The laser-induced damage of aluminum alloy 5083 is affected by its surface treatment. Here, we investigate the effects of aluminum alloy’s ability of laser-induced damage on micro-arc oxidation and composite coatings treatment. Results demonstrate a distinct difference of damage parameters on laser pulse duration at 355 nm between micro-arc oxidation and composite coatings treatment on aluminum alloy, including the laser-induced damage threshold, particulate pollutants for different surface treatment, and the damage morphology, respectively. We now find that the threshold of laser-induced damage is improved a lot through simulative calculation and experiments. Furthermore, the experimental results suggests that surface treatment contribute to the number of particulate pollutants and the microstructure of damaged pit.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Laser-induced damage on aluminum alloy 5083 is a complex issue on different surface treatment. As we know, laser fusion devices have been built in many countries in the world, like the National Ignition Facility (NIF) from the USA, the Laser Mégajoule (LMJ) from the France and the Inertial Confinement Fusion (ICF) laser facility from China [1–3]. However, the stray light in optical system caused by multi-order diffraction and multi-reflection will affect the system. For example, the NIF uses 192 laser beams with 400 mm x 400 mm of each laser beam, the high energy and peak power of a laser beam is 18kJ [500TW]. Many scientists researched the laser-induced damage on optical elements, including fused silica optics and KH2PO4 single crystals (so-called KDP). They researched the growth behavior under multiple irradiation parameters [4–6], growth of sites on the exit and input surface [7,8], and damage mechanisms, precursors, and their mitigation , respectively. Although anti-reflective coatings are deposited on the surface of optical elements to increase transmittance, the reflection coefficient of 0.1% can still bring 18J ghost reflection [10–15]. The LMJ built by French Commissariat à l'Energie Atomique (CEA) uses 240 beams laser facility that are 400 mm x 400 mm each, which can also bring ghost reflection [16–19]. In the high-power laser system, when the fluence is more than 0.1 J/cm2, the pollutants produced by metal surface damage in the system will aggravate the damage of optical elements under the action of intense laser, which will affect the system performance and beam transmission [12,20]. Most of the metal frames are made of aluminum alloy. Thus, the research of anti-laser-induced damage on aluminum alloy under surface treatment is necessary.
It can't be directly irradiated by laser because the Aluminum alloy have low reflectivity and the melting point of it is low . However, after being treated by anodic oxidation, Aluminum alloy 2024 can have low absorptance and high emittance . Silicon dioxide thin film is a very important dielectric film, which has the characteristics of high hardness, good wear resistance, low refractive index, low extinction coefficient and low dispersion. Therefore, it is widely used in semiconductor, optoelectronics, optical components and other fields, as optical film, protective film and insulation film . Many scientists are trying to fabricate silicon dioxide films with high stability, transparency and wear resistance by various methods, such as thermal oxidation , chemical vapor deposition  and magnetron sputtering , etc. Recently, some researchers have studied laser damage of optical materials and thin films through simulation and experiments. By establishing laser damage model, the effects of laser damage are simulated by finite element method. They find the laser-induced damage threshold is related to the existence of impurities [27–29]. In addition, there are many works have been done on film damage. Traditional optical thin films are mainly coated on the surface of optical components as reflective or anti-reflective coatings. High laser-induced damage threshold optical thin films can improve the energy and comprehensive performance of power laser system [30–32]. Much of these work is aimed to investigate the damage and protection of optical elements and electronic components. However, the research about the anti-laser-induced damage on metal frames is still under progressing.
In this work, we demonstrate that the ability of anti-laser-induced damage on aluminum alloy 5083 improves a lot after micro-arc oxidation and composite coatings treatment. In order to reduce the surface reflectivity of aluminum alloy, micro-arc oxidation technology is used to aluminum alloy. A layer of aluminum oxide coating is formed on the surface of aluminum alloy, which can effectively absorb stray light. Similarly, in order to improve the laser damage resistance of micro-arc oxidation on aluminum alloys, silicon dioxide thin films were deposited on the micro-arc oxidation on aluminum alloys, which were called composite coatings. So in our work, we firstly simulate the temperature field and thermal stress field of the substrate and thin film by finite element method, calculate their laser damage threshold, and analyze the failure law of the material. And then, silicon dioxide thin film was prepared on micro-arc oxidation of aluminum alloys. Laser damage tests and characterization analysis were carried out on the films, and laser-induced damage thresholds before and after coating were calculated to verify the correctness of simulation. Finally, the number of particles produced by laser irradiation is compared between the micro-arc oxidation and composite coating films treatment, the anti-laser damage ability is analyzed, and the laser damage morphology is observed. The good consistency between simulation and experiment reveal that the composite coatings can produce fewer particles, and the laser damage resistance of the composite coatings is higher than that of the micro-arc oxidation of aluminum alloy.
