Abstract
1 Introduction
For inertial confinement fusion (ICF)[
NIF is the largest high power laser system in the world. Its FOA development has undergone several integrated verifications and design improvements. The conceptual design[
Actually the main external difference among the representative international FOA lies in the color separation technique and the harmonic conversion crystal. But, the greatest internal challenge to the FOA is to attain high laser performance while maintain low optical damage. High performance and high damage resistance are two demanding aspects of FOA. However, damage problem of final optics at 351 nm is one of the bottlenecks of the high power laser systems. The safe and stable operation of FOA at 351 nm with high fluence is affected by the laser damage, which is related to materials, manufacturing, design and engineering.
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In this article, the design and performance of the FOA in SG-II Upgrade laser facility are mainly reported. The SG-II Upgrade laser facility[
2 Design of the FOA
The FOA is at the end of the whole laser beam line in high power laser driver, which is a key subsystem connecting the laser system with the physical experiments. The FOA has multiple functions including harmonic conversion, color separation, laser focusing, beam sampling, debris shielding, vacuum sealing and so on. Thus it is very challenging to produce such an FOA from scientific and engineering aspect.
2.1 Design specifications
Generally, the FOA has to meet the fundamental needs in two aspects. From the physical aspect, laser energy must be applied into the target hole as much as possible and a certain uniformity of laser focal spot must be achieved. From the laser driver aspect, the triple frequency conversion efficiency must be high enough to obtain high fluence 351-nm laser. Meanwhile the FOA should be in safe and stable operation without serious optical damage. The main design specifications for the FOA are listed in Table
Beam diameter | 310 mm |
Focal length | 2.2 m |
Frequency conversion efficiency | 70% |
Focusability | 30 DL@351nm (95% energy) |
Fluence |
Table 1. Main design specifications for the FOA.
2.2 Physical design
Let us give the design logic of our FOA at the beginning. According to the operation fluence and component processing capacity, the FOA configuration can be determined. Then through the stray light analysis, the general component arrangement can be decided. At the same time the far-field focal spot energy concentration determines the tolerance of the optical axis deviation. At last, in order to avoid the self-focusing and filamentation damage problem, the breakup integral (B integral) in the FOA must be controlled under certain value.
According to the designed laser fluence of and 2 mm separation among the focal points of the different harmonics, the wedged focus lens is chosen in the FOA to achieve the functions of focusing and color separation[
With the increase of laser beams and laser energy, the number of optical components and equipment for measurement in the target systems will also be multiplied. To save space, reduce the B integral and increase the clear aperture, it is the latest trend to use the wedged focus lens in the FOA. A coaxial wedged focus lens is designed in the FOA as shown in Figure
Then the detailed analysis of FOA design is reported. First, it is important to analyze the ghost image[
Second, the focal spot characteristics of the FOA are analyzed. The initial condition for the analysis is as follows: (1) The input light is twelfth order super-Gaussian beams with aperture of . (2) The peak to valley (PV) transmitted wavefront of fused silica component is and that of crystal and debris shield is . The BSG is 8 mm thick and is etched on the rear surface of fused silica substrate. The grating structure is similar to an off-axis Fresnel zone plate. The negative first order light is used for sampling, which accounts for about 0.1% of the whole energy. The BSG is tilted in the focusing light path in order to manage the stray light. But it would cause a coma in the far field. Calculation results show that when the wedged focus lens is reversely rotated by with respect to BSG and the debris shield, the coma in the far field can be balanced and thus a good focal spot can be obtained. The focal spot distribution of FOA is shown in Figure
Third, it is also necessary to control the B integral. B integral is used to evaluate the possibility of small scale self-focusing, which is one of the criteria to design and evaluate the overall performance of a high power laser system. For simplicity, the laser intensity in the FOA is considered as a constant. And the nonlinear index of the component is taken as . The residual laser takes half of all the residual light energy. The intensity of , and lasers can be acquired based on the given conversion efficiency. Then the B integral caused by each wavelength is calculated respectively. At last three B integrals are summed up as the total B integral. The B integral is mainly controlled by the total thickness of the component. Calculation parameters are shown in Table
Fluence | Pulse | Conversion | Filling | Total |
---|---|---|---|---|
( | duration (ns) | efficiency | factor | thickness (mm) |
4 | 3 | 60% | 0.6 | 162 |
Table 2. Calculation parameters for B integral.
