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

Abstract

In the experimental fusion reactor ITER, the plasma-facing component of each optical and/or laser diagnostic needs to be based on reflective optics with at least one mirror (first mirror) facing the thermonuclear plasma. The different kinds of radiation emanating from the burning plasma (neutrons, neutral atoms, electromagnetic radiation) create hostile operating conditions for the first mirrors. Therefore, a special program has been set up under the ITER framework aimed at solving the first mirror problem. This paper will review the main results in this field that have been obtained in the Institute of Plasma Physics, National Science Center “Kharkov Institute of Physics and Technology” (in many cases in cooperation with groups of other countries, as indicated in corresponding parts of the manuscript) during long-term investigations directed to find a solution of this problem,i.e., to find a material and accompanying precautions in order to satisfy the requirements for first mirrors. The main efforts were devoted to finding solutions to overcome the impact of the most severe deteriorating factors resulting in degradation of the optical properties of mirrors: sputtering by charge exchange atoms and deposition of contaminants. The obtained results are focused on: the effects of long term sputtering on mirror specimens fabricated from different metals with different structures (polycrystals, single crystals, metal film on metal substrates, amorphous), the effects of contaminating film and the possible protection to avoid of its appearance, the role of chemical processes for some metal mirrors, and the choice of material of laser mirrors.

Plasma Cleaning of Metallic Mirrors from Carbon-Containing Films – New Possibilities for In Situ Monitoring of the Efficiency of Wall Conditioning in Fusion Devices

“…At present, we have no explanation for these peculiarities of the cleaning process, except for the last slowing down stage. According to [4], a possible reason for this slowing down is the change in the properties of the lowermost layer of the film, where the C film material can enter into chemical interaction with the SS substrate (Fe, Ni, Cr) to form carbides.…”

Optical Coatings as Mirrors for Optical Diagnostics

“…It has to be taken into account for dust deposition on the mirror, the life time will be reduced [30] and is described in the next section for tokamak exposed mirrors. This was leading to extensive research as reviewed by Voitsenya et al [9]. Rh mirrors exposed to D2 plasma show a drop in the reflectivity which can be associated with a formation of a subsurface rhodium deuteride (RhDx, x 2), which has optical constants different to those of Rh [25].…”

“…The mirror reflectivity would also suffer from the rough deposits from W and beryllium grown on the surface; development of the in situ cleaning methods to be operated in the diagnostic modules started and first results on coated mirrors either by laser or plasma cleaning are encouraging [9].…”

“…Rh has also low chemical reactivity, preventing oxide and carbide formation [4,5]. In the case of polycrystalline mirrors composed of many grains with random orientation of faces, the sputtering yield can vary strongly over the material surface depending on the crystallographic plane of each individual grain and its orientation with respect to the incoming particle flux [6][7][8][9]. It is also important to keep the surface relief pattern i.e.…”

“…Moreover, test of amorphous metal alloy mirrors were carried out under ion bombardment. These mirrors have no crystalline structure and after long-term sputtering the initial optical quality was maintained [9,11,12]. However, there is a critical difficulty with the possibility of recrystallization due to hydrogen embrittlement, to significantly reduced performance.…”