ИСТИНА

The development of the first mirror cleaning and recovery system is one of the join challengesfor all optical diagnostics in ITER. The eroded first wall materials are the main source of deposition and associated induced reflectivity degradation of the first mirrors. This work studies 81 MHz capacitively coupled radio frequency (CCRF) discharge as a promising method for removal of the metal deposits. Owing to high toxicity of beryllium and similar chemical properties with aluminum [1], for plasma sputtering experiments we used Al/Al2O3 coatings. Al/Al2O3 sputtering yields are significantly higher than Be/BeO sputtering yields, which allows to rely on the effectiveness of the developed for aluminum film removal techniques applied to beryllium. This technique was studied in the axial-symmetric cylindrical mock-up of plasma cleaning system. Discharge parameters were measured as function of RF power supplied trough a blocking capacitor to molibdenum mirror used as a powered electrode of CCRF. Ion energy distribution (IED) was characterized by retarding field energy analyzer (RFEA) [2]. Ion flux density and sputtering rate were measured by sputtering of thin gold films. The influence of axial magnetic field on the ion flux parameters and uniformity of sputtering rate were investigated experimentally. Both experiments and simulations have revealed marked increase in sputtering rates at the mirror edges. The discharge was characterized with and without λ/4 cooling line. Experiments with λ/4 cooling line resulted in strong overheating of the mirrors and melting of contamination films. Mirror cleaning from Al/Al2O3 deposits was assessed in two different gases, neon and deuterium. Polycrystalline Mo mirror structure degraded under long-term exposure in two ways: crystallites of various orientation became visible after selective sputtering in neon plasmas and mirror surface blisters appeared in deuterium plasmas. Therefore, for long-term cleaning procedures we recommend monocrystalline Mo mirrors and minimum sputtering ion energies. [1] Bodhansky J., et al “Formation of various coatings and their behaviour under particle bombardment” J. Nucl. Mater. 85–86 (1979) 1145 [2] Razdobarin A.G., el Al. “RF discharge for in situ mirror surface recovery in ITER” Nucl. Fusion 55 (2015) 093022