Radio frequency (RF) plasma cleaning technology is emerging as the most promising technique for in-situ cleaning of the first mirror in future Tokamak devices. The self-bias voltage, which determines the ion energy and sputtering behavior, is a critical factor affecting the efficiency and effectiveness of RF plasma cleaning. In order to investigate the impact of self-bias on the recovery of reflectivity of single crystal first mirrors in radio frequency (RF) plasma, the work aims to provide important references and basis for the in-situ recovery of reflectivity of first mirrors in future fusion reactors. Single crystal molybdenum (Mo) samples with a (110) crystal orientation, a diameter of 25 mm, and a thickness of 4 mm were used as first mirror substrates. A 30 nm thick Al/Al2O3 film was coated on the mirror surfaces via magnetron sputtering to simulate impurity contamination in the device. In this experiment, argon (Ar) was used as the working gas with the operating pressure set at 1 Pa. The self-bias voltages were set at -100 V, -200 V, and -300 V, respectively, to clean the Al2O3 deposition on the surface of the single crystal molybdenum (Mo) first mirror. The surface morphology and optical properties of the mirrors were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), ultraviolet-visible spectrophotometry, and non-contact three-dimensional surface profiling. When the self-bias voltage was set to -300 V, complete removal of Al2O3 deposition was achieved after 23 hours of RF argon plasma cleaning. The mirror surface was restored to a flat state, and the total reflectivity of the first mirror was effectively recovered with minimal change in diffuse reflectivity (<1%). At a self-bias voltage of -200 V, the complete removal of Al2O3 deposition required 33 hours of cleaning, and the total reflectivity was restored to the level before coating of Al2O3 deposition. However, at the self-bias voltage of -100 V, even though after 354 hours of cleaning, Al2O3 deposition residues remained on the surface, and the total reflectivity only recovered to 70% of the pre-coating level. Due to the good sputtering resistance of single crystal Mo material, RF plasma cleaning had the least damage to the mirror, and the diffuse reflection coefficient was kept at a low level after cleaning. Higher absolute self-bias voltages (-300 V) enhanced the energy and sputtering rate of impurity particles, leading to faster and more complete removal of deposition and quicker reflectivity recovery. In contrast, the ion sputtering energy at low absolute self-bias (-100 V) was too low to effectively bombard and remove the impurity deposition on the surface of the first mirror. Some of the sputtered impurities were redeposited on the surface of the first mirror. These resulted in the inability to completely remove the Al2O3 deposition layer on the surface of the first mirror even after a long-time (354 h) cleaning. The low roughness and diffuse reflectance of the surface of the first mirror of single crystal molybdenum after cleaning prove that single crystal molybdenum material is an excellent candidate material for the diagnosis of the first mirror in future fusion reactors. When radio frequency plasma is used to achieve in-situ cleaning of the first mirror in future fusion reactor devices such as ITER, the absolute value of self-bias can be appropriately increased to improve cleaning efficiency.
Key words
first mirror /
RF plasma cleaning /
reflectivity /
self-bias /
single crystal Mo /
Al2O3 deposition
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Funding
National Natural Science Foundation of China (122753060); Members of the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2022452); The Outstanding Youth Project of the Natural Science Foundation of Anhui Province (2208085J40)
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