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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.
Schema of the typical structure of an ICF system
Wavefront distortion (a) and its near-field modulation (b)
Heating recovery process of a laser system
Wavefront distortion (a) and corresponding far-field energy distribution(b)
Far-field distribution before and after wavefront correction using climbing algorithm[8]. (a) Open loop; (a) Closed loop
Far-field energy distribution before and after AO correction[13]. (a) AO open loop; (b) AO close loop
Schema of a single laser beam system in SG-Ⅲ
Schema of the laser reversion system in SG-Ⅲ
Influence function in SG-Ⅲ facility.
The correction ability to Zernike aberrations of an AO system with 39 deformable actuators
The 95% energy range after correction
The simulation of the energy distribution on the pin-hole (a) and the near-field energy distribution behind the pin-hole(b)
Schema of the wavefront correction system in SG-Ⅲ facility
The near-field and far-field energy distribution before and after wavefront correction in SG-Ⅲ facility. (a) Far-field open loop; (b) Far-field energy distribution open loop; (c) Far-field close loop; (d) Far-field energy distribution close loop