2. Simulation process
Geometric model of micro-arc oxidation and composite coatings on aluminum alloy 5083 was established in simulation. Under laser irradiation on aluminum alloy, the model is cylindrical, and the two-dimensional axisymmetric model takes the COMSOL Multiphysics as tool for simulations. In order to improve the efficiency of simulation calculation and save calculation time, the diameter and thickness of the model are 2000 μm and 200 μm. The model has met the size requirements due to the 480 μm of laser spot radius. The thickness of aluminum alloy coating is 16 μm under micro-arc oxidation treatment. Besides that, the composite coatings treated aluminum alloy 5083 owns silicon dioxide thin films with thickness of 3 μm. The thermophysical parameters of aluminum alloy for simulation are shown in Table 1. The triangular mesh is used for mesh generation. The local mesh near the laser irradiation center is more compact, and the local mesh near the irradiated surface is also more compact.
Due to the laser-induced damage of finite element were conducted to model the temperature field and thermal stress field in sample. In the simulation and experimental research, the laser used is a Gauss distribution laser. The energy of the Gauss distribution laser is concentrated in the radius of the spot, and the spatial distribution of the intensity is expressed as follows.
When the surface of an object is irradiated by Gauss distribution laser, it will produce uneven temperature rise after absorbing laser heat. When the temperature rises, it will cause thermal deformation and thermal stress of the object, which will lead to the destruction of the object. The thermal shock impedance of materials can be used to express the thermal damage caused by the absorption of laser heat, which shown as follows.
The damage effect of thermal stress on materials irradiated by laser can be judged by Von Mises yield criterion, which is used to evaluate the fatigue failure of materials. The main content of Von Mises criterion is that fatigue failure occurs when the stress exceeds the yield limit of the material, and the equation of Von Mises equivalent stress is as follows.
Firstly, the temperature field of the model is simulated. The pulse duration is 6 ns. Figure. 1(a) shows the simulation results of micro-arc oxidation treatment at 1.2 J/cm2, it can be seen that the heat affected zone is much larger than the composite coating films treatment illustrated in Fig. 1(e) at the same laser fluence, which surface has hardly changed. It is noteworthy that the zone between the micro-arc oxidation and silicon dioxide films layer beneath the surface begins to melt as displayed in Fig. 1(f) (see in the white square frame). It is complicated to explain that the temperature of surface is 1040K, which is lower than the both melting point of the micro-arc oxidation (2327K) and silicon dioxide films layer (2193K). It can be attributed to the thickness of silicon dioxide film is only 3 μm, the depth of heat affected zone is thick than that, and the melting point of silicon dioxide films is higher than that of micro-arc oxidation layer, so the laser penetrates the thin film and attributes to the heat affected zone between the silicon dioxide films and micro-arc oxidation layer more apparently than that On the surface. Nevertheless, the silicon dioxide films didn’t reach its melting point until the fluence grows to 2.12 J/cm2 as observed in Fig. 1(c).
Besides that, it can be seen the temperature of the irradiation center of the micro-arc oxidation on aluminum alloy has reached 2330K and reached the melting point of 2327K at the end of laser loading as shown in Fig. 1(a).So the laser damage threshold of the micro-arc oxidation is 1.2 J/cm2.At the same fluence, the temperature of composite coating films treated aluminum alloy surface is 1040K(see in Fig. 1(c)),when the fluence rises to 2.12 J/cm2, it has reached 2210K that over the melting point of silicon dioxide thin films (2193K). So the laser damage threshold of the composite coatings is 2.12 J/cm2. Laser-induced damage threshold of coated coatings is 0.92 J/cm2 higher than that of only micro-arc oxidation treatment. After that, the variation of temperature and Von Mises stress are simulated (Both are in the Z-axis direction). Figure 2(a) demonstrates the changes of temperature on laser pulse duration at 355 nm from 0 ns to 6 ns. There is a sudden Von Mises stress change at the interface between micro-arc oxidation and silicon dioxide films shown in Fig. 2(b). The Von Mises stress mainly concentrates on the micro-arc oxidation. Owing to the larger thermal expansion coefficient of alumina, the existence of Silicon dioxide film restricts the thermal deformation of alumina coating and produces huge thermal stress, which leads to the shedding of the film and the destruction of the film.
