Cleaning State of the Loop Case for Optical Crystal Module in Final Optics Assembly
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In order to solve the online clean maintenance problem of the optical frequency-doubling crystal module in the final optics assembly, a crystal loop case is developed for better controlling the clean maintenance process. The study presents a detailed research on the cleanliness of the crystal loop case during the online maintenance process and the installation sequence of the crystal modules in the crystal. It first established a fluid simulation model of the crystal loop case system according to the online maintenance process and then analyzed the isolation effect of inlet velocity on the polluted air in view of the cleaning effect of the crystal loop case. Based on the simulation principle of gas–solid two-phase flow and the tracking results of the solid particle contamination, the range of air inflow is given for achieving the best cleaning status of the loop case. The optimal sequence of crystal module is explicit for its installation in the crystal loop case of frequency-doubling crystal module. The experimental setup has been built to examine the cleaning state of the crystal loop case, and the simulation result has been validated. The research on the cleanliness of crystal loop case can provide a useful reference for the ultra-clean manufacturing of line replaceable units and the closed loop control of cleaning in high-power laser facility.
KeywordsLoop case Cleaning state Particle contamination Final optics assembly Ultra-clean manufacturing
Inertial confinement fusion (ICF) as an effective method of achieving controlled fusion is developed as a strategic high technology all over the world . National Ignition Facility in the USA, Laser Mégajoule in France, ShenGuang (SG) facility in China and others have been established over the years. The final optics assembly (FOA) is located at the end of the laser ICF facility, which is one of the key components of the SG facility. In the process of laser target shooting, a variety of large-caliber optical elements are contained in the final optics assembly which has many essential functions, such as frequency conversion, harmonic separation, focusing transmission, beam sampling and so on. The effect of high-power laser will lead to the damage of the optical components, and optical components need to be frequently replaced and maintained. Linear replacement units (LRUs) of SG facility are formed by these optical components and their replacements .
The surface cleanliness of optical components is significant conditions for high throughput of the laser in high-power laser devices [3, 4]. The surface contaminants of optical components can affect the beam quality and reduce the damage resistance of optical components seriously, which result in decreasing load capacity of the laser driver [5, 6]. Researchers suggested that suspended particulates were a pivotal cause in the surface contamination of optical components [7, 8]. Optical components need to avoid that the optical components directly expose in the air containing dust particles when they are removed and installed. It is necessary to grasp closed loop cleaning control of the online clean maintenance process. The crystal loop case for optical crystal module in final optics assembly can not only realize the rapid assembly of the optical components, but also can avoid the pollution of suspended particles during the online maintenance process of the optical components . Pryatel et al.  analyzed the cleaning process, treatment methods and cleaning equipment in NIF and provided the theoretical basis for cleaning transportation equipment design. Wang et al.  investigated the lower part of the LRUs module in the laser device for the position monitoring technology and provided the conditions for the module’s installation process. However, these researchers did not directly involve the problems about calculating flow field and designing the cleaning module. Wong  analyzed the production mechanism of particle contamination in high-power laser system and provided a theoretical basis for the engineering design of the device. The crystal loop case had been designed for the optical components’ assembly in NIF. The loop case design for the optical components’ assembly met the requirement of online clean maintenance for large aperture optical component modules . In order to ensure the well-ventilation in crystal loop case and realize effective online removing of optical elements, it is necessary to analyze the internal flow field state when the loop case is designed. To confirm that the change regulations of the cleaning state inside the loop case can be mastered, the design of the loop case need to be optimized . So far, in the development of high-power laser device, the design and analysis of the loop case for optical crystal module in final optics assembly were seldom reported. The clean state analysis of the loop case and its verification tests are the necessary prerequisite for cleaning design of the high-power laser device and its maintenance equipment. It is of great significance to realize fully the closed loop clean control of systems and promote the development of ultra-clean manufacturing in high-power laser device.
