氖原子对2000 eV X射线激光透明的产生机制

X射线自由电子激光与物质相互作用时出现了X射线透明现象,研究X射线透明现象的产生机制对理解X射线自由电子激光与物质相互作用过程具有重要参考价值.本文基于FAC(Flexible Atomic Code)程序计算的氖原子(离子)对2000 eV X射线的光电离截面和俄歇衰变速率,确定了不同通量密度下氖原子的主要电离方式;通过建立和解速率方程,得到了确定的电离方式中氖元素各电子组态数目比例随时间变化的公式,计算了氖原子在通量密度为2000和10000?~(-2)·fs~(-1)、持续时间为20 fs、光子能量为2000 eV的X射线激光照射下,任意时刻各主要电子组态的原子数目比例和总的氖原子平均光电离截面.建立了价电子完整的空心原子的数目比例随通量密度和曝光时间变化的公式.发现裸核和空心原子都会导致X射线透明,且选择恰当的通量密度和脉冲持续时间可以使价电子完整的空心原子数目比例达到极高.     

Am原子及其离子Am~(q+)(q=1～6)的K,L,M-X射线跃迁能和跃迁几率的理论研究（英文）

使用基于Dirac-Hartree-Fock方法的Grasp2K程序包,计算了Am原子及离子的K,L,M-X射线的跃迁能和跃迁速率。在计算中,包括了Breit相互作用、真空极化和自能等重要效应。目前研究结果与已有的其他实验和理论结果相对误差约为0.04%。此外,我们还首次计算了从Am~(1+)到Am~(6+)离子的K, L, M-X射线的跃迁能和跃迁速率。相对于中性原子,来自低离化态的跃迁能相对于中性的相应跃迁线的能量仅有轻微的偏移,这反映出外层电子几乎不影响内壳层的跃迁性质。     

开放V型系统相对位相对LWI增益瞬态演化的影响

数值计算具有自发辐射诱导相干性的开放V型三能级原子系统密度矩阵运动方程,分析无反转激光(LWI)增益的瞬态演化.结果表明:无论是否存在非相干抽运,探测场和驱动场间的相对位相(Ф)对增益的瞬态演化都有显著的影响;通过选择的取值可以获得更大的瞬态和稳定增益;不存在非相干抽运时,系统达到瞬态增益最大值及获得增益稳定值所需要的时间更长,演化过程中振荡幅度更大,获得的增益更大;原子退出速率r0(原子注入速率比S)的变化将使增益瞬态演化的具体过程发生改变,在r0(S)的一定取值范围内,瞬态和稳定增益随S(r0)的增大而增大.     

脉冲抽运掺镱脉冲光纤放大器理论研究

对主振荡功率放大器(MOPA)方式脉冲抽运双包层掺镱脉冲光纤放大器进行了理论研究,分析了放大中抽运脉冲、激光脉冲和拉曼斯托克斯光脉冲的相互作用过程.对增益光纤中上能级粒子数密度随抽运时间的变化进行了分析,求出了最佳抽运脉冲宽度.随着抽运功率的增加,放大过程中出现的受激拉曼效应(SRS)将抑制激光脉冲能量的增加,当采用最佳抽运功率时激光脉冲的能量可达到最大值.分析了光纤长度、纤芯直径对最佳抽运功率、激光脉冲和一级斯托克斯光脉冲的影响.结果表明,当最佳抽运功率时,采用纤芯较粗、长度较短的增益光纤,可以抑制受激拉曼效应,提高激光脉冲的能量与峰值功率.     

应用ECPSSR理论研究重离子引起原子的内壳层电离

ECPSSR理论是解释离子-原子碰撞内壳层电离最成功的理论之一.我们用VISUAL FORTRAN编写了ECPSSR理论计算程序,修正了ISICS程序中的错误,本程序可以对各种入射离子与靶原子的组合进行计算,给出K,L,M的壳层及次壳层电离截面以及相应的X射线产生截面,并根据需要选择是否对入射离子运动进行相对论修正.采用所编写的程序计算了一些碰撞体系的电离截面和X射线产生截面,并与其他程序的计算结果和实验数据分别进行了比较.     

连续波抽运气体波导产生太赫兹激光的理论研究

基于速率方程理论,建立了光抽运气体波导产生太赫兹(THz)激光的能量转化模型,理论分析并求解得到抽运光吸收系数、THz小信号增益系数以及THz输出功率表达式.计算结果表明,THz输出功率随工作物质气压的升高先增加后逐渐减少,随抽运功率的增加、输出镜反射率的减小而增加;最佳工作气压随抽运功率的增大而增大;激发态粒子数以及THz光子通量随波导截面径向逐渐减小,而THz小信号增益系数逐渐增加;抽运饱和、弱抽运吸收与激发态工作物质对THz激光的吸收是限制激光转化效率提高的根源;基于该模型的计算结果与相关文献中的实验数据符合较好.     

