等离子体与壁材料相互作用产生钨原子的逆光子效率及能级布居实验研究

Tungsten (W) has been chosen as plasma-facing material in the ITER divertor. However, the W impurity which is produced by plasma-wall interaction can significantly influence the core plasma performance because of the strong radiative loss. Therefore, accurate evaluation of the sputtered W atom flux into plasma is important to estimate the components lifetime and the W accumulation in core. Optical emission spectroscopy (OES) is one of the few available diagnostics that provides in situ access to the W sputtering flux, which is based on the application of empirical inverse photon efficiency (S/XB) on neutral W line emission to convert the line intensity into a particle flux. Previous investigations indicate that S/XB values depend not only on electron temperature and electron density, but also on W atomic level population. However, data on photon efficiencies of W atomic lines (W I) are rare and there have been no studies investigating the statistical probability of sputtered W atomic energy-level population, due to the absence of available and quantitative atomic W sources for calibration purposes and suitable diagnostic methods for measurement of W I level population. Therefore, the influence of incident ion energy, electron temperature and electron density on S/XB, and the effect of W I level population on S/XB will be systematically investigated by combination of cavity ring-down absorption spectroscopy and OES in this project, which aim at quantitative study of W sputtering flux. The results will be able to improve our understanding of the relationship between particle flux and its line intensity, and resolve bottlenecks of measuring the sputtered W atomic energy-level population. This work will provide related database and experimental supports for plasma-wall interaction diagnose in tokamak.

钨已被选为ITER偏滤器靶板材料，等离子体与壁材料相互作用过程产生的钨杂质具有很强的辐射损失，会显著影响芯部等离子体性能。因此精确测量溅射产生的钨原子对准确评价钨部件寿命及芯部钨聚积情况十分重要。需通过逆光子效率(S/XB)和钨原子发射光谱强度来原位、定量给出钨溅射通量，其中S/XB不仅依赖于电子温度、电子密度，还依赖于钨原子能级布居。然而因不易获得定量的钨原子源及缺乏诊断钨原子能级布居的方法，关于钨原子S/XB的数据较少且尚无溅射产生钨原子能级布居的研究。本项目拟针对定量测量钨面对等离子体材料物理溅射通量问题，结合发射光谱和光腔衰荡吸收光谱技术，重点研究入射离子能量、电子温度、电子密度及溅射产生钨原子能级布居对S/XB的影响。研究结果将进一步深化对溅射产生的钨原子及其光谱强度之间关系的理解，解决测量钨原子能级布居的瓶颈，为托卡马克装置等离子体与钨壁材料相互作用诊断提供相关数据和实验支撑。

偏滤器区域面对等离子体部件承受着大量的等离子体与壁材料相互作用过程。级联弧等离子体源可模拟产生ITER偏滤器等离子体，用于研究等离子体与壁材料的相互作用。项目执行中课题组不断优化、调整级联弧等离子体源结构，摸索合适的放电参数，获得了Ar气注入或N2/Ar混合注入引起的偏滤器脱靶等离子体状态。为了准确地诊断测量出级联弧等离子体的电子温度和电子密度，分别基于单光栅光谱仪和自行研制的三光栅光谱仪成功地建立了激光汤姆逊散射诊断系统，电子密度探测下限低至1×10^17 m^-3，电子温度探测下限为0.1 eV，电子密度误差小于15%，电子温度误差小于10%，空间分辨率为400 μm，时间分辨率为10 ns，可以精确诊断较低密度等离子体的电子密度和电子温度。经诊断课题组研制的三阴极级联弧等离子体源的电子温度范围为0.7 - 1.5 eV，电子密度范围为10^12 – 10^14 cm^-3。此外课题组采用激光烧蚀钨靶材模拟边缘等离子体在边界局域模或破裂时对钨材料的损伤行为，研究所产生钨等离子体的发射光谱特性。测量时采用共轴收集结构，获得不同激光烧蚀（收集）角度下钨等离子体的发射光谱。研究表明，W I和W II的信号强度、信背比、信噪比、空间分布特性随激光烧蚀（收集）角度呈现不同的趋势。此外研究发现连续背景辐射主要沿着激光烧蚀方向，而W I和W II的发射方向主要沿着样品的法线方向。