Charge-exchange spectroscopic diagnostics for neutral gas in electron beam ion trap
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摘要: 在实验室天体物理研究中，电子束离子阱(EBIT)是极端紫外(EUV)和X射线波段能谱分析的重要实验平台，其中EBIT中心残余的中性气体对离子产生存在显著影响。研究了阱区中心残余中性气体对电荷态分布的影响，发现阱区中心残余中性气体和高电荷态离子之间的电荷/能量交换过程不仅影响离子的电荷分布, 而且对激发函数(离子分布比例随电子能量关系曲线)有着极大的影响。利用电离平衡分析方法成功诊断出阱区中心区域残留的中性气体分子数密度，以及内腔室的真空度。Abstract: Electron beam ion traps(EBITs) play an important role on extreme ultra-violet(EUV) and X-ray spectroscopy in laboratory astrophysics. Residual neutral gas in the EBIT trap center has a significant effect on ion breeding. In this work, we investigated the effect of residual neutral gas on the charge state distribution, as well as the diagnostic for the pressure in the trap center. The charge/energy exchange process between the neutral gas and highly charged ions not only affects the ionic fraction, but also plays an important role in fitting the excitation function (namely the relation curve of ionic fraction as a function of electron energies). By using the ionization equilibrium method on the measured excitation function, we have successfully estimated the residual neutral gas in the ion-electron interaction region of the trap center, as well as the vacuum pressure at the trap center.
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Key words:
- atomic processes /
- electron beam ion trap /
- extreme ultraviolet
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图 2 对Fe13+，Fe16+和Fe17+离子不同理论计算电子碰撞电离截面与实验测量结果比较
Figure 2. Comparison of electron impact ionization cross sections between theories and available experimental data for Fe13+, Fe16 and Fe17+
图 3 电子束离子阱中不同电荷态铁离子在包含和没有包含电荷交换过程(CX)下的分布
Figure 3. Charge state distribution of highly charged iron ions with and without charge-exchange(CX) process
图 4 电子束能量为1590 eV下铁离子的光谱与基于计算线辐射率ε(λj)的全域拟合光谱
Figure 4. Global fitting to the measured spectra at beam energy of Ee=1590 eV
图 5 Fe18+离子的激发函数轮廓与Monte-Carlo拟合曲线
Figure 5. Excitation function profile and Monte-Carlo fitting curve of Fe18+ ion
图 6 优化中性气体密度与归一化因子的等高曲线图
Figure 6. A contour plot for the optimized neutral densities at an electron density of 10-10 cm-3, and the normalization factor
图 7 从Fe17+，Fe18+，Fe19+和Fe20+离子诊断出的电子束离子阱阱区中心最优压强值(中心小方块区域)与归一化因子等高面图
Figure 7. A contour plot of optimized pressure in the center of the electron beam ion trap from the evolution curve of Fe17+, Fe18+, Fe19+ and Fe20+
表 1 实验等离子体中残留气体密度与阱区中心压强
Table 1. Resultant neutral density for the #Fe1008 measurement
ion lg(n0/cm-3) pressure/(10-8 Pa) Fe17+ 6.30±0.56 2.9-2.0+7.5 Fe18+ 6.42±0.64 3.8-2.9+12.3 Fe19+ 6.28±0.60 2.7-2.0+8.2 Fe20+ 6.57±0.64 5.3-4.1+18.0 Fe21+ 6.76±0.56 8.3-6.0+21.7 Fe22+ 6.91±0.66 11.7-9.0+41.7 
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