溴的光电离和辐射复合——平均原子模型速率系数与细致组态速率系数

根据原子自洽场理论(即DCA)，计算了溴原子各种电离度各个壳层的DCA约化光电离截面.与AA模型的平均电子轨道约化光电离截面进行比较，总结出其内在规律.在AA的约化光电离截面的基础上，可以得到电离度分辨的DCA约化光电离截面；进而可以得到细致组态的光电离和辐射复合速率系数.为精密地描述非局域热动平衡(n-LTE)等离子体提供必要的基础. 关键词： 光电离 速率系数 平均原子模型     

高能脉冲C~(6+)离子束激发Ni靶的K壳层X射线

精确测量离子与原子碰撞引起的靶原子内壳层电离截面,对研究原子内壳层过程以及建立合适的理论模型具有重要的意义.现有的实验数据和理论模型大都集中在中低能区,高能区由于受到实验条件的限制,几乎没有相关实验数据的报道,哪种理论更适合描述高能重离子入射的靶原子内壳层电离截面,还需要进行深入的实验研究.采用电子冷却存储环提供能量分别为165,300,350,430 MeV/u的C~(6+)离子束轰击Ni靶,测量Ni的K壳层X射线.分析了实验中探测到的Ni的K_β和K_α射线强度比,发现入射粒子能量的变化对该强度比影响不明显.分别应用两体碰撞近似(BEA)、平面波玻恩近似(PWBA)和ECPSSR理论对Ni的K壳层X射线的产生截面进行理论计算,并将理论结果与实验结果进行比较.     

Ar16+和Ar17+离子与Zr作用产生的X射线谱

将超导离子源提供的10-20keV/q Ar16 和Ar17 离子入射到Zr金属表面,在相互作用中产生的X射线谱表明,高电荷态Ar16 离子在金属表面中性化过程中有可能存在多电子激发,使Ar16 的K壳层电子被激发形成空穴,在退激过程中发射特征Kα-X射线.空心原子Ar的K层发射X射线强度随入射离子的动能而减弱,靶原子Zr的L壳层发射X射线强度随入射离子动能的增加而增强.Ar17 的单离子的Kα-X射线产额比Ar16 单离子的Kα-X射线产额大5个数量级.     

CSR上154 MeV/u C6+离子束分别入射Ti、V、Fe、Ni和Zn靶的X射线辐射

用电子冷却存储环(CSR)提供的能量为165 MeV/u的高能C6+脉冲束分别与Ti、V、Fe、Ni和Zn靶表面相互作用,测量了各靶辐射的K壳层X射线。结果表明,实验探测到各靶的K和K X射线均向高能区发生了不同程度的移动。经分析,X射线发生频移是由靶原子外壳层发生多电离引起的。通过计算各靶K壳层X射线的产生截面,发现该截面随原子序数的增加而减小。将各靶K-X射线产生截面的实验值分别与两体碰撞近似、平面波恩近似和ECPSSR理论值比较,发现现有理论与实验值存在偏差,要准确描述实验结果需要进一步修正。     

A review of Ni-like X-ray laser experiments undertaken using the VULCAN laser at the Rutherford Appleton Laboratory

Recent progress using the VULCAN laser at the Rutherford Appleton Laboratory to pump X-ray lasing in nickel-like ions is reviewed. Double pulse pumping with ∼100 ps pulses has been shown to produce significantly greater X-ray laser output than single pulses of duration 0.1–1 ns. With double pulse pumping, the main pumping pulse interacts with a pre-formed plasma created by a pre-pulse. The efficiency of lasing increases as there is a reduced effect of refraction of the X-ray laser beam due to smaller density gradients and larger gain volumes, which enable propagation of the X-ray laser beam along the full length of the target. The record shortest wavelength saturated laser at 5.9 nm has been achieved in Ni-like dysprosium using double pulse pumping of 75 ps duration from the VULCAN laser. A variant of the double pulse pumping using a single ∼100 ps laser pulse and a superimposed short ∼1 ps pulse has been found to further increase the efficiency of lasing by reducing the effects of over-ionisation during the gain period. The record shortest wavelength saturated laser pumped by a short ∼1 ps pulse has been achieved in Ni-like samarium using the VULCAN laser operating in chirped pulse amplified (CPA) mode. Ni-like samarium lases at 7.3 nm.     

Analysis of tunable picosecond pulse generation from a distributed feedback Ti：sapphire laser

A distributed feedback Ti:sapphire laser (DFTL) pumped by a 532nm Q-switched pulse is proposed for the generation of tunable picosecond pulses. With coupled rate equation model, the temporal characteristics of DFTL are obtained. The numerical solutions show that the DFTL pulse with a 50-ps pulse duration and as much as 3.SmJ pulse energy can be obtained under 40-m J, 5-ns pulse pumping. The dependence of output pulse width on the laser crystal‘s length, pumping pulse duration, and pumping rate is also discussed in detail.     