2.3 Optomechanical design of FOA
Based on the physical aim of FOA, the optomechanical design of FOA is carried out. The FOA should be reliable and easy for maintenance and implementation. The final optics system adopts the modular design. The components including wedged focus lens, crystal, BSG and DDS are replaceable online with certain precision. The FOA is kept at low vacuum sealing to ensure the stable performance of the chemical coating on the component. Meanwhile there is also an adjusting reference interface for 4D interferometer at the entrance of the taper tube, which will be mentioned in Section
3 Integrated performance of the FOA
The SG Upgrade laser facility has eight high power laser beams. They are reflected by the mirrors and then focused into the target chamber center by the FOA. The experimental setup of FOA in the target system is shown in Figure
3.1 Laser fluence
Laser fluence is an important parameter for the FOA. In total 61 shots of large energy laser have been launched in one of the eight beams in SG-II Upgrade laser facility. The output laser energy in the FOA is about 1500–5000 J during the experiment. The laser pulse has a top hat temporal profile with different durations. The maximum laser fluence is . Experimental parameters of laser energy, power and fluence are shown in Figure
Meanwhile, the laser induced damage in the FOA is maintained low in this experiment, which shows that the stray light management technology reported later in Section
3.2 Conversion efficiency
The conversion efficiency is another important factor of FOA. Measurements of and energy and frequency conversion efficiency are shown in Figure
At the same time, the influence of CPP on conversion efficiency is also studied. The efficiency of two shots with CPP is 53.1% and 51.1%. And the efficiency of two shots without CPP is 52.1% and 52.6%. It is obvious that CPP designed for focal spot has only a small influence on the conversion efficiency. By the way, there are also some other factors affecting the conversion efficiency, such as the laser beam quality, the surface figure of the crystal, the transmission loss, and the temperature.
3.3 Laser focus performance
The laser focusability is also one of the important indicators of the FOA. The focal length of the wedged focus lens in SG-II Upgrade laser facility is 2.2 m. The lens is optimized to focus 95% of the laser energy within 2.6 DL theoretically. The angular tolerance around the optical axis is . Sometimes physical experiments need larger focal spot shaped with CPP. The designed focal spot with specific CPP is a round spot with radius of . The detailed design of CPP is not discussed here to keep the paper concise. It is required by the physical experiments that as much of the laser energy as possible is injected into the laser entrance hole of the hohlraum effectively. If the laser focus spot is too large, the laser entrance hole edge material will be heated to plasma, resulting in the pinhole closure effect. The laser focusability is described by the laser perforation efficiency here. The experimental setup of perforation efficiency testing is shown in Figure
A planar target with entrance hole is placed at the target site. The choice for hole size comes from the entrance hole design of the hohlraum for most physical experiments. The laser is injected into an diameter entrance hole. The planar target is designed large enough to block the and lasers. Thus only the laser light could pass through the hole and reach the energy calorimeter. Then we can get the laser perforation efficiency by the ratio of this energy to the energy measured by the BSG. Experimental results are shown in Table
Shot number | Hole ( | Perforation efficiency (%) | |
---|---|---|---|
1 | 2813 | 800 | 97.9 |
2 | 2997 | 800 | 96.7 |
3 | 3105 | 800 | 98.4 |
4 | 2715 | 1000 | 99.2 |
5 | 2471 | 800 | 98.8 |
Table 3. Results of the laser perforation efficiency.
4 Key technology
Although the final experiment results mentioned above were satisfactory, some problems inevitably occurred during the process of project development and debugging in order to realize the high focusability and high fluence performance of the FOA. Our team developed two key technologies to solve the problems based on the original physical design and analysis, which were proved to be very effective to improve the overall performance of the FOA.
