On the Cover of HPL: Research on cleanliness optimization of multisegment disk amplifier based on vectorized flow mode
In high power laser driving devices, it is essential to nullify the quality-reduction of the light beam caused by the deposition of contaminants on the optical elements and the laser damage caused by the contaminants to maintain optical efficiency of each of the multiple beam lines. The cleanliness of the cavity of the multisegment disk amplifier (MSA) has become one of the key factors that restrict the performance improvement of the MSA. Due to the presence of sealing materials, bonding materials, and metal parts in the MSA, large amounts of aerosols will be generated under the irradiation of high flux xenon lamps and high energy laser. Recent work has shown that xenon lamp radiation is the main reason for the damage of the components when the contaminant particles reach the surface of the optical element. Due to xenon lamp radiation, the elevated temperature of the surface contaminants is sufficient to melt or decompose most of the contaminant particles. This will generate local thermal gradients and thermal shocks on the surface of the optical element, causing hairline cracks on the surface of the optical element, which would expand further. Researchers have conducted extensive research on optical component cleaning procedures and steps, environmental requirements for the use of optical components, and the law of the settlement of contaminants.
“Rigorous cleanliness on the National Ignition Facility (NIF) is essential to assure that 99.5% optical efficiency is maintained on each of its 192 beam lines by minimizing obscuration and contamination-induced laser damage.” said James A. Pryatel and William H. Gourdin from Akima Infrastructure Services and Lawrence Livermore National Laboratory.
Since NIF is one of the pioneers in building high power laser drivers, research on the cleanliness in the internal optical components of integrated chip amplifiers abroad was conducted earlier. Based on the accurate cleanliness identification system (SWIPE）and analytical chemistry techniques used for analysis of non-volatile residues and molecular contaminants while studying the antireflective coating of optical elements in the National Ignition Facility, S.C. Sommer et al. from Lawrence Livermore National Laboratory found that the antireflective coating absorbs the airborne molecular contaminations (AMCs). Ghost images would be produced, and the performance of the antireflective coating would be further reduced after a step of loosening. At the same time, the small molecular weight is volatile, but the large molecular weight is volatile only near the vapor-pressure. As the pressure of the spatial filter is about 5-10 torr (1 torr≈133.322 Pa), just near the large molecular weight vapor pressure, this is one of the sources of the AMCs. The measures to ensure the cleanliness in the installation process include mobile clean room, quick connection technology (no other contamination induced activities in the connection process), and positive pressure assembly. The human factors in the installation process have great influence. Based on the idea of modularization and reducing human factor contamination sources, John Horvath from Lawrence Livermore National Laboratory proposed that the installation of MSA should be carried out in the environment with a cleanliness level of class 100 with the maintenance structure of the amplifier being installed at the bottom of the amplifier, and the amplifier should be installed and replaced online by a sealed transport car. Wang Congyu from SIOM proposed a special technology of the combined MSA for Shenguang II laser driver system. Cheng Xiaofeng et al. from Chinese Academy of Engineering studied the design of the fan filter unit at the top of the combined MSA of the Shenguang-III laser driver system, and introduced some technical measures to ensure the effectiveness of the contamination control such as cleaning method, clean detection, and clean protection in detail.
Most of the studies above aim at the cleaning of the optical elements and cleaning control during installation. However, the cleanliness maintenance of the optical elements in operation is a dynamic process and effective flow field optimization is needed to remove the contaminants produced during the operation. There is no mature research report worldwide on the coupling of gas-solid two-phase flow between the contaminants and the clean gas, and the non-whirl flow of the internal amplifier in the MSA.
Since the clean environment of the MSA internal cavity has a great influence on the optical elements inside, it must be blown with nitrogen or air flow so as to reduce the contamination concentration after pumped by the xenon lamp. Therefore, reasonable and effective flow field of the MSA cavity is particularly important. The particles of contamination and clean gases belong to the category of gas-solid two-phase flow. Computational fluid dynamics (CFD), as a powerful tool for flow field analysis, can optimize the flow field with half effort. With the progress of industrial field, especially in the field of large-scale integrated circuits and biomedicine, the design of a clean room and the optimization of the flow field are important prerequisites for ensuring the quality of the products. Bing Wang from Tsinghua University provides a simplified mathematical method to evaluate the average air velocity and particle concentration by using a similar principle in the air pumping clean room for the wind bottom side. The flow distribution indoor is optimized by CFD technology. Se-Jin Yoo from Hanyang University uses Euler algorithm to simulate the settling velocity of particles. Li Yan from Tianjin University and Zhang Weigong from Harbin University of Civil Engineering and Architecture simulate the flow cleanroom and use air age to predict the flow field. The study on the maintenance of the cleanliness of the MSA revolves around the three factors, which are filter device, airflow rate, and flow pattern of gas flow. However, the research on vector flow of the cavity of the MSA remains to be established. The vector flow is not in just a single direction but can be in any direction. The dilution purification mechanism is not only different from the dilution-mixing effects with non-unidirectional cleaning technology, but also from the parallel streamline piston-effect with unidirectional flow. Although the streamlines of the vector flow are not parallel like the non-unidirectional flow cleanroom, they do not cross. The vector flow does not depend on the mixing effect, however, it relies on the oblique flow to discharge clean gas and contaminant particles.
The paper published in High Power Laser Science and Engineering, Vol.6, e1, 2018 (Ren Zhiyuan et al., Optimizing the cleanliness in multi-segment disk amplifiers based on vector flow schemes) studied the numerical model of the vector flow scheme for the MSA. The experiment confirmed the validity of the numerical model. The optimized vector flow scheme of MSA can more efficiently achieve and maintain its required cleanliness level.
In conclusion, with vector flow scheme, there is no obvious eddy flow in the cavity of the multisegment amplifier and on the surface of the optical elements. Therefore, vector flow can achieve a higher level of cleanliness for the amplifier more efficiently and quickly.