什么是同步辐射光源What isSynchrotron radiation light source
同步辐射光源造价如此之高，它到底有何优势？
1、什么是同步辐射光源？What is a synchrotron radiation light source?
" Synchrotron radiation" refers to a continuous band of electromagnetic spectrum including infrared, visible light, ultraviolet, and X-rays. This light has been called "synchrotron radiation", since it was accidentally discovered in an electron synchrotron of the General Electric Company, USA, in 1947. Generations of Synchrotron Radiation Sources."同步辐射"是指包括红外、可见光、紫外线和X射线在内的连续电磁波段。这种光被称为"同步辐射"，因为它在1947年意外地在美国通用电气公司的电子同步器中被发现。一代又一代的同步辐射源。
2、同步辐射光源有什么特点？
折叠宽波段
同步辐射光的波长覆盖面大，具有从远红外、可见光、紫外直到 X射线范围内的连续光谱，并且能根据使用者的需要获得特定波长的光。
折叠高准直
同步辐射光的发射集中在以电子运动方向为中心的一个很窄的圆锥内，张角非常小，几乎是平行光束，堪与激光媲美。
折叠高偏振
从偏转磁铁引出的同步辐射光在电子轨道平面上是完全的线偏振光，此外，可以从特殊设计的插入件得到任意偏振状态的光。
折叠高纯净高亮度
高纯净:同步辐射光是在超高真空中产生的，不存在任何由杂质带来的污染，是非常纯净的光。
高亮度:同步辐射光源是高强度光源，有很高的辐射功率和功率密度，第三代同步辐射光源的 X射线亮度是 X光机的上千亿倍。
折叠窄脉冲
同步辐射光是脉冲光，有优良的脉冲时间结构，其宽度在10-11~10-8秒(几十皮秒至几十纳秒)之间可调，脉冲之间的间隔为几十纳秒至微秒量级，这种特性对"变化过程"的研究非常有用，如化学反应过程、生命过程、材料结构变化过程和环境污染微观过程等。
折叠可精确预知
同步辐射光的光子通量、角分布和能谱等均可精确计算，因此它可以作为辐射计量---特别是真空紫外到 X射线波段计量---的标准光源。
此外，同步辐射光还具有高度稳定性、高通量、微束径、准相干等独特而优异的性能。
What are the characteristics of synchronous radiation light sources?
3.同步辐射的特性Properties of Synchrotron Radiation
The main properties of the synchrotron radiation are the following
1. high intensity;
2. very broad and continuous spectral range from infrared up to the hard x-ray region;
3. natural narrow angular collimation;
4. high degree of polarization;
5. pulsed time structure;
6. high brightness of the source due to small cross section of the electron beam (see Fig. 1.4) and high degree of collimation of the radiation;
7. ultra-high vacuum environment and high beam stability;
8. all properties quantitatively evaluable. All these properties depend on the characteristics of the storage ring and can be calculated by applying classical electrodynamics to the motion of relativistic charged particles.
As already explained, the electron bunches emit radiation as they are radially accelerated by the dipole magnets. This radiation is contained within a fan-like region (see below):
Now this fan-like distribution of radiation (coloured mauve on the diagram) is quite different from that of the radio-transmitter which obviously needs to transmit radio waves in all directions. The reason for this is that with a synchrotron the electrons are travelling very close to the speed of light (e.g. at 2 GeV their velocity is around 0.9999 of the speed of light and the electron mass increases to 4000 times its rest mass). It is a consequence of relativity that at such (relativistic) speeds the spatial bounds of the synchrotron radiation are contracted to a narrow fan in the forward direction of the electron bunches, typically 0.3 mrad (1 mrad ≈ 0.06 degrees) in the vertical plane. However since the electron bunches are circulating horizontally there is a continuum of such fans spread out over an arc (see below) so that the final fan is wider horizontally (typically degrees) than vertically as illustrated above.
This then represents one of the great advantages of synchrotron radiation: that it is condensed into a small angular fan, thus imparting much greater intensity and collimation than can be obtained from conventional laboratory sources.
