閑話同步輻射光源

1、閑話同步輻射光源像科學(xué)史上上演的許多故事一樣， 同步輻射光源的發(fā)現(xiàn)是又一個典型的要趕鷹卻撿只肥兔子 回來的故事。要把其中的來龍去脈講清楚，咱們得向上追溯一個多世紀(jì)。話說 1895 年倫琴 （R?ntgen） 觀測到陰極電子加速運(yùn)動撞擊到陽極的過程中會釋放一種能夠穿過不透明物質(zhì) 并誘發(fā)熒光物質(zhì)發(fā)光的神秘射線。他暫時命名這種射線為 X 射線，后人為紀(jì)念他亦稱其為 倫琴射線。他本人因此在 1901 年捧回史上第一個物理學(xué)諾貝爾獎。倫琴臨終時囑托將其所 有的私人信件及科研手稿焚毀，所以后來有人質(zhì)疑他是否確實(shí)是 X 射線的最初發(fā)現(xiàn)人。當(dāng) 然這是史學(xué)家的事。我們這里關(guān)心的是 X 射線本身。它不僅僅引導(dǎo)人們

2、第一次進(jìn)入了玄妙 的原子分子層次的微觀世界， 從而翻開近代物理學(xué)的新篇章， 同時也向人類展示了它在工業(yè) 和醫(yī)療等實(shí)用領(lǐng)域中的巨大作用。 X 射線的神奇魅力引無數(shù)英雄竟折腰。 人們開始尋求開發(fā) X 射線源。直到現(xiàn)在通常實(shí)驗(yàn)室中用的 X 光源還是基于電子撞擊陽極的產(chǎn)生機(jī)理，即運(yùn)動 電子在撞擊過程中急劇減速而釋放輻射 （韌致輻射或剎車輻射） 及由靶材料原子能級決定的 特征 X 光熒光譜線。 這里的極其關(guān)鍵一步是讓電子生變 （速度）。變則明光環(huán)繞百媚生 - 看 來這是一條從人類社會到自然顛簸不破的普遍真理：-）。由于電子撞擊釋放輻射過程是在固體里進(jìn)行，相當(dāng)多的能量會被最后轉(zhuǎn)化為熱能（低能輻射，也是光）

3、 。受限于陽極靶的散熱 能力，這些 X 光機(jī)能產(chǎn)生的 X 光的通量是很有限的。直到同步輻射光源的出現(xiàn)才繞過了這 個難題。上世紀(jì)早期人們在研究電子回旋加速器的過程中遇到了一個比較頭疼的問題： 電子在加速過 程中會輻射電磁波造成能量損失。 這里輻射產(chǎn)生的實(shí)際上就是同步輻射光。 可惜囿于知識不 足，同步輻射光猶如一塊尚未拋光的璞玉， 未被慧眼相識。 實(shí)際上人們當(dāng)時是把它當(dāng)作可惡 的攔路石， 一心想把它挪走： 因?yàn)檫@種輻射效應(yīng)使加速器的效率降低， 最終給高能電子的產(chǎn) 生設(shè)定了一個上限。1947年4月24日，通用電器（GE）的四位科學(xué)家 Frank Elder, Anatole Gurewitsch,

4、Robert Langmuir, 及 Herb Pollock 在他們新建的 70MeV 的加速器上嘗試一種新 的加速手段。 這個機(jī)器的設(shè)計(jì)能讓人看到電子運(yùn)行的軌道。 實(shí)驗(yàn)剛剛開始， 一個實(shí)驗(yàn)員大喊 叫停。 原來他在電子管中觀察到耀眼的淡藍(lán)色弧形光。大家趕緊跑過來查看，真空正常，不 是電子管受損發(fā)出的光。 很快幾位物理學(xué)家意識到他們觀察到的是同步輻射光。 這是科學(xué)家 世上第一次直接觀察到非自然同步輻射光。 這種光是電子在真空中加速運(yùn)動中產(chǎn)生的， 它有 許多誘人的特點(diǎn)：高光通量（注意沒有伴隨的陽極熱量） ，連續(xù)光譜（通常 X 光管子多是用 熒光分立譜線以求相對高的通量） ，極高的準(zhǔn)直性（這和下面