The laser-induced damage of aluminum alloy is a complex process by simulation. The pulse duration is short (6 ns). Therefore, the finite element analysis can give a good explanation for the thermal damage and thermal stress damage of materials. It is meaningful to better understand the mechanism of laser-induced damage threshold of aluminum alloy 5083. Besides, it’s also help us to guide the experiment by simulation, such as setting laser parameters and fabricating thin films of alumina coating and silicon dioxide film, respectively.
3. Sample preparation and experimental procedure
3.1 Sample preparation
The samples were 50 mm in diameter and 5 mm thick aluminum alloy 5083 treated by micro-arc oxidation and composite coatings. We firstly treat the aluminum alloy by micro-arc oxidation. The loose layer is removed by mechanical polishing because the second layer-silicon dioxide films need a relatively smooth substrate. After polishing, a layer of smooth alumina coating was on the aluminum alloy. Then, silicon dioxide thin films were prepared on micro-arc oxidation of aluminum alloy by Medium Frequency Reactive Magnetron Sputtering, which was called aluminum alloy composite coatings. The micro-arc oxidation treated aluminum alloy are used to make a comparison with the composite coatings. Reactive magnetron sputtering (RMS) is an energy-intensive ion (Ar+) sputtering on the surface of metal or alloy targets. The sputtered metal atoms react with reaction gases to form compound films on the substrate. The experimental equipment used in the experiment of preparing silicon dioxide thin films by magnetron sputtering is two-chamber magnetron sputtering and electron beam coating equipment. The aluminum alloy (after micro-arc oxidation)is with a diameter of 100 mm and silicon target used in the experiment of preparing silicon dioxide thin film is with a diameter of 50 mm. Specific details on sample experimental parameters are listed in Table 2.
3.2 experimental procedure
The experimental system consists of a single longitudinal mode SAGA-S laser Nd: YAG laser, collimated light source, focusing lens, splitter wedges, an EPM2000 energy calorimeter, a sample carrier (two-dimensional adjustable, step-by-step accuracy is 10 um), an optical microscope and a computer. The experimental setup is depicted in Fig. 3. The pulse duration is 6 ns, the laser spot area is 0.7 mm2, and the laser repetition is 1 Hz with the adjustable fluence. The output energy can be adjusted by changing the angle of 1/2 wave plate inside the laser. The laser is a pulsed laser with Gaussian beam profile at the wavelength of 355nm, and the parameters are consistent with the simulation parameters.
To investigate the anti-laser-induced damage ability of micro-arc oxidation and composite coatings treatment on aluminum alloy 5083, the damage testing of aluminum alloy 5083 Micro-arc oxidation before and after composite coatings was conducted. We first adopted the “r-on-1” test mode to test the silicon dioxide thin films because of the small testable area of the sample, which means the same point on the sample is tested by increasing energy gradually until the sample is damaged. The method of laser damage measurement is to irradiate the sample several times with laser fluence of 0.5 J/cm2, 1 J/cm2 and 1.5 J/cm2 because in high power laser system, the fluence of stray light irradiated on metal support is usually less than 2 J/cm2. Each laser fluence irradiates three points and each point irradiates 15 times and then the laser damage threshold of the sample is calculated. A particle collector is placed in front of the sample to record the number of particles with different diameters produced by each irradiation. The sampling period of the particle collector is 30 seconds, and the laser spot area used in the test is 0.8 mm2.The microscopes were used to observe the damage morphology of samples irradiated by different laser energy fluence. The distance between the test points and the experimental points should be three times larger than that of the laser spot. So the distance between the selected test points is 5 mm. The plasma spark method is used to judge the damage. Laser damage threshold of silicon dioxide thin film samples was calculated according to the laser spot area and laser energy used in the test. In order to find an appropriate coating process. The characterization and analysis of thin films were carried out before the experiments.
4. Result and discussion
4.1 The analysis of silicon dioxide thin film
To find the proper parameters for the silicon dioxide thin film for aluminum alloy 5083. Some analyses must be conducted before the laser irradiation. The film thickness is firstly measured by the FORM TALYSURFPGI1240 aspheric profilometer produced by Taylor Hobson Company. In the experiment of preparing silicon dioxide thin films, two polished silicon wafers are used to make thin film - silicon wafer - thin film step, and the thickness of thin films is measured by profilometer. The thickness of the films is shown that the thickness of the films measured in the three experiments is 1.25 μm, 1.7 μm and 2.8 μm, respectively. Under the same sputtering power, the thickness of SiO2 films increases with the increase of sputtering time. Then we choose the sample 3 as the experimental element for its thickness in consistence with simulation.
