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Description
Ion extractor grids are the beam-forming component of the ion source[1]–[3]. It plays a vital role in separation of ions from plasma source and accelerated to the desired energy for Neutral Beam (NB) production required for Tokamak plasma heating and current drive. Steady-State Superconducting Tokamak (SST) -1 NB Injection (NBI) system has rectangular ion extraction area of size 23 cm (W) × 48 cm (H). Beams originating from the extraction plane are horizontally focused (fh) at 5.4 m and vertically focused (fv) at 7 m respectively. This paper describes the MATLAB program developed based on 1D calculation of focusing of multi ion-beamlets by mechanical offset given to the apertures of ion extractor grids. A model of ion extractor grids assembly is presented and showed that effect of thermal expansion of the grids during beam extraction has minimum effect on horizontal and vertical focal length. This calculation is compared with experimental results and found good agreement. Also, the effects of thermal expansion of the grids on beam transmission is studied. It shows, as the grid temperature increases, transmission begins to decline, with values reducing to nearly 85% at ΔT ≈ 25 °C, 84.4% at ΔT ≈ 50 °C, and continuing to fall until it reaches about 83.8% at ΔT ≈ 125 °C. The analysis validates that the thermal expansion model provides a reasonable prediction of experimental behaviour, emphasizing the importance of incorporating effective cooling strategies and mechanical design adjustments to minimize misalignment and ensure stable beam transmission under operational heating conditions.