5、要提到的電子相對論速度運(yùn)動 相關(guān)聯(lián)），最后還有它的時間脈沖特性。經(jīng)過一系列的研究嘗試，人們見識到美玉真顏, 于是不用那種加速器做高能物理了，專門造它們當(dāng)超大的 X 光機(jī)。六十年代開始科學(xué)家建設(shè) 同步輻射光源。至今為止同步輻射光源發(fā)展歷經(jīng)3代。第4代（X光激光）正在籌建中。世界上大約有 22 個國家和地區(qū)共建有（或正在籌建中） 40 多個大大小小的光源中心。其中美 國有近 10 個，其他大部分在歐洲和日本，中國大陸 3 個，臺灣 1 個（第二個剛開始建） ，加 拿大 1 個，。約旦和泰國也都在建造。 光源中心小的只有一個卡車大小； 大的有個把公里。為方便起見， 咱們還是用下面的靜態(tài)圖聊聊同步輻射

6、光是如何產(chǎn)生的。 這圖有點(diǎn)小毛病， 看 看這里的物理大俠們能不能找出來。圖中間與一個小環(huán)相連的直的通道是直線加速器（ LINAC ）。當(dāng)電子由電子槍發(fā)射出來注入 直線加速器加速電子， 并把連續(xù)的電子束切成隔約幾個到幾百個納秒的等間隔電子團(tuán)。然后電子被引導(dǎo)到助動加速環(huán)（booster ring,圖中與直線加速器相接的小環(huán)）進(jìn)一步被加速。當(dāng) 電子速度達(dá)到差不多是光速的設(shè)定值時（例如屯兒里的光源 CLS電子能量2.9GeV ；電子速度是 99.999998% 真空光速；芝加哥的 APS, 7GeV, 99.999999% 真空光速）,電子被注入 圖中大環(huán)，也就是存儲環(huán)（Storage ring），在

7、存儲環(huán)中，電子軌道將被偶極磁鐵（圖中拐彎 處紅色 C 型塊，也稱彎曲磁鐵）彎曲從而保持勻速率繞環(huán)運(yùn)動。彎曲電子軌道也就意味著 產(chǎn)生向心加速度。 前面提過, 加速運(yùn)動的帶電粒子是要釋放輻射的, 當(dāng)粒子以相對論速度 （即 快的跟光速有一比 ）運(yùn)動時它的輻射會聚集在其運(yùn)動方向,這就是同步輻射光的來源。電子 做圓周運(yùn)動, X 光從圓的切線方向散發(fā)出來。 電子損失的能量由存儲環(huán)中高頻腔 （ RF cavity） 補(bǔ)充,這個過程得掌握好時機(jī),得 同步 才行,同步輻射光源也因而得名。除了彎曲形電磁 鐵外，存儲環(huán)上還常插入一些磁鐵陣列（ wiggler和undulator，如圖中存儲環(huán)中左下方的淡 藍(lán)色開口向

8、外的 C 形長塊所示,中間一組紅色塊狀物代表磁鐵陣列） 。這些陣列相當(dāng)于把很 多的小彎鐵綁在一起發(fā)光,亮度能提高很多。說來有趣,這 wiggler 和 undulator 的發(fā)明和舊金山的街道有關(guān)-這個就留作課后作業(yè)吧。這兩種插入件作為光源各有優(yōu)缺點(diǎn)，這里就不多說了。 圖中每串沿存儲環(huán)切線方向 （也是同步輻射光行進(jìn)方向） 的三聯(lián)體的小房子就是一 條束線。輻射光由此引出,經(jīng)束線的光學(xué)元件 （例如硅晶體單色儀,在圖中各束線最接近存 儲環(huán)的小房間，optical hutch ）濾選出想要的頻率光波，再送到末端實(shí)驗(yàn)室（end hutch，圖中中間的小房間）供用戶使用。最后的小房間是用戶操控室。也許有人