X-ray photoelectron spectroscopy (XPS) can be used to detect element composition and quantitative analysis of elements in the thin film, and has a wide range of applications. Therefore, X-ray photoelectron spectroscopy was used to identify the element composition and the element content in the film sample 1. According to the detection results, the ratio of argon and oxygen in the coating process was adjusted to prepare the silicon dioxide film with the required atomic ratio, which can guide the subsequent experimental process.
In the process of identification of thin film sample 1 by X-ray photoelectron spectroscopy (XPS), the wide scanning peaks of sample 1 were obtained. The wide scanning peaks before and after surface etching were shown in Figs. 4(a) and 4(b). According to the corresponding binding energy and wide scanning peak of each element, the main elements in the film before surface etching are O, Si and C, and after surface etching, the elements in the film are mainly Si and O. It can be seen that the prepared film sample produces silicon oxide. After etching on the surface of the sample, the content of C element in the film decreases from the wide scanning peak, which indicates that the surface of the film is contaminated by carbon dioxide in the air during the transfer process.
In order to further determine the chemical composition of oxygen and silicon in the films and the existing state of compounds in the films, high resolution XPS spectroscopic scanning of silicon elements was carried out, and the narrow scanning peaks of the Si2p orbital were obtained. The narrow scanning peaks of silicon were fitted by XPS software according to the binding energies corresponding to the valence states of silicon element and the possible oxide composition of silicon in the film. According to the fitting results, the valence states of silicon in the films are + 4 and + 3, respectively, silicon dioxide and silicon trioxide. The corresponding contents are 54.3% and 45.7%, respectively. The number ratio of oxygen atoms to silicon atoms is 1.7, which has not reached the required atomic ratio of silicon dioxide films. Therefore, the oxygen content is increased in the subsequent experiments, and the argon and oxygen are adjusted. The ratio is changed to 70:30 to produce silicon dioxide films with atomic ratio, through which we can prepare the silicon dioxide thin films on micro-arc oxidation treated aluminum alloy 5083.
4.2 Laser damage testing of aluminum alloy micro-arc oxidation and composite coatings
The micro-arc oxidation and composite coatings of aluminum alloy 5083 were observed by SEM. It can be seen from the image that the morphology of micro-arc oxidation on aluminum alloy is different before and after silicon dioxide thin films (see in Fig. 5). The surface of both is uneven and rough. But the surface of aluminum alloy composite coatings is smoother than that of only micro-arc oxidation. In general, the rough surface quality of the substrate affected the surface quality of the films which called substrate structural defects. But in this composite coatings, the micro-arc oxidation layer was first formed on the aluminum alloy 5083 substrate, and the magnetron sputtering was carried after the removal of loose layer of micro-arc oxidation layer. So the silicon dioxide film on the surface of micro-arc oxidation layer and with it form composite coatings, the quality of which surface is better than only micro-arc oxidation.
The quality of surface of composite coatings is superior to that of micro-arc oxidation, which can be attributed to the double-mixing layers, which means there being two mixing layers between the silicon dioxide film and aluminum alloy 5083 substrate. The process of micro-arc oxidation will form a mixing layer between the substrate and dense layer of oxide film, and another mixing layer will be formed which mixes silica and alumina between the oxide film and silicon doxide thin film. Each suture layer can effectively isolate the pit defects which means the double-mixing layers lead to the better surface quality of composite coatings which is illustrated in Fig. 6.
To investigate the conditions of laser-induced damage, the number of particles with different diameters produced in each laser irradiation was recorded while laser irradiated micro-arc oxidation 5083 and composite coatings of aluminum alloy 5083. After laser irradiation test, the number of particles with a diameter of 0.5 um produced from the sixth to fifteenth times was plotted as a curve, and the number of particles with a diameter of 0.5 μm produced by micro-arc oxidation on aluminum alloy 5083 and composite coatings on aluminum alloy 5083 was compared. The results are shown in Fig. 7, which demonstrates the changes of particles under the conditions of laser fluence of 0.5 J/cm2, 1 J/cm2 and 1.5 J/cm2, respectively.
It can be seen from Fig. 7 that the number of particles produced by aluminum alloy 5083 composite coatings is less than that produced by micro-arc oxidation of aluminum alloy 5083 when the laser fluence is 0.5 J/cm2, 1 J/cm2. However, there is a different result when the laser fluence is up to 1.5 J/cm2, the number of particles produced by both of two ways is almost the same. The removal of loose layer by micro-arc oxidation lead to this result because the loose layer contains lots of particles. When that layer is removed and the fluence of laser grows to 1.5 J/cm2, the two surfaces produce the similar number of particles. Silicon dioxide film can ameliorate the laser-induced damage threshold of aluminum alloy 5083 micro-arc oxidation.
