闲话同步辐射光源

1、闲话同步辐射光源像科学史上上演的许多故事一样，同步辐射光源的发现是又一个典型的要赶鹰却捡只肥兔子回来的故事。要把其中的来龙去脉讲清楚，咱们得向上追溯一个多世纪。话说1895年伦琴（Rntgen） 观测到阴极电子加速运动撞击到阳极的过程中会释放一种能够穿过不透明物质, 并诱发荧光物质发光的神秘射线。他暂时命名这种射线为X射线，后人为纪念他亦称其为伦琴射线。他本人因此在1901年捧回史上第一个物理学诺贝尔奖。伦琴临终时嘱托将其所有的私人信件及科研手稿焚毁，所以后来有人质疑他是否确实是X射线的最初发现人。当然这是史学家的事。我们这里关心的是X射线本身。它不仅仅引导人们第一次进入了玄妙的原子分子层次的

2、微观世界，从而翻开近代物理学的新篇章，同时也向人类展示了它在工业和医疗等实用领域中的巨大作用。X射线的神奇魅力引无数英雄竟折腰。人们开始寻求开发X射线源。直到现在通常实验室中用的X光源还是基于电子撞击阳极的产生机理，即运动电子在撞击过程中急剧减速而释放辐射（韧致辐射或刹车辐射）及由靶材料原子能级决定的特征X光荧光谱线。这里的极其关键一步是让电子生变（速度）。变则明光环绕百媚生 - 看来这是一条从人类社会到自然颠簸不破的普遍真理：-）。由于电子撞击释放辐射过程是在固体里进行，相当多的能量会被最后转化为热能（低能辐射，也是光）。受限于阳极靶的散热能力，这些X光机能产生的X光的通量是很有限的。直到同

3、步辐射光源的出现才绕过了这个难题。上世纪早期人们在研究电子回旋加速器的过程中遇到了一个比较头疼的问题：电子在加速过程中会辐射电磁波造成能量损失。这里辐射产生的实际上就是同步辐射光。可惜囿于知识不足，同步辐射光犹如一块尚未抛光的璞玉，未被慧眼相识。实际上人们当时是把它当作可恶的拦路石，一心想把它挪走：因为这种辐射效应使加速器的效率降低，最终给高能电子的产生设定了一个上限。1947年4月24日，通用电器（GE）的四位科学家Frank Elder, Anatole Gurewitsch, Robert Langmuir, 及 Herb Pollock在他们新建的70MeV的加速器上尝试一种新的加速手

4、段。这个机器的设计能让人看到电子运行的轨道。实验刚刚开始，一个实验员大喊叫停。原来他在电子管中观察到耀眼的淡蓝色弧形光。大家赶紧跑过来查看，真空正常，不是电子管受损发出的光。很快几位物理学家意识到他们观察到的是同步辐射光。这是科学家世上第一次直接观察到非自然同步辐射光。这种光是电子在真空中加速运动中产生的，它有许多诱人的特点：高光通量（注意没有伴随的阳极热量），连续光谱（通常X光管子多是用荧光分立谱线以求相对高的通量），极高的准直性（这和下面要提到的电子相对论速度运动相关联），最后还有它的时间脉冲特性。经过一系列的研究尝试，人们见识到美玉真颜, 于是不用那种加速器做高能物理了，专门造它们当超大

5、的X光机。六十年代开始科学家建设同步辐射光源。至今为止同步辐射光源发展历经3代。第4代（X光激光）正在筹建中。世界上大约有22个国家和地区共建有（或正在筹建中）40多个大大小小的光源中心。其中美国有近10个，其他大部分在欧洲和日本，中国大陆3个，台湾1个（第二个刚开始建），加拿大1个，。约旦和泰国也都在建造。光源中心小的只有一个卡车大小；大的有个把公里。为方便起见，咱们还是用下面的静态图聊聊同步辐射光是如何产生的。这图有点小毛病，看看这里的物理大侠们能不能找出来。图中间与一个小环相连的直的通道是直线加速器（LINAC）。当电子由电子枪发射出来注入直线加速器加速电子，并把连续的电子束切成隔约几个

