Plasma-Chemical Destruction of Transition Metals as Plasma-Facing Materials

Abstract

First mirrors are critical components of optical diagnostic systems for measuring parameters of thermonuclear plasma in tokamaks. The condition and surface quality of first mirrors affect the accuracy of measured data, such as plasma density, temperature, magnetic field magnitude and others. This work investigates the mechanism leading to catastrophic surface destruction of molybdenum and tantalum mirrors used in POINT diagnostic system of EAST tokamak. The destruction of molybdenum and tantalum mirrors—which are transition metals, along with tungsten—is visually observable due to the high reflectivity of their surfaces, initially polished to achieve the required reflective properties. This makes first mirrors a convenient tool for studying surface destruction processes of all transition metals used as the first wall of tokamaks. Thus, in this work, first mirrors with polished surfaces are considered as elements of the first wall of fusion reactors, prepared for detailed study of erosion processes resulting from exposure to thermonuclear plasma. It is noted that it is precisely the mirror surface that allows visual observation of the catastrophic nature of the destruction of molybdenum and tantalum surfaces. Surface studies based on optical microscopy have revealed predominant destruction of surface structures along intergranular spaces and in regions of point defects arising from plasma exposure. X-ray photoelectron spectroscopy made it possible to establish the effect of the return to the first-wall surface of positively charged molybdenum ions and molybdenum-containing radicals. This work presents a destruction mechanism for transition metals that follows from the presented experimental facts. The study emphasizes that catastrophic plasma-chemical destruction occurs with all tokamak components made of transition metals that face the plasma.