Development of technology for fabrication of prototype ion extraction grid for fusion research

Abstract

• Ion extractor grids are the heart of any NBI system. • Ion extractor grid fabrication involves various critical technologies. • Friction welding for joining of dissimilar metals of SS304 L rod and OFHC copper plate. • Electro-deposition technique is best for making embedded coiling channels inside thin OFHC copper plate. • Surface flatness of 100 μm of grid is most important during CNC machining. Steady-state Superconducting Tokamak (SST-1) has a provision for a positive hydrogen ion-based Neutral Beam Injection (NBI) system. This system has a capability of injection of neutral hydrogen beam power of 1.7 MW at 55 kV. This NBI system consists of a 3-grid accel-decel ion extractor system. Each grid has 774 apertures distributed in the area of 23 cm × 48 cm on the OFHC copper plate. During beam operation grid received maximum heat load of ∼1.75 MW/m 2 which is removed by a dense network of 22 semi-circular ( r = 1.1±0.05 mm) cooling channels embedded between the rows of shaped apertures. The water flow velocity inside the cooling channel is 13 m/s and the pressure drop is 9 bar. To maintain the high electric field (∼7 kV/mm) between grids, the required surface flatness of the OFHC copper plate is 100 μm. Another requirement is positional tolerance of aperture of ±60 μm. All these critical requirements imply that the fabrication of the ion extractor grid is very complex and several technologies are involved: (i) Friction Welding (FW) for joining of dissimilar metals of SS304 L rod to the specific location of OFHC copper plate for the fabrication of water manifold of the grid. This has been successfully developed and obtained FW joint strength of 264 MPa (ii) 2.5 mm thickness electro-deposition of OFHC copper for making embedded cooling channels (iii) Precision CNC grid machining. This paper shall describe the development of these technologies for the manufacturing of the extractor grid.