6、到几百个纳秒的等间隔电子团。然后电子被引导到助动加速环（booster ring，图中与直线加速器相接的小环）进一步被加速。当电子速度达到差不多是光速的设定值时（例如屯儿里的光源CLS 电子能量2.9GeV；电子速度是 99.999998% 真空光速；芝加哥的 APS，7GeV, 99.999999% 真空光速），电子被注入图中大环，也就是存储环（Storage ring），在存储环中，电子轨道将被偶极磁铁（图中拐弯处红色C型块，也称弯曲磁铁）弯曲从而保持匀速率绕环运动。弯曲电子轨道也就意味着产生向心加速度。前面提过，加速运动的带电粒子是要释放辐射的，当粒子以相对论速度(即快的跟光速有一比)运

7、动时它的辐射会聚集在其运动方向，这就是同步辐射光的来源。电子做圆周运动，X光从圆的切线方向散发出来。电子损失的能量由存储环中高频腔（RF cavity）补充，这个过程得掌握好时机，得同步才行，同步辐射光源也因而得名。除了弯曲形电磁铁外，存储环上还常插入一些磁铁阵列（wiggler和undulator，如图中存储环中左下方的淡蓝色开口向外的C形长块所示，中间一组红色块状物代表磁铁阵列）。这些阵列相当于把很多的小弯铁绑在一起发光，亮度能提高很多。说来有趣，这wiggler和undulator的发明和旧金山的街道有关 这个就留作课后作业吧。这两种插入件作为光源各有优缺点，这里就不多说了。图中每串沿存

8、储环切线方向（也是同步辐射光行进方向）的三联体的小房子就是一条束线。辐射光由此引出，经束线的光学元件（例如硅晶体单色仪，在图中各束线最接近存储环的小房间， optical hutch）滤选出想要的频率光波，再送到末端实验室（end hutch，图中中间的小房间）供用户使用。最后的小房间是用户操控室。也许有人要问了，建这么个庞然怪物得多少钱哪？不用我说大家也能猜到，钱不会少。举两个例子：我们屯儿里的这个建设费是200M；每年的维护使用要花掉 20M； APS那个建成花掉大约1B；此后每年约需要100M用于正常使用维护-不包括更新升级。说到这儿有人可能要急了，花这么多钱建这么个东西到底有啥特殊的呢

9、？为啥不能用传统的X射线管作光源呢？如前面所言，同步辐射光与传统的辐射光相比有很多优点，其中最重要的同步辐射光特别亮。下面这个图给你一个大致的概念。图中没标出来的，传统实验室的射线管最好大概能达到1010 的数量级。因为光学测量的信噪比大致与光强的平方根成正比，同步辐射光源所提供的强光允许我们做用传统X射线光源无法实现的实验。举个例子, 对于我感兴趣的生物样品，为得到一套可分析数据，在同步辐射光源需要花大约2个小时。对同一样品，如果用传统实验室的X射线管测量，要得到同样质量的数据则大概需要2年!上面的小电影里演示了几个同步辐射光源应用的例子。毫不夸张的说，同步辐射光源的应用已经渗透到科研，工业

10、及生活中的个个领域：从物理，化学，生物，天文，到医学，环境，食品，到电子，材料。第四代同步辐射光源在即。它将给我们带来怎样的光明前景呢？我们拭目以待。-Synchrotron radiationElectromagnetic radiation emitted by relativistic charged particles curving in magnetic or electric fields. With the development of electron storage rings, radiation with increasingly high flux, brightne

11、ss, and coherent power levels has become available for a wide variety of basic and applied research in biology, chemistry, and physics, as well as for applications in medicine and technology. See also Electromagnetic radiation; Particle accelerator; Relativistic electrodynamics.Electron storage ring

12、s provide radiation from the infrared through the visible, near-ultraviolet, vacuum-ultraviolet, soft-x-ray, and hard-x-ray parts of the electromagnetic spectrum extending to 100 keV and beyond. The flux photons/(second, unit bandwidth), brightness (or brilliance) flux/(unit source size, unit solid

13、angle), and coherent power (important for imaging applications and proportional to brightness) available for experiments, particularly in the vacuum-ultraviolet, soft-x-ray, and hard-x-ray parts of the spectrum, are many orders of magnitude higher than is available from other sources.The radiation h

