百千伏超快电子衍射系统的研发

发布时间：2019-03-05 16:31

【摘要】：超快电子衍射与成像技术在最近十多年内得到快速发展,至今已有多个科学机构进行了相关仪器研发及研究工作,取得了许多重要进展。其中直流电子枪的电子脉冲能量由起始的几十千伏发展到数百千伏,而最新应用到该领域的微波电子枪则将能量提升至兆伏量级;电子脉冲的纵向脉宽(时间尺度)则由皮秒量级变至飞秒量级,甚至有望到达阿秒量级。在此基础上,发展起来了包括超快扫描电子显微镜,超快透射电镜等多种时间分辨电子成像技术。研究范围也从晶格探测扩展至表面物理、超导、磁学、等离子、生物、化学等多个领域,逐步显示处了其强大的生命力。在此背景下,我们进行了100kV超快电子衍射系统的研发与搭建。我们先进行了系统设计所需的相关模拟,然后在此基础搭建了一台多功能紧凑型的百千伏超快电子衍射系统,包括100kV直流光阴极电子枪、电子脉冲控制装置、超高真空腔室、样品送入调节装置、探测成像装置、数据自动化采集系统以及外接设备等。通过模拟我们发现当电子脉冲经过电场边界后,其在z方向(纵向)上的扩展速度将会有一个很大幅度的减小,这对电子脉冲后期的行为具有决定性影响。我们修改并发展了传统的Mean field模型,给出了一个描述超快电子衍射系统内部电子脉冲运动的合理方案,使其可以准确描述超快电子束的运动行为,并优于其他模型。电子枪的设计极大程度避免了局部场强突变的出现,使电子枪可在一百千伏电压下稳定工作;增加了反射工作模式,单发电子产额有望达到百万量级,以实现对不可逆过程的探测。特殊的同轴对称结构保证了电场的对称与稳定,良好的磁屏蔽效果保证了电子脉冲的质量,外接保护电阻及保护罩等保证了电子枪工作的安全性。整套装置采用了特殊内嵌安装结构设计,使电子枪阴极至样品的距离在加磁透镜的情况下最短可至130mm,没有磁透镜时最短可小于100mm。装置的腔体设计最高真空度可达10-10Torr,由靶室,电子枪腔室,泵浦反射镜腔室,法拉第筒腔室以及排气腔室等组成。超高真空靶室具有多个窗口,其中侧向的34mm直径法兰窗口特殊设计。特殊设计了装置泵浦激光反射模式,使其沿电子脉冲运动相反的方向入射。样品靶室后期可与其它多个装置连接,实现多功能扩展。样品送入部分具有五维调节范围。样品承载装置——样品架能保证未来多种探测方案的实施,可被换装为其它装置,可升级。装置的探测成像系统包括特殊设计的法拉第筒,电子脉冲成像系统等。采用Labview编写的数据自动化采集系统正在研发中。在进行样品表面时间分辨衍射探测实验时(TR-RHEED),发现衍射条纹出现分裂,距离随延迟时间的变化呈现类高斯分布。等级越高的衍射条纹,其分裂间距越小。我们猜测该现象与样品发射电子有关,并通过分裂间距近似估计样品泵浦后表面电场峰值为107V/m量级。该实验有望同时获得样品晶格信息和表面电场信息,提供一种新的时间分辨电子衍射探测方法。
[Abstract]:In recent ten years, the ultrafast electron diffraction and imaging technology have been developed rapidly. in which the electron pulse energy of the direct current electron gun is developed to several hundred kilovolts from the start of several tens of kilovolts, while the most recent microwave electron gun applied in this field raises the energy to the magnitude of the megavolt; the longitudinal pulse width (time scale) of the electronic pulse is changed from the picosecond magnitude to the femtosecond level, And is even expected to reach the order of a second. On this basis, a variety of time-resolved electronic imaging techniques, including ultrafast scanning electron microscopy and ultrafast transmission electron microscopy, have been developed. The scope of the study also extends from the lattice detection to the surface physics, superconductivity, magnetism, plasma, biology, chemistry and so on, and gradually shows its strong vitality. In this background, we have carried out the R & D and construction of the 100kV ultrafast electron diffraction system. The relevant simulation required by the system design is carried out first, and then a multifunctional compact 100-kilovolt ultrafast electron diffraction system is built, which comprises a 100kV direct current photocathode electron gun, an electronic pulse control device, an ultra-high vacuum chamber and a sample sending and adjusting device, A detection imaging device, a data automatic acquisition system, and an external device, and the like. By simulation we find that the expansion speed in the z-direction (longitudinal direction) will be greatly reduced when the electron pulse passes through the boundary of the electric field, which has a decisive influence on the behavior of the later stage of the electronic pulse. The traditional Mean field model is modified and developed, and a reasonable scheme for describing the internal electron pulse motion of the ultrafast electron diffraction system is given, so that it can accurately describe the movement behavior of the ultrafast electron beam, and is superior to other models. The design of the electron gun greatly avoids the occurrence of local field intensity mutation, so that the electron gun can work stably at a voltage of one hundred kilovolts, the reflection working mode is increased, and the single-shot electron yield is expected to reach the order of millions, so as to realize the detection of the non-reversible process. The special coaxial symmetrical structure ensures the symmetry and stability of the electric field, the good magnetic shielding effect ensures the quality of the electronic pulse, the external protective resistor and the protective cover, and the like, and the safety of the operation of the electron gun is ensured. The complete set adopts a special embedded installation structure design, so that the distance between the cathode of the electron gun and the sample is as short as 130 mm in the case of a magnetic lens, and the shortest length of the magnetic lens can be less than 100 mm. The maximum vacuum degree of the cavity of the device can reach 10-10Torr, and consists of a target chamber, an electron gun chamber, a pump reflector chamber, a Faraday cylinder chamber, an exhaust chamber and the like. The ultra-high vacuum target chamber has a number of windows with a lateral 34 mm diameter flange window special design. The device pump laser reflection mode is specially designed so as to be incident in the opposite direction of the electronic pulse motion. And the later stage of the sample target chamber can be connected with a plurality of other devices to realize the multi-function expansion. The sample feed section has a five-dimensional adjustment range. The sample carrier _ sample rack can ensure the implementation of multiple detection schemes in the future, can be replaced by other devices, and can be upgraded. The detection and imaging system of the device comprises a specially designed Faraday tube, an electronic pulse imaging system, and the like. The data collection system developed by Labview is in the process of R & D. In the time-resolved diffraction (TR-RHEED) of sample surface time-resolved diffraction (TR-RHEED), it was found that the diffraction fringe was split and the distance along with the delay time exhibited a Gaussian distribution. The higher the grade the higher the diffraction fringes, the smaller the splitting pitch. We suspect that the phenomenon is related to the electron emission of the sample, and the peak of the surface electric field after the sample pump is estimated to be on the order of 107 V/ m by the split-pitch approximation. The experiment is expected to obtain sample lattice information and surface electric field information at the same time, and a new time-resolved electron diffraction detection method is provided.
【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O572.322

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