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成丝不稳定性是间接驱动靶丸均匀压缩面临的重要问题,10μm到100μm的靶面光强起伏对其影响尤为严重,须采用合理的束平滑措施加以抑制。提出用光纤系统对啁啾脉冲堆积,然后对组合脉冲做光谱角色散,利用焦斑不同时刻对应频谱成分不同形成的束平滑机制,抑制焦斑内的高频起伏。利用傅里叶变换方法推导出了光栅色散后的组合脉冲表达式。模拟结果表明,焦斑尺寸随色循环数和光栅色散方向上基准脉冲带宽的增加而增大。通过计算包含焦斑能量95%范围内的通量衬比度随时间变化的曲线,得出一个色循环、基准脉冲带宽0.3~0.5 nm下可以获得最佳的焦斑平滑效果。这种新型的时间束平滑方法结构简单且焦斑特性可调,这些优点使得它在提高靶面辐照均匀性方面具有重要的应用价值。
The instability of the filaments is an important issue for the indirect compression of the target. The influence of the intensity fluctuation of the 10μm to 100μm target is particularly serious, and a reasonable beam smoothing method should be used to restrain it. It is proposed that the chirped pulse is piled up by the optical fiber system, and then the dispersion of the spectral angle of the combined pulse is made. The beam smoothing mechanism formed by the different spectral components at different time points of the focal spot is used to suppress the high frequency fluctuation in the focal spot. The combined pulse expression after grating dispersion was deduced by using Fourier transform method. The simulation results show that the focal spot size increases with the increase of the number of color cycles and the increase of the reference pulse width in the direction of grating dispersion. By calculating the curve of flux contrast ratio over time with 95% of the energy of the focal spot, a color cycle is obtained, and the best scorch smoothing effect can be obtained with a reference pulse width of 0.3-0.5 nm. The novel time-beam smoothing method has the advantages of simple structure and adjustable focal spot characteristics, which have important applications in improving irradiation uniformity of the target surface.