9、要問了, 建這么個龐然怪物得多少錢哪？不用我說大家也能猜到,錢不會少。 舉兩個例子：我們屯兒里的這個建設(shè)費(fèi)是200M ;每年的維護(hù)使用要花掉20M ; APS那個建成花掉大約1B ;此后每年約需要100M用于正常使用維護(hù)-不包括更新升級。說到這兒有人可 能要急了,花這么多錢建這么個東西到底有啥特殊的呢？為啥不能用傳統(tǒng)的X 射線管作光源呢？如前面所言, 同步輻射光與傳統(tǒng)的輻射光相比有很多優(yōu)點(diǎn), 其中最重要的同步輻射光 特別亮。 下面這個圖給你一個大致的概念。 圖中沒標(biāo)出來的, 傳統(tǒng)實(shí)驗(yàn)室的射線管最好大概 能達(dá)到10X0的數(shù)量級。因?yàn)楣鈱W(xué)測量的信噪比大致與光強(qiáng)的平方根成正比, 同步輻射光源所提供的

10、強(qiáng)光允許我們做 用傳統(tǒng) X 射線光源無法實(shí)現(xiàn)的實(shí)驗(yàn)。 舉個例子 , 對于我感興趣的生物樣品, 為得到一套可分 析數(shù)據(jù),在同步輻射光源需要花大約 2 個小時。對同一樣品,如果用傳統(tǒng)實(shí)驗(yàn)室的 X 射線 管測量,要得到同樣質(zhì)量的數(shù)據(jù)則大概需要2年!上面的小電影里演示了幾個同步輻射光源應(yīng)用的例子。 毫不夸張的說, 同步輻射光源的應(yīng)用 已經(jīng)滲透到科研,工業(yè)及生活中的個個領(lǐng)域：從物理,化學(xué),生物,天文,到醫(yī)學(xué),環(huán)境, 食品,到電子,材料。 。 第四代同步輻射光源在即。它將給我們帶來怎樣的光明前景呢？我們拭目以待。Synchrotron radiationElectromagnetic radiation

11、emitted by relativistic charged particles curving in magnetic or electric fields. With the development of electron storage rings, radiation with increasingly high flux, brightness, and coherent power levels has become available for a wide variety of basic and applied research in biology, chemistry,

12、and physics, as well as for applications in medicine and technology. See also Electromagnetic radiation; Particle accelerator; Relativistic electrodynamics.Electron storage rings provide radiation from the infrared through the visible, near-ultraviolet, vacuum-ultraviolet, soft-x-ray, and hard-x-ray

13、 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 angle), and coherent power (important for imaging applications and proportional to brightness) available for experiments, p

14、articularly 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 has many features (natural collimation, high intensity and brightness, broad spectral bandwidth, high polarization, pulsed t

15、ime 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 sources of synchrotron radiation in the ultraviolet and x-ray parts of the spectrum became available when high-energy physicists

16、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 variations in spectrum and source shape during each cycle. By contrast, the electron-positron storage rings developed for

17、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 allow the radiation produced in the bending magnets of these machines to leave the ring vacuum system and reach experimental s

18、tations. 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 sources have been completed in several countries. They are called second-generation facilities to distinguish them from the fir

19、st-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 with extended spectral range or with higher flux and brightness than is possible with the ring bending magnets. These devic

20、es, called wiggler and undulator magnets, utilize periodic transverse magnetic fields to produce transverse oscillations of the electron beam with no net deflection or displacement. They provide another order-of-magnitude or more improvement in flux and brightness over ring bending magnets, again op

21、ening up new research opportunities. However, their potential goes well beyond their performance levels, in first- and second-generation sources.Third-generation sources are storage rings with many straight sections for wiggler and undulator insertion device sources and with a smaller transverse siz

22、e 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 higher the photon-beam brightness and coherent power level. With smaller horizontal emittances and with straight sections t