14、as many features (natural collimation, high intensity and brightness, broad spectral bandwidth, high polarization, pulsed time structure, small source size, and high-vacuum environment) that make it ideal for a wide variety of applications in experimental science and technology. Very powerful source

15、s of synchrotron radiation in the ultraviolet and x-ray parts of the spectrum became available when high-energy physicists began operating electron synchrotrons in the 1950s. Although synchrotrons produce large amounts of radiation, their cyclic nature results in pulse-to-pulse intensity changes and

16、 variations in spectrum and source shape during each cycle. By contrast, the electron-positron storage rings developed for colliding-beam experiments starting in the 1960s offered a constant spectrum and much better stability. Beam lines were constructed on both synchrotrons and storage rings to all

17、ow the radiation produced in the bending magnets of these machines to leave the ring vacuum system and reach experimental stations. In most cases the research programs were pursued on a parasitic basis, secondary to the high-energy physics programs.Since about 1980, fully dedicated storage ring sour

18、ces have been completed in several countries. They are called second-generation facilities to distinguish them from the first-generation rings that were built for research in high-energy physics.Special magnets may be inserted into the straight sections between ring bending magnets to produce beams

19、with extended spectral range or with higher flux and brightness than is possible with the ring bending magnets. These devices, called wiggler and undulator magnets, utilize periodic transverse magnetic fields to produce transverse oscillations of the electron beam with no net deflection or displacem

20、ent. They provide another order-of-magnitude or more improvement in flux and brightness over ring bending magnets, again opening up new research opportunities. However, their potential goes well beyond their performance levels, in first- and second-generation sources.Third-generation sources are sto

21、rage rings with many straight sections for wiggler and undulator insertion device sources and with a smaller transverse size and angular divergence of the circulating electron beam. The product of the transverse size and divergence is called the emittance. The lower the electron-beam emittance, the

22、higher the photon-beam brightness and coherent power level. With smaller horizontal emittances and with straight sections that can accommodate longer undulators, third-generation rings provide two or more orders of magnitude higher brightness and coherent power level than earlier sources.One consequ

23、ence of the extraordinary brilliance of these sources is that the x-ray beam is partially coherent. By aperturing the beam, a fully coherent beam can be obtained, but at the expense of flux. Nonetheless, there is still sufficient flux remaining to explore the use and application of coherent x-ray be

24、ams. See also Coherence.Several third-generation rings are in operation. Low-energy (typically 12-GeV) third-generation rings (see illustration) are optimized to produce high-brightness radiation in the vacuum ultraviolet (VUV) and soft x-ray spectral range, up to photon energies of about 23 keV. Hi

25、gh-energy rings (typically 68 GeV) aim at harder x-rays with energies of 1020 keV and above.Layout of the 1.5-GeV Advanced Light Source at Lawrence Berkeley National Laboratory, a low-energy, third-generation synchrotron radiation source. Applications of experimental stations on beam lines are indic

26、ated.The radiation produced by an electron in circular motion at low energy (speed much less than the speed of light) is weak and rather nondirectional. At relativistic energies (speed close to the speed of light) the radiated power increases markedly, and the emission pattern is folded forward into

27、 a cone with a half-opening angle in radians given approximately by 1 = mc2/E, where mc2 is the rest-mass energy of the electron (0.51 MeV) and E is the total energy. Thus, at electron energies of the order of 1 GeV, much of the very strong radiation produced is confined to a forward cone with an in

28、stantaneous opening angle of about 1 mrad (0.06). At higher electron energies this cone is even smaller. The large amount of radiation produced combined with the natural collimation gives synchrotron radiation its intrinsic high brightness. Brightness is further enhanced by the small cross-sectional

29、 area of the electron beam, which is as low as 0.01 mm2 in the third-generation rings.The radiation was named after its discovery in a General Electric synchrotron accelerator built in 1946 and announced in May 1947 by Frank Elder, Anatole Gurewitsch, Robert Langmuir, and Herb Pollock in a letter en

30、titled Radiation from Electrons in a Synchrotron. Pollock recounts:On April 24, Langmuir and I were running the machine and as usual were trying to push the electron gun and its associated pulse transformer to the limit. Some intermittent sparking had occurred and we asked the technician to observe

