Status of the Progress of Fabrication of High Power Prototype Ion Extractor Grid for NBI System

The steady-state superconducting tokamak (SST) neutral beam injection (NBI) system has an ion extractor system that consists of three grids, each having a number of shaped apertures to extract a 90-A hydrogen ion current at 55 kV. During beam operation, the grid received heat load, which is removed by embedded water cooling channels. The ion optical design of grids demands stringent tolerances in various dimensions, which makes the complexity in the engineering design and fabrication. This article describes the status of the progress of the work done toward the development of technologies and routes for the successful fabrication of the prototype acceleration grid (PAG). Friction welding (FW) of a dissimilar metals joint of SS304L rod to oxygen-free electronic (OFE) copper plate was done for making water headers to supply water. scanning electron microscope (SEM) and energy-dispersive x-ray spectroscopy (EDS) analysis of FW samples shows that recrystallization occurred at the weld interface, and a fine grain is formed at the Cu side due to frictional heat and forging pressure. Elemental diffusion takes place across the joint interface, which improves metallurgical bonding between SS and Cu. CMM measurement of the CNC-machined grid shows that the dimensions are as per the design specification. Machined cooling channels and manifolds’ grooves are covered by a technique called copper electrodeposition (ED). Continuity of the cooling channels is checked by a liquid crystal display (LCD) display test, and radiography test (RT) results are satisfactory. Steady-state computational fluid dynamics (CFD) analysis with 0.44-MW/m2heat load (max.) shows that maximum surface temperature is$150~^{\circ }$C with a 0.22-kg/s water flow rate at$20~^{\circ }$C and a velocity of 10.5 m/s. Knowledge gained from the prototype grid (PG) fabrication and acceptance test results shall be used for the fabrication of actual-size ion extractor grids for the NBI system to support Indian fusion program.