吸收谱线Doppler展宽对原子多步光电离的影响

原子法激光同位素分离的核心之一是如何高效地将原子激发电离.本文从原子法激光同位素分离的实际情况出发,研究了原子吸收谱线的Doppler展宽对原子电离率的影响.研究中使用的理论工具是原子激发电离的密度矩阵方程,并利用数值计算方法对方程进行求解.研究结果表明:当吸收谱线有Doppler展宽时,在激光参数不变的条件下原子电离率会降低;且当激光功率固定时,存在使原子电离率为最大的线宽值.这与已发表文献中无Doppler展宽时的计算结果有很大不同.为了追求最佳的原子电离效果,在原子法激光同位素分离系统中激光应该尽可能地工作在最佳线宽条件下.如果激光线宽有不可控的随机波动,在技术上让激光线宽略大于最佳线宽更为有利.无论如何控制激光线宽,尽可能地降低原子吸收谱线的Doppler展宽都有利于原子电离率的提高.     

厚原子蒸气介质中原子选择性光电离的理论研究

激光在厚原子蒸气介质中传播时会产生脉冲形变和延迟现象,这会直接影响原子多步光电离过程中的电离率和选择性.从原子蒸气激光同位素分离的实际出发,对厚原子蒸气介质中的原子光电离过程进行了研究,利用密度矩阵方法描述原子的光电离过程,利用Maxwell方程描述激光在厚介质中的传播,建立了介质中同时存在两种同位素的激光传播电离方程组,考察了原子蒸气参数和激光参数对厚介质中平均电离率和平均选择性的影响.研究结果表明:对于较厚的原子蒸气介质,激光功率的增加使平均电离率升高,平均选择性下降;对于相对较薄的介质,适当地降低激光功率可以同时提高平均电离率和平均选择性.存在一个正的激光延时使原子蒸气中目标同位素的平均电离率达到最大.尽量延长激光脉冲的宽度不仅可以同时提高目标同位素的平均电离率和平均选择性,还可以降低对激光脉冲之间相对延时的控制精度.     

Laser isotope separation

This work describes the atomic route to laser isotope separation. This is a process which uses intense pulsed lasers to photoionize one isotopic species of a chemical element, after which these ions are extracted electromagnetically. The paper describes only the isotopic enrichment of uranium for nuclear fuel cycles. It makes brief mention of the traditional cascade processes used at present, and then turns to the atomic physics of laser photoionization. The principles of the laser isotope separation process, important spectroscopic experiments and considerations which determine the design of a plant are described. This review concentrates on the laser ionization process, few details of vaporization of uranium metal or the extraction of ions are given.     

Optimization of feed flow parameters in separation of neodymium isotopes by atomic vapor selective laser photoionization and analysis of results

Experimental data for separation of neodymium isotopes by atomic vapor selective laser photoionization are analyzed. Atom scattering in the working volume and the Doppler shift of the atom absorption line are shown to be the basic deselecting processes in the experimental cell studied. A data processing technique that allows one to determine the effect of either deselecting process on the product enrichment and yield is suggested. The experimental dependence of the target isotope concentration in photoions present in the separating chamber on the angular divergence (collimation) of the atomic vapor flow along the laser beam is found. A method for determining the vapor flow optimal angular collimation is developed.     

A study of isotopically selective photoionization of ytterbium atoms for laser isotope separation

Atomic-vapor laser isotope separation (AVLIS) is studied experimentally and theoretically for ytterbium vapors. The optimum ionization scheme and the process dynamics are determined. The photoionization scheme uses the transitions $$6^1 S_0 \mathop \to \limits^{555.648nm} 6^3 P_1 \mathop \to \limits^{581.067nm} (7/2,3/2)_2 \mathop \to \limits^{582.79nm} (52353cm^{ - 1} ).$$ For a numerical study of photoionization dynamics, the mathematical model of the AVLIS process is used, which is based on the density matrix formalism and Maxwell's equations. Selective photoionization of the¹⁶⁸Yb isotope is simulated numerically. The yield and selectivity of the process are determined. It is shown that the length of the photoionization region is limited because the laser radiation is absorbed by atoms of¹⁷¹Yb and¹⁷³Yb isotopes. The advantage of the laser method over the method using an electromagnetic separator is demonstrated.     

基于气体放电等离子体射流源的模拟离子引出实验平台物理特性

离子引出过程是原子蒸气激光同位素分离中非常重要的物理过程之一,而其中关键的等离子体参数(等离子体初始密度和电子温度等)均会对离子引出特性产生影响.基于千赫兹电源驱动的氩气高压交流放电等离子体射流源,建立了离子引出模拟实验平台-2015 (IEX-2015),开发了用于诊断氩等离子体参数的"碰撞-辐射"模型,对等离子体射流区的电子温度和电子数密度等关键参数进行了测量.结果表明,电源输入功率和驱动频率以及工作气体流量均会对等离子体射流区的电子温度和数密度产生影响;在真空腔压强为10~(-2)Pa量级下,射流区电子数密度和电子温度的可调参数范围分别为10~9—10~(11)cm~(-3)和1.7—2.8 e V,这与实际离子引出过程中的等离子体参数范围相近.在此基础上,开展了不同引出电压、极板间距和电子数密度条件下初步的离子引出实验,所得到的离子引出电流变化规律亦与实际原子蒸气激光同位素分离中的离子引出特性定性一致.上述研究结果验证了在IEX-2015上开展离子引出模拟实验的可行性,为后续深入开展离子引出特性的实验研究准备了良好的条件.     

