Determination of divertor stray light in high-resolution main chamber Hα spectroscopy in JET-ILW

Abstract

The theoretical model suggested for ITER main chamber Hα spectroscopy is applied to the high-resolution spectroscopy (HRS) data of recent JET ITER-like wall (ILW) experiments. The model is aimed at reconstructing the neutral hydrogen isotope density in the SOL, as well as the isotope ratio, by solving a multi-parametric inverse problem with allowance for (i) the strong divertor stray light (DSL) on the main-chamber lines of sight (LoS), (ii) substantial deviation of the neutral atom velocity distribution function (VDF) from a Maxwellian in the SOL, and (iii) data for the direct observation of the divertor. The JET-ILW HRS data on resolving the power at the deuterium and hydrogen spectral lines of the Balmer-alpha series is analysed, with direct observation of the divertor from the top and with observation of the inner wall along the tangential and radial LoS from the equatorial ports. This data allows the spectrum of the DSL and the signal-to-background ratio for the Balmer-alpha light emitted from the far SOL and divertor in the JET-ILW to be evaluated. The results support the expectation of the strong impact of the DSL upon the ITER main chamber Hα (and visible light) spectroscopy diagnostics.