23、hat can accommodate longer undulators, third-generation rings provide two or more orders of magnitude higher brightness and coherent power level than earlier sources.One consequence of the extraordinary brilliance of these sources is that the x-ray beam is partiallycohere nt. By aperturi ng the beam

24、, a fully cohere nt beam can be obta in ed, but at the expe nse of flux. Nonetheless, there is still sufficient flux remaining to explore the use and application of cohere nt x-ray beams. See also Cohere nee.Several third-ge nerati on rings are in operati on. Low-e nergy (typically 1-2-GeV) third-ge

25、 nerati onrings (see illustrati on) are optimized to produce high-bright ness radiati on in the vacuum ultraviolet(VUV) and soft x-ray spectral ran ge, up to phot on en ergies of about 2-3 keV. High-e nergy rings(typically 6 GeV) aim at harder x-rays with energies of 10 -20 keV and above.urfm undmet

26、rology andx-ray rmcrotem廠r ZAynGhrmron Aatonite and nrAterlalsand surfa-cfl scltncaahNn忙 nd molKularbflim linradio-frqjenEyfield*rnatvriBl* Ei*Fic4 nti biology.m- rayX-rty mkraprotw、_iray apllu deYBlopmeatLayout of the 1.5-GeV Advaneed Light Source at Lawrenee Berkeley National Laboratory, a low-e n

27、ergy, third-ge neratio n syn chrotro n radiati on source. Applicati ons of experime ntal stati ons on beam lines are in dicated.The radiati on produced by an electro n in circular moti on at low en ergy (speed much less tha n the speed of light) is weak and rather non directi on al. At relativistic

28、en ergies (speed close to the speed of light) the radiated power in creases markedly, and the emissi on pattern is folded forward into a cone with a half-opening angle in radians given approximately by 丫 -1 = mc2/E, where mc2 is the rest-mass en ergy of the electro n (0.51 MeV) and E is the total en

29、 ergy. Thus, at electro n en ergies of the order of 1 GeV, much of the very strong radiation produced is confined to a forward cone with an in sta ntan eous ope ning an gle of about 1 mrad (0.06). At higher electr on en ergies this cone iseven smaller. The large amount of radiati on produced comb in

30、ed with the n atural collimati on gives synchrotron radiation its intrinsic high brightness. Brightness is further enhanced by the small cross-sectional area of the electron beam, which is as low as 0.01 mm2 in the third-generation rin gs.The radiati on was n amed after its discovery in a Gen eral E

31、lectric syn chrotron accelerator built in 1946 and announced in May 1947 by Frank Elder, Anatole Gurewitsch, Robert Langmuir, and Herb Pollock in a letter en titled Radiati on from Electr ons in a Syn chrotr on. Pollock reco un ts:On April 24, Lan gmuir and I were running the mach ine and as usual w

32、ere trying to push the electron gun and its associated pulse transformer to the limit. Some intermittent sparking had occurred and we asked the tech ni cia n to observe with a mirror around the protective con crete wall. He immediately sig naled to turn off the syn chrotr on as he saw an arc in the

33、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 Chere nkov radiati on, but it soon became clearer that we were see ing Ivanenko and Pomera nchuk radiatio n.M87s Energetic Jet., HST image. The blue light from the

34、 jet emerging from the bright AGN core, towards the lower right, is due to syn chrotr on radiati on.Stream ing out from the cen ter of the galaxy M87 like a cosmic searchlight is one of n atures most amazing phenomena, a black-hole-powered jet of electrons and other sub-atomic particles traveli ng a

35、t n early the speed of light. I n this Hubble telescope image, the blue jet con trasts 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. Lying at the cen ter of M87, the mon strous black hole has swallowed

36、up matter equal to 2 billion times our Suns mass. M87 is 50 million light-years from Earth.Chwnd 舊 X 車 ayM87s En ergetic Jet. The glow is caused by syn chrotron radiati on, high-e nergy electr ons spirali ng along magnetic field lines, and was first detected in 1956 by Geoffrey R. Burbidge in M87 co

