托卡马克高Z面壁材料近无损伤双脉冲激光诱导击穿光谱诊断研究

Laser-induced breakdown Spectroscopy (LIBS) has been regarded as an advanced in-situ diagnostic method for plasma facing materials (PFMs) in tokamak due to its unique features such as remote sensing, real-time monitoring and multi-element analysis. However, the relatively low sensitivity is the main challenge of the in-situ LIBS system used to identify the trace deposited high Z impurities and retained fuel in PFMs. It is fallacious to achieve higher emission signal by increasing the incident laser energy density, because excessive laser ablation rate will damage the surface of PFMs. The ablation from the solid target is directly proportional to laser intensity. The ablated high Z impurities can enhance radiation losses and dilute the fuel ions, both effects impairing the performance of a tokamak reacting plasma. In this project, a collinear double-pulse laser-induced breakdown spectroscopy (DP-LIBS) method will be developed with the aim of overcoming the sensitivity and spatial resolution shortcomings of the conventional single pulse laser-induced breakdown spectroscopy (SP-LIBS) technique in vacuum conditions. A systematic study of improving the detection sensitivity and spatial resolution simultaneously dependence on the inter-pulse delay, laser energy and laser wavelengths will be carried out in vacuum conditions, and the results will be compared with the ones obtained with a single laser pulse of energy corresponding to the sum of the two pulses. These results can provide directly the detail information of the composition and micro-structure of PFMs for EAST. This collinear DP-LIBS nearly non-destructive technique would help us to develop a more promising system to monitor the fuel retention and impurity deposition on PFMs of large-scale tokamak devices.

激光诱导击穿光谱 (LIBS)技术是一种极具潜力的托卡马克装置壁状态原位诊断技术，目前由于高真空环境下其对高Z金属壁探测灵敏度较低的弱点制约了该技术的推广应用。通过加大入射激光能量以提高LIBS信号强度的实验方案并不可取。激光能量的提高会导致材料烧蚀深度的增大，严重威胁壁材料基底安全。实验不当会对壁材料造成损伤，并产生大量高Z杂质粒子，对聚变等离子体约束产生不良影响，甚至引发等离子体破裂。本课题提出一种共轴双脉冲激光诱导击穿光谱（DP-LIBS）近无损伤分析技术，第一束激光用于解吸或微烧蚀壁材料表面滞留层，另一束激光用于增强原子/离子激发。通过改变双脉冲间隔时间、能量配比、波长等方式同步提高探测灵敏度和纵向空间分辨率。本项目的顺利实施将提升现有EAST装置高Z壁材料LIBS原位诊断系统的分析能力，对深入理解托卡马克装置高参数、长脉冲运行过程中杂质沉积、燃料滞留等关键问题提供重要的实验支撑。

托卡马克作为磁约束可控热核聚变研究领域的主流实验装置，在其通往商用聚变堆的建设道路上仍然面临着诸多挑战。其中等离子体与壁材料相互作用过程制约了托卡马克装置偏滤器靶板、第一壁等关键材料的使用寿命，关乎芯部等离子体污染、破裂等关键物理问题。发展精确的偏滤器靶板、第一壁表面沉积杂质监测技术对理解高参数托卡马克装置刮削层/偏滤器PWI物理过程具有非常重要的意义。本项目立足于升级现有EAST托卡马克装置LIBS原位诊断能力的实际需求，发展了一种同轴DP-LIBS技术，确保第一壁、偏滤器靶板近无损伤的前提下，可以有效提高真空环境下对高Z杂质元素的探测灵敏度。项目组经过系统的DP-LIBS优化实验获得了最佳实验参数。波长1064nm，能量密度7J/cm2的纳秒激光脉冲脉冲对纯钨材料单脉冲烧蚀深度约10nm，可确保LIBS分析过程对壁材料近无损伤；DP-LIBS的双脉冲能量密度都设为3.5J/cm2，脉冲间隔为250ns时沉积杂质的特征光谱信号增强最为明显。项目组选取2017年EAST整轮辐照后的H段高场侧HI2-3第一壁（纯钨标记瓦）样品表面的沉积杂质层进行DP-LIBS验证实验，结果显示DP-LIBS技术与现有SP-LIBS技术相比杂质元素锂、钼、燃料的信号分别增强了3.9，4.2和5.4倍。同时，DP-LIBS探测到了SP-LIBS无法探测的痕量钨，铁，钛等杂质。项目组发展了基于单标样修正的CF-LIBS新算法，即OPC-LIBS技术。真空环境下对EAST-like壁样品（钨钼合金、PLD钨钼薄膜等材料）进行了LIBS定量分析，获得了钨、钼等元素定量分析精度优于50%的实验结果。项目组取得实验成果为EAST装置壁诊断设备升级提供了重要的技术储备，有助于突破现阶段复杂服役工况下LIBS远程定量化分析的技术瓶颈。