Effect of exposure inside the Large Helical Device vessel on the optical properties of stainless steel mirrors

Abstract

During the third experimental campaign, three stainless steel mirrors were exposed inside the Large Helical Device (LHD) vacuum vessel and, in order to measure the change in their spectral reflectance R(λ), the effect of this exposure was measured at normal incidence (λ = 200-700 nm). The mirrors located in the divertor region and in the diagnostic port became coated with carbon-based films; however, the mirror placed closest to the plasma confinement volume became cleaner than it had initially been. The characteristics of the films were obtained by different techniques. The deposited films were slowly removed by repeatedly exposing the mirrors to low-energy ions of deuterium plasma, and R(λ) was measured after every exposure. In the present paper we compare the reflectance behaviour during this cleaning procedure and the reflectance calculated in the framework of a model assuming that a half-transparent film is formed on stainless steel substrate. The n(λ) and k(λ) data for carbon-based films were taken from experiments and the literature.

Experimental Simulation of the Behaviour of Diagnostic First Mirrors Fabricated of Different Metals for ITER Conditions

“…22) without degradation of the mirror optical properties because of the low ion energy. A similar procedure with exploitation of a low temperature ECR plasma was successfully used to remove a carbon layer that appeared during exposition of mirrors in the LHD heliotron [66], the TRIAM-1 tokamak [67], in the test stand DSM-2, and similarly, of mirrors exposed in DIII-D and TEXTOR in the toroidal stand TOMAS [68]. A high efficiency of chemical cleaning of hydrocarbon films was demonstrated in ref [69].…”

“…The characteristics of these films are described in refs [65,66], and [53], respectively. The effect of long term sputtering by plasma ions on the appearance of diffusive components was studied for mirrors of different materials by the use of the DSM-2 stand (in IPP NSC KIPT).…”

Calculation of Geometry Matrices for IRVBs for Application to 3D Tomography of Radiative Phenomena in LHD

“…If the location of the subvoxel has intersected the first wall then it is not added to T and the T sv of all subsequent FoV subvoxels are also neglected. This is justified since the reflectivity of the stainless steel wall is negligible for UV and x-ray wavelengths which dominate the radiation in LHD [3]. This is repeated for each channel of each IRVB giving one geometry matrix for each IRVB.…”

Diagnostics for steady state plasmas

“…Carbon based films of up to 50 nm thickness have been found after exposure of stainless steel mirror samples in LHD. The properties are strongly depending on the location ranging from hard films mainly consisting of Fe (40 %), C (40 %), and O (10 %), which were difficult to remove in D glow discharges, to soft films mainly consisting of C (90 %) and H (10 %) [271]. The deposition strongly depends on the individual poloidal position in the torus and the temperature of the surface.…”