But what is this radiation? As already remarked it extends from the radio-frequency to the X-ray regions of the electromagnetic spectrum although we will be principally interested in the X-ray region. Such radiation is often referred to as white (since it contains all wavelengths); its performance is measured by a power spectrum which is a plot of radiation intensity versus wavelength, λ; a typical version is shown below:
This characteristic shape is common to all dipole radiation. It shows a maximum typically at about λ = 10 Å and spreads out either side, slowly on the high wavelength side (termed soft radiation) all the way to the radio-wave region, and rather abruptly on the short wavelength side (termed hard radiation since it penetrates matter more easily). The intensity appears to be measured in strange units, and this therefore requires some explanation. All forms of electromagnetic radiation show both wave-like and particle-like properties, and so sometimes we may prefer to describe the radiation as a stream of particles, rather than as a propagative wave. We refer to these particles as photons (which have discrete amounts of energy E given by hc/λ - yet another fundamental principle of physics). A common goal is to maximise the number of X-ray photons of a desired wavelength hitting a sample, and so the intensity of a synchrotron radiation beam has been traditionally expressed in units of photons per second per 0.1% bandwidth per mrad2; this is so that comparisons with other sources can be made, accounting for the time of collection (per second), that the white source contains all wavelengths (per % bandwidth), and that the radiation is spread out over a fan (per mrad2). It is customary now also to refer to brilliance (photons -1 0.1% bandwidth-1 mrad-2 mm-2) whereby the effective size of the electron bunches (per mm-2) is also taken into consideration. Whatever the units, one should realise that we are invariably talking in terms of numbers like 1012 photons per second and this can represent an enormous transmission of energy; a point of some concern with fragile specimens.
The radiation also has some other remarkable properties: It is horizontally polarised in the plane of the electron orbit and circularly polarised above and below the orbit. As the schematic below illustrates, whereas with laboratory sources the X-ray electric vector vibrates in all directions perpendicular to the propagation of the X-ray; with the synchrotron this vibration is horizontally polarised. This has advantages which can be put to use in both synchrotron diffraction and spectroscopy.
| Synchrotron | Laboratory |
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Finally, the X-ray beam is not actually continuous in time but fires in extremely short bursts. This is simply a property of the electron bunches; the X-rays are produced only as the bunches pass through the dipole magnet. Depending on the number, size and speed of the bunches, each flash typically lasts for 100 ps (100 × 10-12 second) and is repeated typically every 1-300 ns (1 ns = 10-9 second). This time structure to the radiation can also be put to use in certain specialised experiments.
