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神龙一号直线感应加速器(LIA)产生的强流高功率的脉冲电子束与X光转换靶作用后可以产生高剂量的X光,同时由于转换靶的被烧蚀破坏在靶面产生回流离子,该回流离子的存在影响到电子束的聚焦。设计了4套法拉第筒及其对应的偏压电路,法拉第筒被放置在神龙一号X光转换靶上游不同位置,分布在电子束轴线两侧,电路设计最高偏压为1 kV;对神龙一号LIA的X光转换靶面产生的回流离子进行了实验测量,分别得到回流正离子密度约在1021/m3,离子运动速度可达2~3 mm/s。计算比较表明,该离子流强度与神龙一号靶前电子束流相差很大,只有电子束流强的0.27%,对神龙一号电子束聚焦不会造成影响。 相似文献
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强流直线感应加速器(LIA)能够产生2—3kA、10-20MeV、约80ns(FWHM)的强流高功率的脉冲电子束,经过数十米的传输、聚焦成毫米量级的束斑后打击到轫致辐射靶上,来产生高剂量的X光。同时电子束打靶使靶面沉积大量的能量导致靶面的温度骤然升高,引起靶表面物质或杂质(如碳、氢、水蒸气与靶材料等)被汽化电离而产生等离子体。强流电子束在靶前附近产生的强空间电荷电场(场强可达MV/cm)把离子从靶面拉出,以逆着电子束的方向前进,被称为回流离子。人们提出回流离子与电子束发生作用,会导致电子束被提前聚焦,在预定的靶面形成散焦。 相似文献
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采用束包络方程分析了单脉冲和多脉冲情况下回流离子对强流相对论电子束聚焦的影响。分析结果表明,单脉冲情况下通过缩短焦距,仍可以获得较小的积分焦斑,而在多脉冲情况下回流离子将导致电子束完全散焦。通过数值模拟和实验研究了利用薄膜阻挡回流离子的可行性,对不同薄膜在电子束作用下的温升及动力学行为的模拟结果表明,在1.06 μs的时间尺度内,薄膜虽然发生了不同程度的膨胀,但是仍然有足够的材料可以阻挡离子回流。在神龙一号加速器上,通过法拉第筒测量了靶前放置和不放置薄膜情况下的离子信号,实验证实了薄膜至少能够将离子约束在薄膜和转换靶之间长达数十μs。 相似文献
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建立在单粒子运动学基础上的回流离子理论预测: 强流电子束轰击在
辐射转换靶上可能产生正离子. 这些离子在电子束空间电荷场作用下
回流, 会造成电子束过聚焦, 改变焦斑大小, 从而影响X光机的照相分辨率.
然而大量的实验没有发现相关的现象. 本文分析认为, 电子束
打靶时, 在靶表面可能会形成离子鞘层过渡区. 该区域可以抵消束流空间
电荷场对回流离子的驱动作用, 因此, 回流离子可能以等离子体集团扩散的
方式运动. 这种物理图像得到的结论是离子对聚焦的影响可忽略, 和已有的
实验结果相吻合. 相似文献
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T. Graber S. Anderson H. Brewer Y.‐S. Chen H. S. Cho N. Dashdorj R. W. Henning I. Kosheleva G. Macha M. Meron R. Pahl Z. Ren S. Ruan F. Schotte V. Šrajer P. J. Viccaro F. Westferro P. Anfinrud K. Moffat 《Journal of synchrotron radiation》2011,18(4):658-670
BioCARS, a NIH‐supported national user facility for macromolecular time‐resolved X‐ray crystallography at the Advanced Photon Source (APS), has recently completed commissioning of an upgraded undulator‐based beamline optimized for single‐shot laser‐pump X‐ray‐probe measurements with time resolution as short as 100 ps. The source consists of two in‐line undulators with periods of 23 and 27 mm that together provide high‐flux pink‐beam capability at 12 keV as well as first‐harmonic coverage from 6.8 to 19 keV. A high‐heat‐load chopper reduces the average power load on downstream components, thereby preserving the surface figure of a Kirkpatrick–Baez mirror system capable of focusing the X‐ray beam to a spot size of 90 µm horizontal by 20 µm vertical. A high‐speed chopper isolates single X‐ray pulses at 1 kHz in both hybrid and 24‐bunch modes of the APS storage ring. In hybrid mode each isolated X‐ray pulse delivers up to ~4 × 1010 photons to the sample, thereby achieving a time‐averaged flux approaching that of fourth‐generation X‐FEL sources. A new high‐power picosecond laser system delivers pulses tunable over the wavelength range 450–2000 nm. These pulses are synchronized to the storage‐ring RF clock with long‐term stability better than 10 ps RMS. Monochromatic experimental capability with Biosafety Level 3 certification has been retained. 相似文献
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Fabrication and testing of a newly designed slit system for depth‐resolved X‐ray diffraction measurements 下载免费PDF全文
John Sinsheimer Nathalie Bouet Sanjit Ghose Eric Dooryhee Ray Conley 《Journal of synchrotron radiation》2016,23(6):1296-1304
A new system of slits called `spiderweb slits' have been developed for depth‐resolved powder or polycrystalline X‐ray diffraction measurements. The slits act on diffracted X‐rays to select a particular gauge volume of sample, while absorbing diffracted X‐rays from outside of this volume. Although the slit geometry is to some extent similar to that of previously developed conical slits or spiral slits, this new design has advantages over the previous ones in use for complex heterogeneous materials and in situ and operando diffraction measurements. For example, the slits can measure a majority of any diffraction cone for any polycrystalline material, over a continuous range of diffraction angles, and work for X‐ray energies of tens to hundreds of kiloelectronvolts. The design is generated and optimized using ray‐tracing simulations, and fabricated through laser micromachining. The first prototype was successfully tested at the X17A beamline at the National Synchrotron Light Source, and shows similar performance to simulations, demonstrating gauge volume selection for standard powders, for all diffraction peaks over angles of 2–10°. A similar, but improved, design will be implemented at the X‐ray Powder Diffraction beamline at the National Synchrotron Light Source II. 相似文献
