Abstract
• Doppler Shift Spectroscopy diagnostics for ITER DNB at Indian Test Facility is conceptualised for neutral beam characterisation. • Modelling of the system suggests that 36 Line of Sights is required for a complete mapping of the beam. • A novel method of deconvolution of beam focusing effect in the total width is performed. • The error in divergence estimation of the 7 mrad focused divergent beam is less than 10%. INTF (Indian Test Facility) is a negative hydrogen ion source based neutral beam test facility to characterise 100 keV,60 A Diagnostic Neutral Beam (DNB), conceptualised for ITER. Several diagnostics are under development to monitor and characterise the performance of ion source and the beam properties. Doppler Shift Spectroscopy (DSS) diagnostics shall be used to estimate the beam divergence, beam inhomogeneity and the stripping losses of the extracted H − ion beam. In this work, the conceptualisation of the DSS diagnostics and the performance evaluation of the proposed system is presented. A forward model is developed to predict the signal strength and signal to noise ratio for various beam operation scenarios and for different optical setups. The model also estimates the expected error in the line width analysis carried for estimating the beam divergence. To reduce the error in the estimated beam divergence, a study on the error propagation was carried out by considering a range of viewing angles, different types of light collection systems and optical instruments. An optimum viewing angle, the light collection system and the instrumentation is chosen accordingly. An endoscopic design concept for each LOS has been adopted and the mechanical assembly of the same is presented in this work. The present work also reports that for a focused divergent beam, using conventional methods of line width analysis might lead large errors and impacts the beam divergence measurements obtained by DSS. A suitable algorithm has been developed to deconvolve the beam focusing spectral width contribution, thereby improving the preciseness of the beam divergence estimation.
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