【摘要】：超快電子衍射與成像技術(shù)在最近十多年內(nèi)得到快速發(fā)展,至今已有多個(gè)科學(xué)機(jī)構(gòu)進(jìn)行了相關(guān)儀器研發(fā)及研究工作,取得了許多重要進(jìn)展。其中直流電子槍的電子脈沖能量由起始的幾十千伏發(fā)展到數(shù)百千伏,而最新應(yīng)用到該領(lǐng)域的微波電子槍則將能量提升至兆伏量級;電子脈沖的縱向脈寬(時(shí)間尺度)則由皮秒量級變至飛秒量級,甚至有望到達(dá)阿秒量級。在此基礎(chǔ)上,發(fā)展起來了包括超快掃描電子顯微鏡,超快透射電鏡等多種時(shí)間分辨電子成像技術(shù)。研究范圍也從晶格探測擴(kuò)展至表面物理、超導(dǎo)、磁學(xué)、等離子、生物、化學(xué)等多個(gè)領(lǐng)域,逐步顯示處了其強(qiáng)大的生命力。在此背景下,我們進(jìn)行了100kV超快電子衍射系統(tǒng)的研發(fā)與搭建。我們先進(jìn)行了系統(tǒng)設(shè)計(jì)所需的相關(guān)模擬,然后在此基礎(chǔ)搭建了一臺多功能緊湊型的百千伏超快電子衍射系統(tǒng),包括100kV直流光陰極電子槍、電子脈沖控制裝置、超高真空腔室、樣品送入調(diào)節(jié)裝置、探測成像裝置、數(shù)據(jù)自動化采集系統(tǒng)以及外接設(shè)備等。通過模擬我們發(fā)現(xiàn)當(dāng)電子脈沖經(jīng)過電場邊界后,其在z方向(縱向)上的擴(kuò)展速度將會有一個(gè)很大幅度的減小,這對電子脈沖后期的行為具有決定性影響。我們修改并發(fā)展了傳統(tǒng)的Mean field模型,給出了一個(gè)描述超快電子衍射系統(tǒng)內(nèi)部電子脈沖運(yùn)動的合理方案,使其可以準(zhǔn)確描述超快電子束的運(yùn)動行為,并優(yōu)于其他模型。電子槍的設(shè)計(jì)極大程度避免了局部場強(qiáng)突變的出現(xiàn),使電子槍可在一百千伏電壓下穩(wěn)定工作;增加了反射工作模式,單發(fā)電子產(chǎn)額有望達(dá)到百萬量級,以實(shí)現(xiàn)對不可逆過程的探測。特殊的同軸對稱結(jié)構(gòu)保證了電場的對稱與穩(wěn)定,良好的磁屏蔽效果保證了電子脈沖的質(zhì)量,外接保護(hù)電阻及保護(hù)罩等保證了電子槍工作的安全性。整套裝置采用了特殊內(nèi)嵌安裝結(jié)構(gòu)設(shè)計(jì),使電子槍陰極至樣品的距離在加磁透鏡的情況下最短可至130mm,沒有磁透鏡時(shí)最短可小于100mm。裝置的腔體設(shè)計(jì)最高真空度可達(dá)10-10Torr,由靶室,電子槍腔室,泵浦反射鏡腔室,法拉第筒腔室以及排氣腔室等組成。超高真空靶室具有多個(gè)窗口,其中側(cè)向的34mm直徑法蘭窗口特殊設(shè)計(jì)。特殊設(shè)計(jì)了裝置泵浦激光反射模式,使其沿電子脈沖運(yùn)動相反的方向入射。樣品靶室后期可與其它多個(gè)裝置連接,實(shí)現(xiàn)多功能擴(kuò)展。樣品送入部分具有五維調(diào)節(jié)范圍。樣品承載裝置——樣品架能保證未來多種探測方案的實(shí)施,可被換裝為其它裝置,可升級。裝置的探測成像系統(tǒng)包括特殊設(shè)計(jì)的法拉第筒,電子脈沖成像系統(tǒng)等。采用Labview編寫的數(shù)據(jù)自動化采集系統(tǒng)正在研發(fā)中。在進(jìn)行樣品表面時(shí)間分辨衍射探測實(shí)驗(yàn)時(shí)(TR-RHEED),發(fā)現(xiàn)衍射條紋出現(xiàn)分裂,距離隨延遲時(shí)間的變化呈現(xiàn)類高斯分布。等級越高的衍射條紋,其分裂間距越小。我們猜測該現(xiàn)象與樣品發(fā)射電子有關(guān),并通過分裂間距近似估計(jì)樣品泵浦后表面電場峰值為107V/m量級。該實(shí)驗(yàn)有望同時(shí)獲得樣品晶格信息和表面電場信息,提供一種新的時(shí)間分辨電子衍射探測方法。
[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.
【學(xué)位授予單位】：中國科學(xué)院大學(xué)(中國科學(xué)院物理研究所)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：O572.322

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