Theoretical study on isotope separation of an ytterbium atomic beam by laser deflection

Isotope separation by laser deflecting an atomic beam is analyzed theoretically. Interacting with a tilted onedimensional optical molasses, an ytterbium atomic beam is split into multi-beams with different isotopes like172Yb,173Yb, and174Yb. By using the numerical calculation, the dependences of the splitting angle on the molasses laser intensity and detuning are studied, and the optimal parameters for the isotope separation are also investigated. Furthermore, the isotope separation efficiency and purity are estimated. Finally a new scheme for the efficient isotope separation is proposed. These findings will give a guideline for simply obtaining pure isotopes of various elements.     

A Scaling Law of Photoionization in Ultrashort Pulses

By means of the numerical solution of time-dependent Schr6dinger equation, we verify a scaling law of photoionization in ultrashort pulses. We find that for a given carrier-envelope phase and duration of the pulse, identical photoionizations are obtained provided that when the central frequency of the pulse is enlarged by k times, the atomic binding potential is enlarged by k times, and the laser intensity is enlarged by ka times. The scaling law allows us to reach a significant control over direction of photoemission and offers exciting prospects of reaching similar physical processes in different interacting systems which constitutes a novel kind of coherent control.     

Isotope separation of some rare-earth elements by two step photoionization

The experimental results of the laser isotope separation (LIS) of europium, samarium and gadolinium atoms by two step selective photoionization are reported. The atom vapour of Eu, Sm, Gd atoms has been produced by the heating of a crucible containing the metal. A dye laser has been used as a selective exciter of the atoms. For the photoionization N₂-laser or mercury lamp have been used.     

Theory of gain and mode locking in free-running class-B lasers with simultaneous oscillation of three longitudinal modes

The operation of a three-mode laser with a homogeneous broad bandwidth is modeled as three different harmonic oscillators coupled to each other by the usual Maxwell–Bloch equations. The main aim is to extend and compare the laser variables when it moves from a single to a three-mode state. A multi-component model for the cavity electric field and the atomic population inversion is introduced to describe the simultaneous oscillation of the laser central and adjacent modes. The phase relations and the frequency shifts of these components in mode-locking conditions are determined. We also investigate the effect of the laser pumping rate and the mode frequency separation on the amplification of modes. Finally, the amplification gains of all three modes are calculated and their agreement with the energy conservation law is demonstrated.     

背景气体对金属原子二维平面蒸发过程的影响

在原子蒸气法激光分离同位素中,金属原子蒸气宏观物理性质的空间分布会直接影响到分离过程的电离率和原料利用率.本文从分离过程的实际需求出发,建立了双组分气体的Bhatnagar-Gross-Krook模型方程组,并利用数值计算方法对方程进行求解,研究了背景气体对二维平面蒸发过程中原子蒸气宏观物理性质和蒸发速率的影响.研究结果表明:随着背景气体密度的增加,远离蒸发源位置处的金属原子蒸气密度增大,速度减小,温度升高,而近蒸发源位置处原子蒸气的性质则几乎不受影响,因而蒸发速率基本上不随背景气体密度发生变化.另外,随着尾料板温度的升高和对原子蒸气吸收率的增加,金属原子蒸气宏观物理性质受背景气体的影响逐渐下降.理论计算的结果对于分离装置的真空设计和光斑分布设计有较为重要的参考意义.     

碱金属原子的光激发与光电离

本文研究了碱金属原子在三步激光脉冲作用下的光激发和光电离过程的动力学特性, 重点关注和比较了锂和铯原子的异同. 针对多种激发模式, 本文不但建立了其原子布居数在各个跃迁态的速率方程组, 还给出了各相关态的光激发和光电离过程的解析解. 通过精心设计并选择了特殊情况, 显著简化了解析解的数学表达式, 从而凸显和讨论了其物理内涵. 通过自行编程, 系统地计算和观察了各种激发模式对锂原子的光激发和光电离过程的可能影响, 研究和讨论了电离率随激光参数的变化规律. 在相同激发模式下, 比较和分析了采用两种不同激发路径所导致的各态原子布居率的变化, 凸显了改变原子参数所产生的作用. 探讨了锂和铯原子在类似的激发条件下在电离率方面的差别. 最后, 基于本文的研究结果, 本文指出了优化电离率的多种途径.     

