Vacuum Insulation and Achievement of 980 keV, 185 A/m2 H− Ion Beam Acceleration at JAEA for the ITER Neutral Beam Injector

Abstract

Vacuum insulation of −1 MV is a common issue for the HV bushing and the accelerator 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/m2, 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 modification improved the voltage holding capability in vacuum, and the MeV accelerator succeeded in sustaining −1 MV stably. Furthermore, 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/m2 H− ion beam acceleration has been demonstrated, which is close to the ITER requirement.