We should finish this part by summarising the five main features of synchrotron radiation:
It is intense. This can enable measurements to be conducted at great speed and with superior statistics.
It is highly collimated (divergence in the order of mrads). This results in less wastage of radiation in its passage through the optical components towards the sample, and greater eventual resolution in measurement due to its spatial precision.
It has a smooth continuous spectrum. This gives a choice of conducting experiments with white radiation, or offering any single wavelength by the use of monochromators (monochromators are discussed later).
It is horizontally polarised.
It has a precise "flashing" time structure.
All these features except the last are regularly exploited in synchrotron powder diffraction.
以下的一张slide（自己的留着做报告吹牛时用，放上的是上海光源肖体乔老师课题组在用户会议上的报告PPT）可以解释这两个问题：
3、全世界有多少同步辐射装置？
还是上权威的地图分布最直观最靠谱。图片来自Nature Photonics 9, 281 (2015)，数据来自 http://www.lightsources.org 一个国际同步辐射合作组织的网站。
从图中可以看出，全世界截至2015年大约有47个同步辐射装置，分布于23个国家和地区。中国境内有北京正负电子对撞机兼用的一代光源，合肥同步辐射光家实验室二代光源，上海同步辐射光源（三代光源）及台湾新竹的同步辐射光源。此外，预计今年年底会在北京怀柔开工建设3.5代准衍射极限环北京高能光源（衍射极限环被称为第四代），新闻联播上报道过，就是那个48亿人民币预算的大家伙。见下图（图片来于项目首席科学家董宇辉研究员）：
知名的同步辐射光源有美国的ALS、APS、BNL、CHESS、SSRL、NSLS-II，日本的SPring-8，欧洲的ESRF，DESY旗下的BESSY、英国钻石光源，加拿大光源、瑞士光源、意大利蒂利亚斯特光源等。大概都长一个样子（个人一直觉得位于Cupertino的苹果新总部就是个同步辐射储存环）
题外话：大伙看出来了，美国有8台同步辐射装置，咱中国只有4台，大陆目前只有1台三代同步辐射光源大科学装置，所以，资源还是很稀缺的。最近教育部、湖北省及武汉市政府、武汉大学在大力推动武汉光源的立项与建设工作，作为同步辐射人，我们是喜闻乐见的。
4、同步辐射到底具有哪些优势？
其实问题已经回答完了，第一张图上列出的同步辐射的特点便是它的优势。
常规来讲，同步辐射上运用的方法学大多分为衍射、吸收、散射三大类。
如解析蛋白质的结构，为晶体衍射；常规CT为吸收衬度成像；小角散射等。
只对成像具有一定的研究，对结构生物学与各类谱学不太精通。
同步辐射上常用的成像方法有CT（计算机断层扫描成像术）、纳米CT、STXM（扫描透射X射线显微成像术）、TXM（透射X射线显微成像术）、CDI（相干衍射成像），其中纳米CT和STXM及TXM是有交叉的，在STXM及TXM二维成像的基础上旋转样品，获得一系列角度下的投影即可重构出三维图像，分辨率为纳米量级，故称为纳米CT。
CT、纳米CT一般采用吸收衬度或相位衬度原理成像；STXM及TXM一般采用吸收衬度及元素吸收边进行成像；CDI采用横向相干的X射线进行衍射成像。
因为光源相干性好、通亮高，所以获得的图像能取得更佳的衬度，图像的实空间分辨率可以更高，这便是在成像中的优势。更为关键的是，同步辐射是一个大科学装置，是一个复杂的研究平台，各类方法与技术是可以同时使用的。比如说冷冻电镜里的Cryo系统可以引入成像系统，实现冷冻干燥下的细胞成像；在线的金刚石对顶砧(DAC)可以实现材料的高温高压下原位成像研究；荧光探测器可以加载到成像系统的CCD探测器旁，进行成像的同时实现荧光谱，元素识别的研究～～ 说多了，还有很多idea，留着自己做发paper用
上海同步辐射装置(Shanghai Synchrotron Radiation Facility，简称 SSRF)，是一台世界先进的中能第三代同步辐射光源，总投资计划12亿人民币。上海同步辐射装置的电子储存环电子束能量为3.5GeV(35亿电子伏特)，仅次于世界上仅有的三台高能光源(美、日、欧各一台)，居世界第四。
HEPS于2017年12月获得国家发展改革委批复立项，是我国“十三五”期间建设的，为国家重大战略需求和前沿基础科学研究提供技术支撑平台的国家重大科技基础设施，是中国科学院和北京市立足于推动落实国家“创新驱动战略”，共建怀柔科学城的核心装置，也是北京怀柔综合性国家科学中心最重要的重大科技基础设施。
它的整体建筑外形似一个放大镜，寓意为探测微观世界的利器。建成后的HEPS，将是我国第一台高能同步辐射光源，也是世界上亮度最高的第四代同步辐射光源之一。
该项目由中科院高能所承担建设，主要建设内容包括加速器、光束线站及辅助设施等，建设周期6.5年，新建建筑面积12.5万平米。
HEPS主加速器的周长为1360.4米，电子束流能量为60亿电子伏特，设计亮度高于1×1022 phs/s/mm2/mrad2/0.1%BW。主加速器采用7BA弯铁消色散的结构单元，可实现电子束流的水平自然发射度小于60皮米·弧度，这也是第四代衍射极限光源的主要特点。
HEPS具有建设不少于90条高性能光束线和实验站的能力，首期拟建设其中14条用户最急需的光束线及对应的实验站。HEPS建成后，可提供能量高达30万电子伏特并具有高亮度、高相干性等特点的同步辐射光，并具备纳米量级空间分辨、皮秒量级时间分辨、毫电子伏特量级能量分辨能力，将为国家发展战略和工业核心迫切需求的相关研究，提供开展多维度、实时、原位的表征的研究平台。
HEPS的建设是推动我国同步辐射光源领域研究达到世界前沿的重要举措，建成后将会全面提升我国在科技和产业领域的原始创新能力，也会成为我国重要的国际科技合作与基础科学研究平台。