After laser irradiation experiments on micro-arc oxidation and composite coatings of aluminum alloy 5083, the damage morphology of the samples irradiated by different laser fluence (0.5J/cm2, 1J/cm2 and 1.5J/cm2) was observed by ultra-depth-of-field microscopy. The results are shown in Fig. 8, the magnification of which is 200 times. It can be found that the damage area of the sample is growing with the increase of the laser fluence, and the damage degree is much more clearly. The damage morphology of micro-arc oxidation on aluminum alloy 5083 has changed after being treated with silicon dioxide film. Besides that, the morphological damage of micro-arc oxidation on aluminum alloy 5083(see in Figs. 8(a)–8(c)) and composite coatings on aluminum alloy 5083(see in Figs. 8(d)–8(f)) at the same fluence is compared, which can be seen the damage of composite coatings is obviously smaller than that of only micro-arc oxidation at the same fluence. After coating treatment, the physical properties of the surface of 5083 micro-arc oxidation of aluminum alloy have changed, the melting point has increased, and the ability of absorbing laser and conducting heat on the surface has also improved.
The micro-arc oxidation and composite coatings of aluminum alloy 5083 were irradiated several times under the condition of 1.5 J/cm2 laser fluence. The irradiation times were 1, 5, 10, 20 and 50 times. The damage morphology was shown in Fig. 9 and Fig. 10, and the amplification times were 200 times. It can be seen from Fig. 9 and Fig. 10 that under the same laser fluence, the damage of the sample becomes more and more obvious with the increase of laser irradiation times. Moreover, the damage area of the sample increases gradually at first and then tends to be stable with the increase of irradiation times. When irradiated 50 times, the damage morphology of 5083 composite coatings was obvious. From the damage morphology, we can see that the surface treatment on aluminum alloy 5083 can effectively enhance it anti laser-induced damage threshold. The double-mixing layers was formed to isolate the substrate pit defects which is more excelled than only a mixing layer formed by micro-arc oxidation.
In addition to the particulate pollutants and damage morphology discussed above, we also pay attention to the microstructure of damage pit with micro-arc oxidation and composite coatings. The differences of them are apparent under scanning-electron microscope (SEM) examination. SEM images of the center of pits showing the different damage mechanisms under the same laser irradiation which are shown in Figs. 11(a) and 11(b), respectively. The higher spatial resolution images in the blue frame. We all know the top layer of composite coatings is silicon dioxide thin film, Silicon dioxide is deposited on the surface of aluminum alloy 5083 with micro-arc oxidation by magnetron sputtering, and the target atoms form silicon dioxide film on the surface. When the surface is damaged by laser irradiation, the change of surface microstructure is the shedding of individual particles. However, the surface treated by micro-arc oxidation will flake and crack in the form of broken scales under irradiation.
In this paper, laser damage simulation of micro-arc oxidation and composite coatings of aluminum alloys is carried out by finite element method, laser-induced damage threshold is calculated, and the temperature and thermal stress variation law produced by laser irradiation is obtained. Silicon dioxide thin film is prepared by magnetron sputtering technology, and the micro-arc oxidation and composite coatings of aluminum alloys are irradiated by multiple laser irradiations. The number of particles produced by the laser irradiation changes significantly after sputtering dioxide silicon films, which is quite different with the only micro-arc oxidation layer. The results of simulation and experiment are compared and analyzed. The main conclusions are as follows: A laser damage model of aluminum alloys 5083 micro-arc oxidation and silicon dioxide composite coatings system is established. The simulation results show that silicon dioxide thin film can improve the laser-induced damage threshold of aluminum alloys 5083 only treated by micro-arc oxidation; multiple irradiation of aluminum alloys 5083 micro-arc oxidation and composite coatings under different laser fluence is carried out. By comparing the number of particles produced by laser damage, it is found that the composite coatings can produce fewer particles. The double-mixing layers can effectively eliminate the uneven pits geometrically on aluminum alloy 5083, and the laser-induced damage threshold of the composite coatings is higher than that of the micro-arc oxidation on aluminum alloy.
National Natural Science Foundation of China (NSFC) (51535003, 21573054), State Key Laboratory of Precision Measurement Technology and Instruments (PILAB1701), the Joint Funds Key Project of the National Natural Science Foundation of China (U1537214).
We thank Jiaxuan Chen, Shusen Guo and for assistance in the execution of experiments and the preparation of samples. This work was performed under the auspices of the Center for Precision Engineering (CPE) of Harbin Institute of Technology (HIT).
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