Controlling nonadiabatic spectral redshift of high-order harmonic using two orthogonally polarized laser fields

We investigate the nonadiabatic spectral redshift of high-order harmonic of He driven by two time-delayed orthogonally polarized laser fields. It is found that the nonadiabatic spectral redshift can be observed by properly adjusting the time delay of the two laser fields when the controlling pulse is added in the raising part of the driving pulse in the vertical direction. That is because the controlling pulse in the vertical direction prevents the ionized electrons from returning to the vicinity of parent ions and then reduces the recombination probability. This leads to the high-order harmonic generated mainly in the falling part of the driving pulse. Meanwhile,we also find that the quantity of redshift can be effectively controlled through accommodating the positive time delays. In addition, this scheme can also be used to produce nonadiabatic spectral blueshift.     

boldmath Broadband water window supercontinuum generation with a ω+3ω/2 multicycle two-colour pulse

We propose a method to generate a high-efficiency broadband water window supercontinuum with a ω+3ω/2 multicycle two-colour pulse. Our results reveal that the 3ω/2 laser pulse can simultaneously modulate the acceleration step and the ionization step, which not only broadens the bandwidth but also enhances the yield of the generated supercontinuum. An ultra-broadband supercontinuum from 290 eV to 555 eV covering the whole water window is generated. Using this method, we expect that an isolated 62-as pulse with a minor pre-pulse can be directly obtained.     

激光诱导击穿光谱在核分析实验教学中的应用

为了适应核物理与核分析实验教学的需要,本文将激光诱导激光光谱测量以创新实验的形式引入到核工程类专业实验中.通过将纳秒激光脉冲与不同靶材作用,测量样品中各类元素的LIBS谱线,结合理论分析提取激光等离子体的电子温度和等离子体密度.假设单一元素谱线获得的等离子体密度为混合样品等离子体密度,计算得到未知重元素Fe的原子谱线展宽参数.     

Density measurement through elastic electron scattering with a gaseous target at the Jefferson Lab

We report the density measurement through e-³He elastic scattering with a 1.23 GeV electron beam in Jefferson Lab experiment E06-010. The extracted ³He density is (9.26±0.06) amagats and the N₂/³He ratio is (1.49±0.08)%. In addition, these results are consistent with the deduced target densities based on pressure broadening measurement.     

High-order harmonic generation with a two-color laser pulse

We theoretically investigate the electron dynamics of the high-order harmonics generation process by combining a near-infrared 800 nm driving pulse with a mid-infrared 2000 nm control field.We also investigate the emission time of harmonics using time-frequency analysis to illustrate the physical mechanisms of high-order harmonic generation.We calculate the ionization rate using the Ammosov-Delone-Krainov model and interpret the variations in harmonic intensity for different control field strengths and delays.We find that the width of the harmonic plateau can be extended when the control electric field is added,and a supercontinuum from 198 to 435 eV is generated,from which an isolated 61-as pulse can be directly obtained.     

Role of excited states in the elliptically polarized harmonic generation

We theoretically investigate the contribution of the excited state to the ellipticity of the harmonics from H＋ at different orientation angles irradiated by a linearly polarized laser pulse. It is found that the first excited state has a significant influence to the ellipticity of the harmonics, and the contribution of higher excited states to the ellipticity can be neglected. Moreover, the conclusion is not dependent on the laser intensity.     

飞秒激光低压镍等离子体光谱的实验研究

分别对低于0.1 MPa的空气和氩气环境气体条件下的飞秒激光镍等离子体光谱进行了实验研究。结果表明,在两种环境气体中,激光镍等离子体光谱均表现为连续谱和原子线状谱的叠加。随着环境气压的降低,由于电子密度的下降,从而使得低压条件下的谱线比高压条件下的谱线具有更好的分辨特性。同时,谱线强度经历了由缓慢增强发展到迅速降低的演化过程。另外,还讨论了环境气体成分对谱线强度的影响。     

Experimental studies of THGEM in different Ar/CO_2 mixtures

In this paper, the performance of a type of domestic THGEM (THick Gaseous Electron Multiplier) working in Ar/CO₂ mixtures is reported in detail. This kind of single THGEM can provide a gain range from 100 to 1000, which is very suitable for application in neutron detection. In order to study its basic characteristics as a reference for the development of a THGEM based neutron detector, the counting rate plateau, the energy resolution and the gain of the THGEM have been measured in different Ar/CO₂ mixtures with a variety of electrical fields. For the Ar/CO₂(90%/10%) gas mixture, a wide counting rate plateau is achieved from 720 V to 770 V, with a plateau slope of 2.4%/100 V, and an excellent energy resolution of about 22% is obtained at the 5.9 keV full energy peak of the ⁵⁵Fe X-ray source.     

飞秒激光低压N2等离子体特性的实验研究

在低于一个标准大气压的条件下对飞秒激光产生的N₂等离子体光谱进行了实验研究.结果表明, 各种样品气压下的激光N₂等离子体光谱均表现为连续谱和线状谱的叠加.随着样品气压的降低, 连续谱和原子谱线的强度经历了由缓慢增强发展为缓慢降低再到迅速降低的过程, 而正一价离子谱线强度呈现逐步增强的特征.在气压低于0.3 atm (1 atm=101325 Pa)时, 出现了正二价态的离子谱线. 给出了低压N₂等离子体对于飞秒激光传输和能量吸收的物理特性, 并初步讨论了低压等离子体通道特性.这些结果有助于加深了解飞秒激光等离子体的特性和机理, 特别是给出了在实验上测量不同价态离子光谱的条件, 为今后的研究提供了有益的实验依据. 关键词： 飞秒激光 气压 等离子体光谱 激光传输     

基于Micromegas探测技术的中子CT可行性研究(英文)

在培育新品种过程中能够实时掌握了解土壤中植物根系的形态以及其生长情况将为植物学家提供很多不可或缺的信息。对基于 micromegas 探测技术的中子 CT 在这一新应用的试验环境进行了仿真模拟, 通过三维图像重建后,得到了令人满意的预期结果。首先通过 Geant4 和 Garfield 模拟计算利用聚乙烯薄膜作为中子转换层的 micromegas 中子探测器, 得到了非常理想的位置分辨, 说明基于micromegas 探测技术建立中子 CT 照相系统的可行性。 然后利用MCNP仿真模拟14 MeV 中子CT 的实验环境, 最后由Matlab程序进行图像重建。In this paper, the possibility using micromegas(Micro Mesh Gaseous Structure) as neutron detector in 14 MeV neutron computed tomography(CT) has been simulated. The results show that the micromegas neutron detector has high spatial resolution and is a good candidate for neutron radiography. The three dimensional images of plant roots in soil are successfully and clearly obtained by the 14 MeV neutron CT with micromegas as a neutron detector. In the present simulation, MCNP is employed for 14 MeV neutron transport in the sample and Matlab for the 3 D photograph reconstruction.