37、n firm ing a predict ion by Hannes Alfv n and Nicolai Herlofs on in 1950, and Iosif S. Shklovskii in 1953.Stream ing out from the cen ter of the galaxy M87 like a cosmic searchlight is one of n atures most amaz ing phe nomena, a jet of electr ons and other sub-atomic particles traveli ng at n early

38、the speed of light. In this Hubble telescope image, the blue jet contrasts with the yellow glow from the comb ined light of billi ons of un see n stars and the yellow, poin t-like clusters of stars that make up this galaxy.國家同步輻射實(shí)驗(yàn)室 座 落在中華人民共和國安徽省合肥市南郊，中國科學(xué)技術(shù)大學(xué)的新校園內(nèi)，占地約10公頃。1989年4月建成出光，是一臺專用真空紫外和軟X射

39、線、特征波長24?的同步輻射光源。主要設(shè)備包括200MeV直線加速器和一個 800 MeV電子儲存環(huán)。直線加速器總長35米，由電子槍、予聚焦器、聚束器和四個六米加速區(qū)段組成。總功率為70兆瓦的五只速調(diào)管向直線加速器提供微波功率。被加速的電子經(jīng) 88米長輸運(yùn)線注入到儲存環(huán)里。儲存環(huán)周長 66米，由彎轉(zhuǎn)磁鐵、四極磁鐵、六極磁鐵、注入系統(tǒng)、高頻系統(tǒng)、超高1.200MeV電子直線加速器該直線加速器是一臺常規(guī)的行波直線加速器。它的加速結(jié)構(gòu)是常阻抗、2 n /3莫的盤荷波導(dǎo)結(jié)構(gòu)。加速系統(tǒng)包括預(yù)注入器和4個加速單元。每個加速單元由兩個3米均勻加速節(jié)構(gòu)成。直線加速器的總長為 35米。由5個速調(diào)管提供微波功率。

40、直線加速器位于地下隧道內(nèi)，它 的電子束流中心軌道所在平面比儲存環(huán)的電子軌道水平面低3.2米。200MeV電子直線加速器除作為電子儲存環(huán)的注入器外，還為核物理、輻射化學(xué)、放射生物學(xué)、醫(yī)學(xué)等領(lǐng)域的科學(xué)工作者提供能量為 20- 30MeV，70-220MeV，最大束流為130mA的電子束流。為適應(yīng)電 子束的用戶，在直線加速器后面裝有開關(guān)磁鐵和將電子束流引向核物理大廳（500平方米）的束流輸運(yùn)線，使上述領(lǐng)域的科學(xué)工作者在注入期間也可以同時分享電子束流進(jìn)行研究。其主要參數(shù)如表1所示。表1直線加速器的主要參數(shù)能量200MeV脈沖束流50mA束流脈沖寬度0.1-1s束流脈沖重復(fù)頻率50Hz能散度0.8%微波

41、頻率2856.04MHz加速腔工作溫度42 C )2C真空度（有束流時）2*10-7Pa（無束流時）5*10-7Pa2. 800MeV電子儲存環(huán)電子儲存環(huán)是同步輻射光源的主體。它有4個周期（或2個周期），每個周期有3塊彎轉(zhuǎn)磁鐵和8塊四極磁鐵，屬于 TBA聚焦結(jié)構(gòu)。全環(huán)有12塊彎轉(zhuǎn)磁鐵和32塊四極磁鐵，周長為66米。該環(huán)有4個3.36米的長直線節(jié)分別用于安裝注入系統(tǒng)、高頻腔和插入元件；有24個1米長的中直線節(jié)用于安裝脈沖沖擊磁鐵、束流診斷設(shè)備、真空測量元件等。全環(huán)有14個六極磁鐵用于校正色品，以克服束流的頭尾不穩(wěn)定性。每塊彎鐵上附有一個水平校正線圈，每個四極鐵上附有一個校正線圈，它們分別用于束流軌道的水平校正和垂直校正。儲存環(huán)的真空室是用無磁不銹鋼鋼板焊接而