激光聚焦位置对半球腔约束等离子体光谱增强特性的影响

光谱信号增强是提高激光诱导击穿光谱技术分析性能的重要手段之一，对等离子体进行空间约束由于装置简单且约束效果好而常被采用，等离子体的特性会直接影响空间约束的效果，而等离子体的特性与实验系统中激光的聚焦情况密切相关，为研究激发光源的聚焦情况对半球形空腔约束等离子体光谱增强特性的影响，通过控制透镜到样品之间的距离(LTSD)来改变激光的聚焦位置，分别在无约束和有半球形空腔约束两种实验条件下，烧蚀合金钢产生等离子体，采集15个不同LTSD位置时等离子体的时间演变光谱，得到谱线强度和增强倍数随着LTSD和采集延时的二维空间分布图。研究结果发现：无约束情况下，谱线强度分别在LTSD为94和102 mm时出现峰值，在采集延时小于8 μs时，谱线强度的最大值在LTSD为94 mm的位置，采集延时大于8 μs后，谱线强度的最大值出现在LTSD为102 mm的位置；当用半球空腔约束等离子体，谱线强度先后在采集延时范围为4~10和12~15 μs出现第一次增强和第二次增强。谱线强度出现第二次增强的主要原因是被半球腔内壁反射的冲击波与等离子体相互作用后会继续向前传播，遇到另一侧的腔壁再次被反射，进而对等离子体产生二次压缩。分析增强倍数随LTSD和采集延时的二维变化关系发现，第一次增强的最大增强倍数随LTSD的变化没有明显规律，增强倍数在2~6之间波动；谱线第二次增强时的增强倍数相对较高，最大增强倍数随着LTSD变化呈现出先增大再减小，然后再小幅增加后降低的变化规律，在LTSD为96 mm时达到最大值，两条谱线的最大增强倍数约为6倍。分析出现最大增强倍数对应的延迟时间发现，第一次增强出现的最优延迟时间在6~9 μs之间变化，当LTSD在85~93 mm范围时，最优延迟时间保持不变，当LTSD在94~105 mm时，出现先降低再增大的变化规律；第二次增强出现的延迟时间主要在14~15 μs，随着LTSD的变化没有明显的变化规律。

Effect of Laser Focusing on Laser-Induced Plasma Confined by Hemispherical Cavity

Spectral enhancement is one of the key methods to improve Laser-Induced Breakdown Spectroscopy (LIBS) analysis performance. Spatial confinement of plasma is often used due to its simple device and better confinement effect. The characteristics of plasma will directly affect the spatial confinement. The properties of the plasma are closely related to the focusing of the laser in the experimental system. In order to study the effect of the laser focusing on the spectral enhancement of the plasma confined by a hemispherical cavity, the condition of the laser focusing was changed by adjusting the distance between the lens and the sample (Lens to Sample Distance, LTSD). Under the experimental configurations without and with confinement, the alloy steel sample was ablated to produce plasma, and the time evolution spectra at 15 different LTSD positions were collected. The two-dimensional spatial distributions of the spectral line intensity and enhancement factor with LTSD and acquisition delay were obtained. The results had shown that the spectral line intensity of the plasma without confinement peaks when the LTSD was 94 and 102 mm, respectively. When the acquisition delay was less than 8 μs, the maximum value of the spectral line intensity was at the LTSD of 94 mm. The maximum intensity appeared at the LTSD of 102 mm when the delay time was greater than 8 μs. Moreover, the line intensity has two sequential enhancements when the hemispherical cavity confined the plasma. The delay time ranges corresponding to these two enhancements were 4~10 and 12~15 μs. The main reason for the second enhancement is that the shockwave reflected by the inner wall of the hemispherical cavity will continue to propagate after interacting with the plasma and it will encounter the other side of the cavity wall and be reflected again secondary compress the plasma. The two-dimensional distribution of the enhancement factor with LTSD and delay time was analyzed. It is found that the maximum enhancement factor of the first enhancement has no obvious trend with the change of LTSD and the enhancement factor fluctuates from 2 to 6. The maximum enhancement factor of the second enhancement first increases and decreases as the LTSD changes and decreases after a small increase. The enhancement factor is relatively high. It reaches the maximum when the LTSD is 96 mm, and the maximum enhancement factor is about 6. The delay time corresponding to the maximum enhancement factor was defined as the optimal delay time. It is found that the optimal delay time for the first enhancement varies from 6 to 9 μs. When the LTSD is in the range of 85~93 mm, the optimal delay time remains unchanged. When the LTSD varies from 94 to 104 mm, the optimal delay time of the first enhancement first decreases and then increases. However, the optimal delay time of the second enhancement maintains at a range from 14 to 15 μs, and there is no obvious change with the change of LTSD.