组合式片状放大器洁净度优化与矢量化流动方式吹扫

安装过程中保证洁净的措施包括:移动洁净室、快接技术(连接过程中无其他引入污染的活动)和正压装配,而安装过程中人的因素影响很大。

基于模块化思想以及减少人为因素污染源,美国劳伦斯利弗莫尔实验室的John Horvath提出放大器的安装须在洁净度为百级的环境中进行,放大器底部须安装维护的结构,并通过密封转运小车进行放大器的在线安装与更换。

上海光机所联合实验室的王聪瑜针对神光Ⅱ激光驱动器,提出组合式片状放大器超净特种装校工艺技术方案。中国工程物理研究院的程晓锋等研究了神光Ⅲ主机装置组合式片状放大器顶部设计风机过滤单元,并详细介绍了洁净清洗、洁净检测、洁净保护等保障污染控制效果的技术手段。

以上技术针对的大多是光学元件本身的清洗,以及安装过程中的洁净控制。然而运行中的光学元件的洁净度维护是动态的过程,需要有效的流场优化设计以清除装置运行过程中产生的污染物。污染物与清洁气体之间的气固两相流耦合运动方式,以及组合式片状放大器内部流场的无漩涡流动方式的研究,国内外未见有比较成熟的研究报道。

由于组合式片状放大器内部腔体的洁净环境对其中的光学元件影响重大,同时氙灯抽运之后必须进行氮气或空气吹扫以降低污染物浓度,因此有效的流场设计就显得尤为重要。污染物颗粒与清洁气体之间属于气固两相流范畴,计算流体动力学(CFD)作为流场分析的有力工具,可令流场优化工作事半功倍。

随着工业的进步,在大规模集成电路以及生物医药领域,洁净室的设计和流场优化排布,是产品质量的重要保证。清华大学的Bing Wang在上供风底侧抽风洁净室中,利用相似原理,得到了简化的数学方法来评估平均速度和粒子浓度,并通过了CFD技术优化了室内流场排布。韩国汉阳大学Se-Jin Yoo利用欧拉算法模拟了颗粒沉降速度。天津大学的李岩,哈尔滨建筑大学的张维功等对矢流洁净室进行了模拟,用空气龄来判断流场推进过程。

影响洁净环境有三大因素:过滤装置、送风量和气体流动流型,国内外学者对主放大器洁净度维持的研究基本围绕这三个因素展开。但是目前,针对矢量化流动的主放大器内部洁净室的流动研究尚处于起步阶段。

所谓矢流推进,即气流并非保持单一方向, 而是在任何方向上都有可能。其净化机理既不同于非单向流洁净室的稀释掺混作用,也不同于单向流洁净室时均流线平行的活塞作用。它的流线并不平行,这一点和非单向流洁净室相同,但不同的是流线不发生交叉,因此不是靠掺混作用,仍然靠推出作用, 只是不同于单向流的平推,而是斜推作用。通过洁净气体矢量化流动产生的气流的斜推,将室内空气排至室外以达到净化空气的目的。

任志远等发表于High Power Laser Science and Engineering 2018年第1期的论文“Optimizing the cleanliness in multi-segment disk amplifiers based on vector flow schemes”,研究了矢量化吹扫组合式片状放大器的数值模型,通过实验确认了数值模型的有效性,经过优化的组合式片状放大器的矢量化吹扫模式可以更高效地达到并维持其所要求的洁净度等级。

综上所述,片状主放大器内部采用矢量化流动方式洁净吹扫,可以令主放大器内部以及光学元件表面流场分布无明显湍流,从而更快速有效地使组合式片状放大器内部及光学元件表面达到要求的洁净度。

多程主放大器在矢量化洁净模式下的流场分布。(a)和(b)是光学元件表面流场形态,(c)和(d)是主放大器内部腔体流场形态。无论是光学元件表面和内部腔体的流场,其分布都无明显紊流,洁净气体的流动非常顺畅匀滑。

Research on cleanliness optimization of multisegment disk amplifier based on vectorized flow mode

“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.

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.

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.

Streamlines for the flow field of the multisegment disk amplifier. Figure (a) and (b) are flow field on the surface of optical elements, Figure (c) and (d) are flow field inside the multisegment disk amplifier. In either circumstance, there is no obvious turbulence in the flow field distribution, and the flow field of the clean gas is very smooth.