LD预泵浦条件与Cr4+,Nd3+:YAG微片激光器的输出特性

 建立了LD预泵浦的被动调Q平平腔Cr⁴⁺,Nd³⁺:YAG微片激光器模型,分析了预泵浦参数与激光脉冲时间特性的关系,设计了LD预泵浦的Cr⁴⁺,Nd³⁺:YAG微片激光器实验,测量了不同预泵浦参数下激光单脉冲以及脉冲序列的时间特性。实验结果与数值计算结果基本符合：调节预泵浦参数使每个泵浦脉冲刚好对应输出单个激光脉冲时,固定脉冲泵浦宽度,脉冲泵浦速率随连续泵浦速率的增加呈一次函数减小;固定连续泵浦速率,脉冲泵浦速率随脉冲泵浦宽度的增加呈非线性函数减小。结果还表明,激光脉冲半高全宽随整体泵浦速率的增加而减小,峰值功率随整体泵浦速率的增加而增加;当整体泵浦速率在阈值附近变动时,激光单脉冲时间特性的变化最为明显。     

飞秒光脉冲光纤放大增益特性的实验研究

报道了掺Ｅｒ＾３＋光纤激光器输出１．５３１μｍ波长飞秒激光脉冲增益放大的实验研究结果,将自起振相加脉冲摹参Ｅｒ＾３＋光纤激光器输出的飞秒激光脉冲注入掺Ｅｒ＾３＋光纤放大器中进行放大,分别采用正向和逆向抽地这的方式,得到了最高放大倍数５５倍（１７．４ｄＢ）和６４倍（１８．１ｄＢ）的增益,对应的最大单脉冲能量（峰值功率）分别为０．３８４ｎＪ（７５２Ｗ）０．４５２ｎＪ（１２９５Ｗ）,脉冲重复率为２０．８     

Measurement of temporal durations of transient collisional excitation X-ray lasers

We have measured temporal durations of Ni-like transient collisional excitation X-ray lasers (4d4p, J=01) using a high-speed X-ray streak camera coupled with a flat field grating spectrometer. The pumping laser pulse consisted of a 400-ps prepulse and a 4.8-ps main pulse derived from a Nd:glass laser system with chirped pulse amplification. A quasi-traveling-wave pumping using a six-step mirror was adopted to synchronize the X-ray laser pulse and the onset of gain in a plasma medium. The X-ray laser outputs from three different materials, silver (Z=47), palladium (Z=46) and molybdenum (Z=42), showed that the X-ray laser duration became shorter as the atomic number decreased. PACS 42.55.Vc; 52.50.Jm; 52.70.La     

掺Er3+飞秒光纤放大器的特性研究

依据二能级速率方程和光传输方程,理论计算了Er3+光纤放大器中Er3+光纤的最佳长度Lc. 以全光纤飞秒激光器输出的孤子脉冲作为种子源,以理论值Lc作为实际采用的光纤长度,实验测量了Er3+光纤放大器在同向泵浦和反向泵浦下信号光的输出功率,得到同向泵浦的增益为20.0dB, 反向泵浦的增益为20.2 dB.同时测量了脉冲宽度随泵浦功率的变化,实验结果表明,随着泵浦功率的增大,输出光脉冲的宽度的压缩效应经历逐渐增强后逐渐减弱的过程.在同向泵浦方式下,可以得到更短的输出脉宽,其最短脉冲宽度为260 fs.     

Pump pulse characteristics of quasi-continuous-wave diode-side-pumped Nd:YAG laser

The influence of pumping laser pulse on the property of quasi-continuous-wave (QCW) diode-side-pumped Nd:YAG laser is investigated theoretically and experimentally. Under remaining a fixed duty cycle, the average output power increases, and the corresponding thermal focal length shorten with the increase of the pump pulse duration, which attributes to the decrease of the ratio of pulse buildup time to the pulse duration. At a pump power of 146 W, the laser output power changes from 65.1 W to 81.2 W when the pulse duration is adjusted from 150 μ s to 1000 μ s, confirming a significant enhancement of 24.7%. A laser rate equation model incorporating the amplified spontaneous emission is also utilized and numerically solved, and the simulated results agree well with the experimental data.     

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.     

掺Er3+氟化物光纤振荡器中红外超短脉冲的产生

基于非线性薛定谔方程建立了氟化物(ZrF₄-BaF₂-LaF₃-AlF₃-NaF, ZBLAN)光纤振荡器中产生中红外超短脉冲的理论模型, 在此基础上研究了中红外超短脉冲在氟化物光纤振荡器中形成的物理机理, 数值模拟了氟化物光纤振荡器中中红外超短脉冲的演化过程. 分析了腔内净色散和小信号增益系数对振荡器中锁模脉冲产生的影响, 并给出了参数设置范围. 研究发现: 当掺Er³⁺氟化物光纤长度, 小信号增益系数, 不饱和损耗为一定值时, 腔内净色散在一定范围内才会出现稳定的锁模脉冲, 且随着腔内净色散增加脉冲宽度变宽, 光谱变窄, 峰值功率降低; 当掺Er³⁺氟化物光纤长度及不饱和损耗一定, 腔内净色散量为合理值, 小信号增益系数在合理的范围时可以得到稳定的锁模脉冲, 且随着小信号增益系数的增加脉冲宽度变宽, 光谱变宽, 峰值功率增加.