Wall reflection modeling for charge exchange recombination spectroscopy (CXRS) measurements on Textor and ITER

Abstract

Contamination of optical signals by reflections from the tokamak vessel wall is a matter of great concern. For machines such as ITER and future reactors, where the vessel wall will be predominantly metallic, this is potentially a risk factor for quantitative optical emission spectroscopy. This is, in particular, the case when bremsstrahlung continuum radiation from the bulk plasma is used as a common reference light source for the cross-calibration of visible spectroscopy. In this paper the reflected contribution to the continuum level in Textor and ITER has been estimated for the detection channels meant for charge exchange recombination spectroscopy (CXRS). A model assuming diffuse reflection has been developed for the bremsstrahlung which is a much extended source. Based on this model, it is shown that in the case of ITER upper port 3, a wall with a moderate reflectivity of 20% leads to the wall reflected fraction being as high as 55-60% of the weak signals in the edge channels. In contrast, a complete bidirectional reflectance distribution function (BRDF) based model has been developed in order to estimate the reflections from more localized sources like the charge exchange (CX) emission from a neutral beam in tokamaks. The largest signal contamination of ∼15% is seen in the core CX channels, where the true CX signal level is much lower than that in the edge channels. Similar values are obtained for Textor also. These results indicate that the contributions from wall reflections may be large enough to significantly distort the overall spectral features of CX data, warranting an analysis at different wavelengths.

Physically principled reflection models applied to filtered camera imaging inversions in metal walled fusion machines

“…A similar model was used by Banerjee et al with a simplified wall model for modelling spectral diagnostic reflections in Textor and ITER 12 . The Fresnel term, F (n, k), is the analytic solution to Maxwells equations for reflections from a smooth surface [29][30][31] .…”

“…Reflection effects are also a major issue with spectroscopic diagnostics on ITER where reflected light could contaminate spectral signals and jeopardise the usefulness of measurements [12][13][14][15][16][17][18] . Some of these studies utilised LightTools 19 , a commercially available ray-tracer, for simulating the magnitude of reflected light and to test possible mitigation strategies.…”

Emission of Fast Hydrogen Atoms in a Low Density Gas Discharge—The Most “Natural” Mirror Laboratory

“…Light reflectance at metallic surfaces is a natural phenomena, which was observed in many kinds of laboratory plasmas, including fusion, Grimm-type, or rf-plasma discharges [35][36][37]. Until now, however, this effect has been associated with the negative role by preventing the accurate derivation of plasma parameters from spectroscopic measurements [38,39]. In this paper, we tried to formulate the plasma conditions at which the light reflectance became so dominant that the emission of atoms could approximate the mirror laboratory with the aim to derive the optical properties of the surface in the absence of additional light sources.…”

Morphology and Optical Property Changes of Nanostructured Tungsten in LHD

“…Moreover, it was found that the influence of stray light would not be negligible also in divertor region [6]. Even for charge exchange recombination spectroscopy, theoretical modeling demonstrated that the stray light issue can be serious in ITER, because the beam attenuation is much greater than that in author's e-mail: kajita.shin@nagoya-u.jp the present devices due to high density and the large device size [7].…”