改善激光在靶面的辐照均匀性,一直是惯性约束聚变(ICF)驱动器中非常关键的课题,在过去的30多年里已经发展了多种光束匀滑技术。其中空间匀滑技术可以控制焦斑的包络形状并改善大空间尺度的辐照均匀性,时间匀滑技术则可消除焦斑内部高空间频率的强度调制。综述了国内外现有的各种光束匀滑技术及其进展,并对“神光Ⅱ”激光装置的光束匀滑要求、拟采用的光束匀滑路线及其中关键技术单元的研究进展做简单介绍。

The target irradiation uniformity is improved by using two-dimensional smoothing by spectral dispersion and a lens array (LA). A pattern of steep edges and good middle-scale uniformity is obtained with a diffraction-weakened lens array. When two-dimensional spectral dispersion is employed in the laser chain, the beams experience spectral dispersion in two vertical directions, and the fine stripes caused by beamlets interference are smoothed out greatly. If the effect of lateral thermal conduction smoothing is taken into account, the high spatial-frequency intensity fluctuation can be eliminated further. Two-dimensional theoretic results indicate that a flat-topped and steep-edged target pattern can be achieved in this scheme. Because the position of the target does not require adjustment with high precision, the implement of this scheme is comparatively convenient.

A scheme of combining technology of lens array (LA) and smoothing by spectral dispersion (SSD) is introduced to improve the irradiation uniformity in laser fusion based on the earlier works on LA. The feasibility of the scheme is also analyzed by numerical simulation. It shows that a focal pattern with flat-top and sharp-edge profile could be obtained, and the irradiation nonuniformity can fall down from 14% with only LA to 3% with both SSD and LA. And this smoothing scheme is depended less on the incidence comparing to other smoothing methods. The preliminary experiment has demonstrated its effectiveness.

利用二维光谱色散平滑技术和透镜列阵(LA)来改善激光驱动器中靶面的辐照均匀性。通过消衍射透镜列阵可得到包络陡峭且中小空间尺度均匀性较好的焦斑。当在光路中加入二维光谱色散平滑单元后,光束在两个互相垂直的方向发生光谱色散,多光束干涉所引起的细密条纹也将在很大程度上被抹平,如果把横向热传导平滑效应也考虑在内,高空间频率的强度波动可进一步被消除。二维理论模拟结果表明采用该方案可获得顶部平坦边缘陡峭的焦斑,而且该方案无需仔细调整靶面的位置,实际应用较方便。

A scheme of combining smoothing by spectral dispersion (SSD) technology with lens array (LA) is introduced in laser produced shock wave experiments. The feasibility of the scheme is analyzed by numerical simulation. It is shown that the beam uniformity in near field could be improved by the phase modulation and spectral dispersion, and a smoothed pattern with flat-top and sharp-edge profile could be obtained in the focal plane. The irradiation nonuniformity of the focal pattern depends on the design of both SSD and LA but less on the incident beam in comparison with other smoothing methods. Experiments are conducted by adopting the schemes of LA and SSD, distributed phase plate (DPP) and SSD. The results are beneficial for better smoothing in laser-plasma interaction experiments in the future.

It is simulated numerically the transmission of beams phase modulated and spectrally dispersed by the unit for smoothing by spectral dispersion put in the laser driver for inertial confinement fusion. The results show that the diffraction spot of a spectrally dispersed beam becomes larger and its near-field uniformity is improved, while there exists area where the intensity is nearly even within the far-field spot. The effects of the main parameters of the electro-optic phase modulator and the grating in the unit on the diffraction characteristics of the beams are further discussed, and it is found that intensity modulation emerges in the far-field spot under some conditions.

A scheme employing a lens array and the technique of smoothing by spectral dispersion is presented for uniform irradiation on targets in inertial confinement fusion. Quasi-near field spots with a profile of sharp edge and flat top can be obtained with a diffraction-weakened lens array, while the speckles imposed on the spot by beamlets interference can be smoothed by spectral dispersion. Simulation results indicate that this scheme is a good candidate for improving the irradiation uniformity. The dependence of the smoothing performance on the characteristics of phase modulation and dispersion is further discussed, and it is found that parameters of the system of spectral dispersion and lens array must be chosen carefully to make optimal compromise between uniformity and usable energy of the laser beam.