Nonlinear polarization response of an atomic gas medium in the field of a high-intensity femtosecond laser pulse

Polarization response that appears in silver vapors in the field of a high-intensity femtosecond Ti:sapphire laser has been studied by the direct numerical integration of the time-dependent Schrödinger equation. The regions of applicability have been determined for perturbation theory and the power series expansion of the polarization in the field. The contribution of free electrons to the response at the frequency of the interacting field has been calculated, which is due to the photoionization process and limits the Kerr effect. An important contribution of laser-excited atomic states to nonlinear atomic responses of neutral atoms has been demonstrated.     

The Photoionization of Atomic Hydrogen in an Intense Circularly Polarized Laser Field

We calculate nonperturbatively the irfluence of a strong circularly polarized laser beam on the hydrogen atom energy levels by making use of the time-independent formalism proposed by one of us. The photoionization cross section of the hydrogen atom irradiated by this laser beam and the angular distribution of photoelectrons are also calculated. From the numerical results we clearly see the intensity dependence of the whole photoionization process,including the intensity dependence of the photoelectron energies.     

The Photoionization of Atomic Hydrogen in an Intense Circularly Polarized Laser Field

We calculate nonperturbatively the irfluence of a strong circularly polarized laser beam on the hydrogen atom energy levels by making use of the time-independent formalism proposed by one of us. The photoionization cross section of the hydrogen atom irradiated by this laser beam and the angular distribution of photoelectrons are also calculated. From the numerical results we clearly see the intensity dependence of the whole photoionization process,including the intensity dependence of the photoelectron energies.     

Rare-earth-doped materials for applications in quantum information storage and signal processing

Realization of practical quantum memory and optical signal processing systems critically depends on suitable materials that offer specific combinations of properties. Solid-state materials such as rare-earth ions doped into dielectric crystals are one of the most promising candidates for several quantum information storage protocols, including quantum storage of single photons. This article provides an overview of rare-earth-doped material properties and summarizes some of the most promising materials studied in our laboratory and by other groups for applications in quantum information storage and for ultra-wide bandwidth signal processing. Understanding and controlling spectral diffusion in these materials, which ultimately limits the achievable performance of any quantum memory system, is also briefly reviewed. Applications in quantum information impose stringent requirements on laser phase and frequency stability, and employing a narrow spectral hole in the inhomogeneous absorption profile in these materials as a frequency reference can dramatically improve laser stability. We review our work on laser frequency and phase stabilization and report our recent results on using a narrow spectral hole as a passive dynamic spectral filter for laser phase noise suppression, which can dramatically narrow the laser linewidth with or without the requirement of active feedback.     

生物组织中光子微区扩散理论研究

为合理描述生物组织中光子微区的扩散情况,提出在相函数中引入独立控制参量来改进现有的一般扩散近似理论。以平板结构为例,分别利用改进的一般扩散近似理论、标准扩散近似理论以及一般扩散近似理论,对光子在不同厚度中的扩散进行数值模拟,并与蒙特卡罗方法的模拟结果进行了比较,从而论证了改进的一般扩散近似理论能很好地描述生物组织中光子微区扩散。在此基础上,进一步探讨了系统参量对独立控制参量的影响。结果表明：最佳独立控制参量会随着数值孔径的增加而下降、随着光束半径的增加而增加。本研究对微区生物组织光学特性测量技术的研究具有重要的指导意义。     

镥的激光共振电离同位素选择性研究

本文在速率方程基础上通过数值模拟方法 ,对镥的激光共振电离通道 :5d6s2 2 D3 /2 (5 73.6 5 5nm)→5d6s6 p4F3 /2 (6 4 2 .5 18nm)→ 6s6 p2 4P1/2 (6 4 3.5 4 8nm)→Autoionizationstate的激光诱导同位素选择性进行了研究。在实际实验条件下用这一方法计算得到的激光波长对激光诱导同位素选择性的关系与实验结果相符合。探讨了在偏振激光作用的情况下各种激光参数 (波长、带宽和激光强度 )对激光诱导同位素选择性的影响 ,并提出了在一定实验条件下激光共振电离质谱计较为准确地测定同位素比值的方法。这一理论方法 ,同样适用于研究其它元素的激光共振电离同位素选择性和选择激光同位素分离电离通道     

Experimental evaluation of the measurement of isotope ratios by time of flight mass spectrometry with laser ablation

Time of flight mass spectrometry (TOF-MS) in combination with laser ablation (LA) applied for low isotope composition measurements is limited by energy and spatial dispersion of ions, spatial charge density, and some other factors. In order to reduce these factors that make the extraction of the plasma ions difficult, we used a second laser to create ions by non-isotope-selective photoionization of neutral particles. The photoionization was carried out in uranium vapor beam by the third harmonic of a pulsed Nd:YAG laser. This work presents the results of tentative experiments to estimate the characteristics of the LA-TOF-MS method with non-selective photoionization.