31、with a mirror around the protective concrete wall. He immediately signaled to turn off the synchrotron as he saw an arc in the tube. The vacuum was still excellent, so Langmuir and I came to the end of the wall and observed. At first we thought it might be due to Cherenkov radiation, but it soon bec

32、ame clearer that we were seeing Ivanenko and Pomeranchuk radiation.M87s Energetic Jet., HST image. The blue light from the jet emerging from the bright AGN core, towards the lower right, is due to synchrotron radiation.Streaming out from the center of the galaxy M87 like a cosmic searchlight is one

33、of natures most amazing phenomena, a black-hole-powered jet of electrons and other sub-atomic particles traveling at nearly the speed of light. In this Hubble telescope image, the blue jet contrasts with the yellow glow from the combined light of billions of unseen stars and the yellow, point-like c

34、lusters of stars that make up this galaxy. Lying at the center of M87, the monstrous black hole has swallowed up matter equal to 2 billion times our Suns mass. M87 is 50 million light-years from Earth.M87s Energetic Jet. The glow is caused by synchrotron radiation, high-energy electrons spiraling al

35、ong magnetic field lines, and was first detected in 1956 by Geoffrey R. Burbidge in M87 confirming a prediction by Hannes Alfvn and Nicolai Herlofson in 1950, and Iosif S. Shklovskii in 1953.Streaming out from the center of the galaxy M87 like a cosmic searchlight is one of natures most amazing phen

36、omena, a jet of electrons and other sub-atomic particles traveling at nearly the speed of light. In this Hubble telescope image, the blue jet contrasts with the yellow glow from the combined light of billions of unseen stars and the yellow, point-like clusters of stars that make up this galaxy.=国家同步

37、辐射实验室座 落在中华人民共和国安徽省合肥市南郊，中国科学技术大学的新校园内，占地约10公顷。1989年4月建成出光，是一台专用真空紫外和软X射线、特征波长 24的同步辐射光源。主要设备包括200MeV直线加速器和一个800 MeV电子储存环。直线加速器总长35米，由电子枪、予聚焦器、聚束器和四个六米加速区段组成。总功率为70兆瓦的五只速调管向直线加速器提供微波功率。 被加速的电子经88米长输运线注入到储存环里。储存环周长66米，由弯转磁铁、四极磁铁、六极磁铁、注入系统、高频系统、超高真空系统、束流测量及控制系 统等组成。 1. 200MeV电子直线加速器 该直线加速器是一台常规的行波直线加速

38、器。它的加速结构是常阻抗、2/3模的盘荷波导结构。加速系统包括预注入器和4个加速单元。每个加速单元由两个3 米均匀加速节构成。直线加速器的总长为35米。由5个速调管提供微波功率。直线加速器位于地下隧道内，它的电子束流中心轨道所在平面比储存环的电子轨道水 平面低3.2米。200MeV电子直线加速器除作为电子储存环的注入器外，还为核物理、辐射化学、放射生物学、医学等领域的科学工作者提供能量为20- 30MeV，70-220MeV，最大束流为130mA的电子束流。为适应电子束的用户，在直线加速器后面装有开关磁铁和将电子束流引向核物理大厅 (500平方米)的束流输运线，使上述领域的科学工作者在注入期间

39、也可以同时分享电子束流进行研究。其主要参数如表1所示。 表1 直线加速器的主要参数 能量 200MeV脉冲束流 50mA 束流脉冲宽度 0.1-1s 束流脉冲重复频率 50Hz能散度 0.8%微波频率 2856.04MHz加速腔工作温度 420.2真空度 (有束流时)2*10-7Pa(无束流时)5*10-7Pa 2. 800MeV电子储存环 电子储存环是同步辐射光源的主体。它有4个周期(或2个周期)，每个周期有3块弯转磁铁和8块四极磁铁，属于TBA聚焦结构。全环有12块弯转磁铁和32 块四极磁铁，周长为66米。该环有4个3.36米的长直线节分别用于安装注入系统、高频腔和插入元件；有24个1米长的中直线节用于安装脉冲冲击磁铁、束 流诊断设备、真空测量元件等。 全环有14个六极磁铁用于校正色品，以克服束流的头尾不稳定性。每块弯铁上附有一个水平校正线圈，每个四极铁上附有一个校正线圈，它们分别用于束流轨道的水平校