水窗及水窗附近软X射线激光研究

Water window refers to the wavelength range, in which water is essentially transparent, from K absorption edge of oxygen at 2.3nm to that of carbon at 4.4nm. Soft x-ray laser in and near water window has demanding application prospect in living cell imaging, biological holographic imaging and hot dense plasma diagnoses. However, there is not yet a saturated output results report. The applicant had gained a certain power output of nickel-like tantalum soft x-ray laser at 4.5nm with SG-II high power laser device, which was near water window. The laser's gain length product was about 5.5, did not reach saturation. The driving requirements of nickel-like tantalum soft x-ray laser were preliminarily understood, through the optimization of experimental conditions. The applicant come to recognize that the main factor which limits higher intensity output is the lack of drive capacity of SG-II laser device. In recent years, laser drive capacity has been greatly improved with the gradual establishment of SG-II upgrade device and picosecond's petawatt device. The project propose experimental study using the two devices to carry out soft x-ray lasers in and near water window. According to the characteristics of the two devices, and based on years of experience on soft x-ray laser, three research approaches are proposed. The further works are communicating with various departments including of theory, devices, etc. to select and optimize the suitable scheme and strive to obtain saturation output of soft x-ray lasers in and near water window.

水窗指波长在氧的K边（2.3nm）到碳的K边（4.4nm）之间的波段。水窗及水窗附近的软X射线激光在生物活细胞成像、全息以及等离子体诊断等众多领域具有重要的应用前景，但至今尚未有饱和输出的报道。申请人曾利用"神光II"激光装置驱动获得了类镍钽4.5nm水窗附近软X射线激光输出，增益长度积5.5，未达到饱和。通过对驱动条件的优化，初步了解了类镍钽软X射线激光的驱动要求，进而认识到难以获得更高强度输出的主要限制因素是"神光II"激光装置的驱动能力不足。近年来，随着"神光II"升级装置和皮秒拍瓦装置的逐步建立，输出能力大幅提高。本项目拟定利用两个装置开展水窗及水窗附近软X射线激光的实验研究。针对两个装置的特点，结合多年软X射线激光的研究经验，初步拟定了三种研究思路。进一步将通过与理论、器件、靶场等各部门的沟通，优化选择合适的方案，力争获得饱和输出的水窗及水窗附近软X射线激光。

自从1984年实验室等离子体软X射线激光成功演示以来，如何获得更短波长、直至水窗波段的软X射线激光就是一个研究的热点。所谓水窗是指波长在氧（O）的K吸收边（2.3nm）到碳（C）的K吸收边（4.4nm）之间的波段，这个波段及附近的光对不同有机分子具有不同的透过率，因此水窗及水窗附近软X射线激光在生物活细胞全息、成像等方面具有独特的优势；同时它还是软X射线波段的高亮度相干光源，具有广泛的应用前景。但对于以等离子体作为增益介质的软X射线激光，波长越短对驱动条件要求越高，难度也越大。几十年来，不少实验室为此而努力，但至今尚未有饱和输出的结果报道。.本项目在多年软X射线激光实验研究的基础上，提出利用输出能力更高的“神光II”升级及皮秒拍瓦激光装置来驱动产生水窗或水窗附近软X射线激光，主要瞄准类镍钽4.5nm或类镍钨4.3nm的软X射线激光。项目立项时，激光装置尚未最后建成，根据当时已知的条件，设计了三种可选的方案。三种方案各有特点，从技术上都具备可行性。但装置建成后，发现如果采取前两种方案，需要装置进行较大的适应性调整，时间、成本均不允许，因此只能采用方案三，即长短脉冲联合斜入射驱动的方式进行实验研究。经过认真的准备和精心的实验，我们获得了比较明确的水窗附近类镍钽4.5nm软X射线激光的输出结果，但输出强度比较低。另外在实验中，还发展改造了针孔相机、光栅谱仪等设备，解决了在超短超强激光实验中遇到的问题，不但很好完成了本项目实验的工作，还获得更大的应用。.尽管水窗附近软X射线激光应用前景广阔，但本项目获得光源输出强度很低，难以用于开展应用研究。但通过实验，初步了解了皮秒激光驱动水窗及水窗附近短波长软X射线激光的条件，为今后更好开展研究打下了一定的基础。如果能够获得更强、乃至饱和输出输出的水窗附近短波长软X射线激光，一定会有很好的应用。