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Tabulated mass absorption coefficient data from the XCOM, Chantler and ion beam analysis (IBA) Handbook are compared, and differences are shown against X‐ray energy and target atomic number. Over the X‐ray energy range 1–40 keV systematic differences of several tens of percent are noted between these datasets, particularly for the earlier IBA Handbook dataset. The influence of these different mass absorption coefficients on the X‐ray yields for thick target proton‐induced X‐ray emission (PIXE) are investigated and compared as a function of X‐ray energy and target atomic number. For contemporary experimental PIXE users trying to work quantitatively over a broad range of elements and X‐ray energies, differences between PIXE results obtained using these three separate mass attenuation coefficient datasets can be larger than the typical accuracy limits of ±3% to ±5%. There are systematic differences in the mass attenuation coefficients of 5–10% between the XCOM and Chantler, while the differences for the IBA Handbook dataset can be larger (up to 40% and greater) at high X‐ray energies. At this time, we recommend the dataset of Chantler as it is more recent, and the synchrotron experimental results seem to favour it over the older XCOM and IBA Handbook data. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
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The degradation of cell performance of polymer electrolyte fuel cells under monochromatic X‐ray irradiation at 13.5 keV was studied in galvanostatic and potentiostatic operation modes in a through‐plane imaging direction over a range of two orders of magnitude beam intensity at the TOMCAT beamline of the Swiss Light Source. The performance degradation was found to be a function of X‐ray dose and independent of beam intensity, whereas the degradation rate correlates with beam intensity. The cell performance was more sensitive to X‐ray irradiation at higher temperature and gas feed humidity. High‐frequency resistance measurements and the analysis of product water allow conclusions to be drawn on the dominating degradation processes, namely change of hydrophobicity of the electrode and sulfate contamination of the electrocatalyst. 相似文献
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侧向背光照相能直接反映靶表面扰动幅度的变化情况。在神光Ⅱ装置上,实验利用侧向背光照相技术,对烧蚀面扰动引起的内界面扰动增长进行了观测。实验结果表明,观察到的内界面扰动幅度大于期望值。分析认为,造成内界面较大扰动增长的原因主要是2维效应。X光辐照的主要是烧蚀面的中间部分,烧蚀面扰动引起的内界面的扰动就呈现出一幅从中间的扰动区域逐渐过渡到四周的图像。由此,提出了新的靶优化设计方案,应尽可能减小沿背光方向的样品尺寸。 相似文献
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M. K. Tiwari L. Alianelli I. P. Dolbnya K. J. S. Sawhney 《Journal of synchrotron radiation》2010,17(2):237-242
In this paper the first practical application of kinoform lenses for the X‐ray reflectivity characterization of thin layered materials is demonstrated. The focused X‐ray beam generated from a kinoform lens, a line of nominal size ~50 µm × 2 µm, provides a unique possibility to measure the X‐ray reflectivities of thin layered materials in sample scanning mode. Moreover, the small footprint of the X‐ray beam, generated on the sample surface at grazing incidence angles, enables one to measure the absolute X‐ray reflectivities. This approach has been tested by analyzing a few thin multilayer structures. The advantages achieved over the conventional X‐ray reflectivity technique are discussed and demonstrated by measurements. 相似文献
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D. Starodub P. Rez G. Hembree M. Howells D. Shapiro H. N. Chapman P. Fromme K. Schmidt U. Weierstall R. B. Doak J. C. H. Spence 《Journal of synchrotron radiation》2008,15(1):62-73
The resolution of X‐ray diffraction microscopy is limited by the maximum dose that can be delivered prior to sample damage. In the proposed serial crystallography method, the damage problem is addressed by distributing the total dose over many identical hydrated macromolecules running continuously in a single‐file train across a continuous X‐ray beam, and resolution is then limited only by the available molecular and X‐ray fluxes and molecular alignment. Orientation of the diffracting molecules is achieved by laser alignment. The incident X‐ray fluence (energy/area) is evaluated that is required to obtain a given resolution from (i) an analytical model, giving the count rate at the maximum scattering angle for a model protein, (ii) explicit simulation of diffraction patterns for a GroEL–GroES protein complex, and (iii) the spatial frequency cut‐off of the transfer function following iterative solution of the phase problem, and reconstruction of an electron density map in the projection approximation. These calculations include counting shot noise and multiple starts of the phasing algorithm. The results indicate counting time and the number of proteins needed within the beam at any instant for a given resolution and X‐ray flux. An inverse fourth‐power dependence of exposure time on resolution is confirmed, with important implications for all coherent X‐ray imaging. It is found that multiple single‐file protein beams will be needed for sub‐nanometer resolution on current third‐generation synchrotrons, but not on fourth‐generation designs, where reconstruction of secondary protein structure at a resolution of 7 Å should be possible with relatively short exposures. 相似文献