PSI - Issue 50

S.A. Filin et al. / Procedia Structural Integrity 50 (2023) 91–99 S. A. Filin at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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drainpipe enters the pre-treatment unit (if there is a significant amount of contaminants in it) or directly into the distillation tank (if there is a small amount of contaminants). Preliminary cleaning of the washing mixture "freon-114B2 - acetone - tert-butanol" is as follows. It enters the filtration through a polymer membrane. Turbine provides mixing of the solvent. After filtration, the washing mixture is condensed in the lower cavity under the polymer membrane, passing through a radiator filled with liquid nitrogen for cooling. Then it enters the cubic (lower) part of the distillation column for final cleaning, or is stored in a special tank before further use. Preliminary cleaning allows ensuring the purity of the washing mixture up to 10 -2 G/cm 3 , and the final cleaning in the distillation column - up to 10 -8 G/cm 3 . The amount of washing mixture regenerated by the distillation column allows doing processing in a stream of a continuously supplied washing mixture with a period of one cleaning cycle (up to a complete change of the contaminated solvent) from 30 to 45 minutes, depending on the degree of contamination and dimensions of the mirrors. The dissolution rate of the main technological contaminants was analyzed. The duration of the cleaning process is 4-8 hours, depending on the degree of contamination and dimensions of the mirrors. The degree of cleaning of the mirrors during their processing was assessed using an "EF-3MA" brand electronic fluorimeter. The input and output control of the chemical purity of the surface was carried out using an "LPE-3M-1" brand laser photoelectric ellipsometer. The stabilization of the washing mixture was carried out by three methods. 1. Adding tert-butanol to the washing mixture (up to 2 weight %), including in the form of an azeotrope "freon 114B2 - tert-butanol". It inhibits the process of thermal dissociation of the washing mixture. 2. The use of a temperature-controlled heat exchanger, which ensures the separation of a significant part of the gaseous products of thermal decomposition of the washing mixture during its condensation. 3. The use of an ion-exchange filter, that allows removing thermal decomposition products and water from impurities present in the washing mixture at the solubility level at 25- 30 °C. The investigated copper mirrors were in contact with the chemically active laser medium (CO 2 , N 2 , He, triethylamine) during operation. The diameter of the mirrors was 160 mm, the radius of curvature was 40 m. Mirror № 1 was made by diamond turning method followed by finishing with diamond paste, and mirror № 2 - using free abrasive. Mirror parameters: surface shape N = 2; shape error ΔN = 0.2; optical purity class P = V. The characteristics of the laser resonator, the operating modes of the laser, and the technique for measuring the radiation characteristics are described in (Rogalin (2013)). The pulse duration was 4- 5 µs by the base, the beam aperture was 10 x 10 cm 2 . Long-term operation of mirrors in gas discharge medium containing O 2 2 , CO - , N 2 ions, various nitrogen oxides, triethylamine decomposition products, and various combinations thereof leads to the deposition of a dark film on the surface of the mirrors, which noticeably worsens the optical properties of the mirror in the visible and IR regions of the spectrum. Cleaning the mirrors with standard solvents and wiping with cotton wool significantly worsened the optical purity. The experimental procedure for testing the efficiency of cleaning the mirrors of a pulsed CO 2 laser with an output energy of up to 800 J (Apollonov et al. (1985); Drobot et al. (1990)) after long-term operation in a chemically active laser gas discharge medium was as follows. Before installation in a laser cell, the value of the specular reflection coefficient (SRС) was measured at λ = 10.6 μm. After the operation cycle (~10 3 starts), the mirrors were removed and SRС was determined again. Then, the mirrors were restored in the installation for 4 cleaning cycles (4 h), after which the final cleaning was carried out with ethanol. The results showed that cleaning the mirrors resulted in complete removal of the dark film. In this case, SRС increased to 99.0 ± 0.2%, which is somewhat higher than the initial value (after the polishing process), i.e., the radiation losses on the mirror decreased by 1.5 – 2 times compared with the initial value and by 2 – 4 times - compared with the value, that the mirror had after working in the resonator. The control of optical parameters ( N, ΔN, P ) after processing on the setup showed, that they remained unchanged. Cleaning did not worsen the parameters of the mirrors.