2 The Design of Loop Case and Analysis of Air Flow Field
2.1 The Design of Loop Case
2.2 Simulation Model and Critical Gas Flow Velocity
Figure 4 shows that the maximum velocity on the monitoring plane correlates well with the inlet speed. Due to the impact of the loop case structure, the slight fluctuation of the maximum velocity on the monitoring plane ranges from 0.25 to 0.4 m/s. When the inlet speed is less than 0.2 m/s, the maximum velocity of outlet 2 is greater than zero. Therefore, the outside polluted gas can enter the loop case from outlet 2. When the inlet speed exceeds 0.2 m/s, the maximum velocity of outlet 2 is less than zero and the outside gas cannot enter the loop case. Meanwhile, by analyzing average velocity of each monitoring surface, the results show that the average velocity of each monitoring plane is linearly related to the inlet velocity of the cleaning gas. The analysis indicated that average velocity of each monitoring plane was linearly related to the inlet velocity of the cleaning gas. As the inlet velocity increases, the absolute value of the average velocity increases. The maximum velocities of the three monitoring planes are all less than zero when the inlet speed is bigger than 0.2 m/s. The result indicated that clean gas could isolate outside pollution. Therefore, the speed of 0.2 m/s is the critical air velocity to guarantee the cleaning state of the loop case in maintenance.
3 The Effect of Air Intake on Cleanliness
With the variation of the air inflow of positive pressure clean gas in final optics assembly, the trajectories of particles can be changed significant correspondingly. The intake air volume can affect the volume of the eddy current area inside the loop case and amount of the eddy current. The eddy current resulted in irregular diffusion movement of suspended particulates . If the suspended particulates cannot be removed from the inside of the loop case, they will remain on the surface of the crystal elements. The suspended particulates would induce and aggravate the damage of the optical components under strong laser light . The damage of optical elements is also a significant source of particle contamination. From the above analysis, the result can be found that optical elements can effectively isolate the outside polluted gas when the inlet speed reaches 0.2 m/s or intake capacity is 650 L/min. In addition, in order to remove surface debris at a rate of nearly 100% efficiency (> 60 μm), some scholars had studied a 1-s high-speed (76 m/s) air pulse from commercially available “air knife” crossing the mirror to its surface . Nevertheless, in the actual work process, the particles outside the loop case will also pollute the crystal components. The particles from residual materials and production in the components during the operation are also likely to diffuse into the loop case. The particles remain inside the loop case under the action of positive pressure cleaning gas and further result in contamination of the crystal elements surface. Therefore, it is necessary to further analyze the particles changes inside the loop case with the intake air of the clean gas and determine the optimum air intake for the minimum residual particles in the loop case under the condition of isolating positive pressure protection.
Figure 5 shows that the number of particles trapped on the surface is zero and the number of suspended particles is less when the inlet velocity is in the range of 0.2–0.25 m/s or 0.45–0.5 m/s. Once the inlet velocity exceeded 0.5 m/s, the number of particles trapped on the surface increases dramatically. The inlet velocity increase will increase the probability of the particles depositing on the surface of the crystal elements. Therefore, taking the external gas isolation effect into account, the intake velocity of 0.45–0.5 m/s or intake air volume of 1464–1626 L/min is chosen as the optimum parameter. Because the inlet velocity is too small to effectively remove the particulate matter in the loop case and also cannot form the isolation of the external pollution gas. When the inlet velocity is too high, the swirl area and the number of swirls in the loop case will increase, which aggravates the diffusion cycles of particulate pollutants, and it is not conducive to its cleanliness. So the wind speed is 0.45–0.5 m/s when the particles contamination in the loop case is the least.
4 The Influence of Crystal Mounting Sequence on the Clean State of the Loop Case
Simulation models for various sequences of installation
Only install crystal 1 into the assembly
Only install crystal 2 into the assembly
Only install crystal 3 into the assembly
Install both crystals 1 and 3 into the assembly
Install both crystals 1 and 2 into the assembly
5 Experimental Validation of Clean State of the Loop Case
In this paper, the influence of the intake air volume on the clean state inside the loop case is obtained by the simulation. The simulation results show that the minimum air intake volume is 650 L/min which can isolate the outside polluted gas. Based on the theory of gas–solid two-phase flow, the optimal air intake volume is chosen as range from 1464 to 1626 L/min which can keep excellent cleaning state inside the loop case by using the simulation results of the solid particles’ trajectory. It determines the optimum installation sequence of crystal elements in the loop case. Finally, the study sets up an experimental detection system, which detects the clean state of loop case. The simulation results were verified by experiments. The results of this study can provide a theoretical basis for the engineering design of the loop case and formulation of clean process.
This research work was jointly supported by the State Key Program of National Natural Science Foundation of China (Grant No. 51535003) and the National Natural Science Foundation of China (Grant No. 51575138).
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