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摘要
在惯性约束聚变(ICF)高功率激光装置中，自适应光学波前控制技术是确保装置安全顺畅通光以及光束质量达标的关键技术之一。本文介绍了我国ICF激光装置中波前控制技术从概念的提出到大规模应用的研究和发展历程，重点介绍了在装置不同发展阶段针对装置的需求所研究和发展的关键系统技术，包括基于远场焦斑优化的爬山法波前控制技术、基于双波前传感器数据融合的全装置波前控制技术，以及旋转腔激光装置结构中基于双变形镜的全系统波前控制技术，并介绍了相关技术在装置上的应用结果。
Abstract
In the high-power laser system for inertial confinement fusion, wavefront control is one of the key technologies for the laser system to ensure it operates safely and reaches the beam quality criteria. In this article, the development of the wavefront control technology from its first being putting forward for the ICF laser system to its application in the latest ICF laser system in China was introduced. During the development of the ICF facilities, the wavefront control methods are varying to satisfy the varied demands promoted by these facilities. Based on different facilities, the methods and the application results are illustrated, including the climbing wavefront method for far-field spot optimization, the full-facility wavefront control method based on the data fusion acquired from two wavefront sensors, and the full-system wavefront control method with bi-deformed mirrors in the rotation chamber laser structure.
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Overview
Overview: Inertial confinement fusion (ICF) is one of the most important controllable nuclear fusion processes by confining particles using the inertial effects. A fuel target, typically in the form of a pellet containing the mixture of deuterium and tritium, is converted to plasma by heating and compressing with high-energy beams of laser light. These plasmas explodes and produces sufficient shock waves to compress and heat the fuel at the center, which makes the fusion reaction occur. This article introduces the typical structure of an ICF system first. The effects of wavefront distortion of the laser beams are analyzed. It shows that the wavefront distortion may affect the near-filed beam quality and reduce the efficiency of the triper device and focusing. Therefore, wavefront distortion must be effectively controlled to meet the needs of the safety of facilities and the physical parameters to realize the ICF process.
The wavefront control methods are illustrated by introducing the development from SG-Ⅰ (LF12), SG-Ⅲ prototype (TIL) to SG-Ⅲ facility. In LF12, climbing algorithm was utilized in adaptive optics (AO) to optimize the far-field quality. It disturbed the elements of the controller to get the highest peak energy, and the peak energy increased 3 times with the AO system. It was the first time that wavefront aberration in ICF system was compensated by AO. It revealed that it was possible to employ the active wavefront control method in these high-energy laser systems to improve their optical quality. Since then, AO system became a standard component in the ICF facilities.
In TIL, 45-element deformed mirrors with the size of 70 mm×70 mm were employed in the prime-amplification system. Bi-wavefront-sensor fusion technique was utilized for system-wide wavefront control. Two Shack-Hartmann wavefront sensors were used. One was located in the parameter-diagnose package to compensate the wavefront aberration in the prime-amplification system. The other one was placed to the fuel target for the system wavefront measurement, and the result was passed to the first sensor for close-loop control. Without AO, 95% of the total energy was gathered in the zone of size 15 times diffraction limit (DL), and the Strehl ratio was 0.02. After compensation, the values were 7 and 0.46, respectively, and the focusing performance was enhanced significantly.
The "U-type reversion + 90° rotation + aperture transformation" configuration, one of the main characters that makes the SG-Ⅲ different from all other ICF facilities in the world, brought challenges to the AO system. 39-element deformed mirrors with the size of 340 mm× 340 mm were used as chamber mirrors for the wavefront compensation of the static and dynamic aberration in the prime-amplification system. 77-element deformed mirrors, placed close to the fuel target, correct the residual wavefront correction after the prime-amplification system. The result showed that both the near-field and far-field distributions were improved remarkably, 95% energy was concentrated in 10DL, which guarantees the full-energy operation stage of the SG-Ⅲ facility.
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图 2 波前畸变(a)及近场调制(b)
Figure 2. Wavefront distortion (a) and its near-field modulation (b)
图 4 一组波前(a)和远场环围能量分布(b)
Figure 4. Wavefront distortion (a) and corresponding far-field energy distribution(b)
图 10 SG-Ⅲ装置中的变形镜影响函数。(a)不旋光的影响函数；(b) 90°旋光及口径变换的影响函数
Figure 10. Influence function in SG-Ⅲ facility.
图 11 39单元AO系统对Zernike像差的校正能力
Figure 11. The correction ability to Zernike aberrations of an AO system with 39 deformable actuators
图 13 小孔上的能量分布(a)及小孔后的近场能量分布模拟(b)
Figure 13. The simulation of the energy distribution on the pin-hole (a) and the near-field energy distribution behind the pin-hole(b)
图 14 SG-Ⅲ装置波前校正系统方案
Figure 14. Schema of the wavefront correction system in SG-Ⅲ facility
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参考文献
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