采用啁啾脉冲堆积的时间束平滑技术

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。

Temporal Beam Smoothing Technique Using Chirped Pulse Stacking

In the indirectdrive target compression process, filamentation instability should be considered seriously, especially light intensity fluctuation of size 10～100 μm on the surface of target, and a proper beam smoothing method must be adopted. A new method, using fiber system to stack chirped pulses, treating the composite pulses with angular spectral dispersing, depressing the high frequency fluctuation in the focal spot with the beam smoothing mechanism of focal spot at different time determining different spectral components, is proposed. Using Fourier transform method, the expression for the composite pulses dispersed by a grating is deduced. Simulation results show that the size of the focal spot increases with the number of color cycles and the basic pulse bandwidth in the dispersing direction. Through calculating the flux contrast including 95% focal spot energy with time, it is found that the optimal smoothing effect is achieved with one color cycle and 0.3～0.5 nm basic pulse bandwidth. This new kind of temporal beam smoothing technique has the advantages of simple configuration and adjustable focal spot performance, and is of important application value in improving irradiation uniformity on target surface.