelerator, whose target is the acceleration of a H- ion beam of 1 MeV, 200 A/mS, the gap between the grid support was extended to suppress breakdowns triggered by electric field concentration at the edge and corner of the grid support. This modi- fication improved the voltage holding capability in vacuum, and the MeV accelerator succeeded in sustaining -1 MV stably, b-~rthermore, it appeared that the H- ions beam was deflected and a part of the beam was intercepted at the acceleration grid, This causes high heat load on the grids and breakdowns during beam acceleration. To suppress the direct interception, a new grid was designed with proper aperture displacement based on a three dimensional beam trajectory analysis, As a result, 980 keV, 185 A/ms H- ion beam acceleration has been demonstrated, which is close to the ITER requirement.