4.1 Measurement and adjustment technology of the wedged focus lens
The wedged focus lens is a key component in the FOA. The wedge angle measurement affects the focusing performance of high power laser. Once the processing angle or the working gesture of the wedged focus lens deviates from the given one, the PV transmitted wavefront of the wedged focus lens will increase a lot. Then the focal spot will be enlarged. There is not a mature method to measure the wavefront and wedge angle of the wedged focus lens because of its special shape as far as we know. Here we propose a set of measurement and adjustment technology[
4.1.1 Measurement of the wedged focus lens in the manufacturing process
The wedged focus lens can be treated as a large aperture flat convex lens. But there is a big angle between the main axis of the wedged focus lens and the optical axis of the input laser beam. Figure
In Figure
After that, replace the prism with the wedged focus lens, as shown in Figure
Next a focus lens is added to the 4D interferometer. Make sure that the beam from the 4D interferometer focuses at the focal spot of the wedged focus lens as shown in Figure
4.1.2 Offline debugging of FOA
Through processing under the measurement control mentioned in Section
4.1.3 Online adjustment of the wedged focus lens in the FOA
After offline installation of the FOA, the positioning of the wedged focus lens is consistent with the design scheme. However it may not be the best positioning for working because there is also wavefront deformation induced by other components in the FOA such as frequency conversion crystal, BSG and DDS. So it is necessary to fine adjust the wedged focus lens online to get the optimal transmitted wavefront of the whole FOA.
The online adjusting optical path is shown in Figure
This control technology is first carried out in one of the FOAs in the SG-II Upgrade facility. As shown in Figure
4.2 Stray light management technology based on ground glass
There are six large aperture components in the FOA. Through multiple reflection and focusing of the input laser by the optical surface, there will be the focus of the stray light, i.e., the ghost image. For the high power laser facility, these ghost images irradiating on materials will induce damage to some extent.
Generally speaking, there are usually two ways to evade the stray light. On one hand, choose the distance and angle of each component to ensure that the ghost image does not fall on the optical element surface or the inside of the FOA. On the other hand, use the stray light trap to protect the mechanical structure surface from the ghost image light. Besides the two ways, we invent a protecting technology to manage the stray light by scattering and absorption based on ground glass[
This protecting device is like an armor of the FOA, which can provide the all-around protection of the FOA from the stray light. Combination of ground glass and neutral absorbing glass is mainly used to protect against the first order stray light, which is above . The single ground glass is used to protect against the second order and even higher order stray light, which is below . The ground glass is designed with a sinusoidal surface to reduce the intensity of the stray light as shown in Figure
At the same time, ground glass is hydrofluoric acid (HF) etched assisted by the ultrasonic during the manufacturing processes, which can improve the laser damage resistance of the ground glass. So the ground glass would not be damaged by large amount of stray light irradiation. The morphology of ground glass before and after hydrofluoric acid etching in the electron microscope is shown in Figure
5 Discussion
It is a great challenge for us to build the FOA in SG-II Upgrade laser facility under the condition of limited space and short focal length. Through the optimization design, the FOA in the engineering practice has achieved a pretty good performance, which is close to the level in international high power laser facilities such as NIF and LMJ. Physical experiment effect is the touchstone to test the FOA in high power laser facilities. A series of experiments have been carried out in SG-II Upgrade laser facility such as fast ignition and proton imaging, which have already obtained excellent results. This shows that our team has acquired the capability to design and build a high power laser facility with a high level of performance. There is also a high energy petawatt laser beam facility in National Laboratory on High Power Laser and Physics. A lot of innovative physical exploration research can be launched by the SG-II Upgrade laser facility together with the petawatt laser beam.
Nowadays, one of the challenges in the high power laser facility is the UV damage in the FOA. With the increase of the laser fluence, the damage problem is becoming more and more prominent, which seriously influences the costs and performance of the physical experiments. The main problem is that there are many factors related to the damage initiation, such as laser beam quality, surface quality of the component, and working environment. The main work in the future is to determine the laser damage inducement under different laser fluences and then improve the load capacity of the FOA correspondingly. By the way, the higher load capacity of the FOA first needs better beam quality to support. It puts forward a higher requirement for not only optical component surface quality and cleanliness level but also active control ability of the beam quality. Our facility will be continuously upgraded to meet the growing demands from the physical experiment.
6 Conclusion
In conclusion, the design and performance of the FOA in SG-II Upgrade laser facility are mainly reported here. The experimental results of FOA show that the design requirements including the laser fluence, frequency conversion efficiency and the perforation efficiency of the focus spot have been achieved, which well meet the needs of physical experiment. In order to ensure the FOA performance, two key technologies are also developed including the detection and adjustment technique of the wedged focus lens and the stray light management technique based on ground glass protection. In the near future, a lot of experimental work will be carried out to improve the load capacity of the FOA. At last, SG-II Upgrade laser facility is open to international users. We are looking forward to more international cooperation to meet new challenges together.
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