Design and overview of fabrication tests for the 1 MV bushing for ITER NB system

Abstract

The ITER NB System consists of two injectors each delivering 16.7 MW to the plasma in the form of neutral deuterium atoms accelerated at 1 MeV. Each injector has a single negative ion source connected to a 1 MV electrostatic accelerator. The accelerated negative ions pass through a neutraliser, then an electrostatic deflection system, where residual ions, exiting after neutralisation, are dumped. The neutral part of the beam continues to the torus through a duct connecting to the torus vessel. The 1 MV bushing is an insulated cylindrical, feedthrough (about 2 m in diameter and 2.5 m in height), which forms the physical boundary between the gas-insulated transmission line and the torus primary vacuum. An innovative design has been developed fulfilling all the requirements, including 1 MV class insulation in vacuum for a large number of services, vacuum compatibility, operation under neutrons and gamma radiation, safety requirements for pressure containment. In particular, the design allows a nitrogen pressurised guard volume that prevents any leakage of the external insulation gas (sulphur hexaflouride, potentially damaging for platinum/palladium catalysts of the tritium recovery system) inside the torus primary vacuum, and relieves the alumina insulator from substantial structural functions. The article describes the design, the selection of the materials and the mechanical analyses. To validate the design a reduced scale prototype, including all the relevant mechanical and electrical features of the bushing insulator, has been successfully manufactured and tested. The article reports an overview of this development.