Synchrotron Radiation and Synchrotron Radiation Light Source
Shanghai Synchrotron Radiation Facility, SSRF, is a third-generation of synchrotron radiation light source, and would be the invaluable tools for Chinese scientific research and industry community. Up to now, SSRF is the biggest scientific platform for science research and technology development in China, and more than hundreds of scientists and engineers from universities, institutes and industries in domestic and even overseas can do research, experiments and R&D by using SSRF each day.
The synchrotron radiation was firstly observed in synchrotron accelerator in 1947. When a circulating electron beam is deflected by the bending magnets in a storage ring, an intense flux of electromagnetic radiation, called synchrotron radiation, is produced. Relativistic effects cause this synchrotron radiation to be emitted in a narrow cone centered about the electron beam direction. Originally, synchrotron radiation was viewed primarily as an annoyance because of the need to compensate for the electron beam energy loss by using a powerful radio-frequency accelerating system. However, it is soon recognized that synchrotron radiation is useful for a wide range of physical, chemical, and biological experiments, quite unrelated to the original purpose of these electron storage ring.
Although the scientific community has taken great advantage of the synchrotron radiation produced by these first-generation light sources (originally constructed for high energy physics, such as BSRF in BEPC in Beijing), the properties of accelerators were not at all optimized for this purpose. From 70s, developed countries, such as the United States, Japan, Germany and England, have designed and constructed electron storage rings specifically dedicated to producing synchrotron radiation. Most storage ring in this so-called second-generation of synchrotron radiation light sources, such as NHLS in Hefei, having big electron beam emittance of 150nm.rad, have become operational in the recent past years. These second-generation facilities, while built expressly for the production of synchrotron radiation, were designed primarily for photon beam lines from bending magnets. Thus, their figure of merit was primarily the integrated flux of photons.
The third-generation of synchrotron radiation light sources, of which the constructed ALS, APS, SPEAR3 in the United States, Spring-8 in Japan, ESRF in Europe, BESSYII in Germany, SLS in Swiss, ELETTRA in Italy, SSRC in Taiwan of China, PLS in South Korea, CLS in Canada, the constructing DIAMOND in British, SOLEIL in France and SSRF in Shanghai, the spectral properties of the photon beams have been considerably enhanced firstly by much smaller beam emittance of 3~20 nm.rad, secondly by utilizing special magnetic insertion devices called wiggler and undulators that are placed in the straight sections of the storage ring. And another twelve third-generation of synchrotron light sources are under design. To the end of year 2010, we expect that more than ten thousands of scientists and engineers will use synchrotron radiation light sources do research each day in the world.
The advanced performance and international position of SSRF
1) The advance performance of SSRF
The high cost performance ratio: The energy of storage ring is 3.5GeV, which is the highest value in the medium-energy light source. Its performance is optimized in the wide used X-ray energy region. By using advanced technology of insertion devices, not only high brilliance synchrotron radiation light with 1~5keV photon energy can be generated, but also high brilliance hard X-ray with 5~20 keV photon energy can be generated, which have the similar properties as which is generated by high-energy light source (6~8GeV).
Full waveband: The wide wavelength range and continuous adjustment, which is from far-infrared to the hard X-ray. We can discover the unknown parts of science that other light source can not applied by using homochromatic light at different wavelength.
High intensity: The total power is 600 KW, which is tens of thousands times higher than that of X-tube. The light flux is more than 1015 photon/(S.10-3bw). The high intensity and flux lead to the possibility of reducing the time of obtaining the experimental datum, hard controlling experiment and the application of the medicine, technology.
High brilliance: Its brilliance is hundreds of millions of times higher than that of X-tube. The light brilliance of main spectra region is 1017-1020 photon/(S.mm2.mrad2.10-3bw). It offers the high spatial resolution and high momentum resolution and fast time resolution condition of gain the out-breaking achievement.
The excellent structure of pulse: The pulse width is only some Ps, it can work with single bunch and multiple bunch modes. The two close pulse interval reaches the range of ns and ms, so that it can offer the credible data for the process of chemical dynamic, live, material and the pollution of the atmosphere.
High polarization: The synchrotron light in the electron orbit plane of SSRF is full polarized, and the light off the electron orbit plane is elliptical polarization, meanwhile it is good tools to research the optical activity biological molecule, medical molecule and dicromatism magnetic material.
Quasi-coherence: The high brilliance light from the SSRF insertion device has partial coherence. It lead a new way for microscope holographic imaging in many advanced subjects.
High stability: It can apply stable beam and last many hours. The beam position stability is about 10% of the facula.
High efficiency: About more than 60 beamlines and more than hundreds experimental station will be put up. Therefore it will offer the consumer more than 5000 hour/year. Everyday, it contact hundreds scientists and engineers come from the different fields and they can use the synchronic radiation light day and light.
Flexibility: SSRF can run as single bunch, multiple bunch, high flux, high luminosity, narrow pulse modes and so on. It also can change the running mode according to the consumer¡¯s demands, so that it can satisfy the different demands.
Forward Looking: It has advanced scientific aim of the first beamline stations. It will satisfied the demands in many scientific fields and scientific lifetime larger than 30 years.
2) Compare and standing in the world after construction
With the electron beam energy of 3.5GeV, just lower than APS, ESRF and SPring-8, storage ring emittance of 4 nm.rad, SSRF will be the fourth energy high in the world. Its performance is better than the existing third generation of synchrotron radiation light source with the same energy region, and it is also one of the best light sources in the world.
The size and cost of SSRF is accord with the situation of our country, and have a good cost performance ratio in a wide photon energy region. Its brilliance in hard X-ray region (5~20keV) is close to 6~8 GeV light source, which is very large and expensive. And from 1keV to 5keV photon energy region, its brilliance is on the top of the world.
Comparable with the third generation of synchrotron radiation light sources in the United States and Europe, SSRF, together with Spring-8 (8GeV) in Japan, PLS in South Korea, TLS in Taiwan of China and Indus-II in India, will become the world-class synchrotron radiation light source platform, and cover all the photon energies from low to high energy in Asia.
Scientific lifetime larger than 30 years.
Target and challenge of SSRF
1). Target of SSRF
As a third generation of synchrotron light source, SSRF¡¯s electron energy is 3.5Gev, which is fourth in the world, only next to Spring-8 of Japan (8GeV), APS of America (7GeV) and ESRF of European Community (6GeV). SSRF consist of a 150MeV LINAC, a booster that can increase the electron energy from 150MeV to 3.5GeV in 0.5 second, and a 3.5GeV electron storage ring. The 20 cell, 4 fold structure is used to store an electron beam (average current 300mA) with low emittance (minimum 4 nmrad) and long lifetime ( >10 hours). By using advanced insertion device, synchrotron radiation light with high flux and high brilliance will be produced, and the range of the photon energy is from 0.1 to 40keV, which have the call of the users. The brightness of the photon is higher than the 19th power of 10. There are 40 bending magnets, 16 standard straights (6.5 meters) and 4 long straights (12 meters) along the ring, so more than 60 beamlines could be installed in the ring, where 26 of them will be based on insertion devices(by installing two mini-gap undulators for several straights), 36 lines are based on bending magnets and several infrared beamlines are available too. SSRF will also include 7 initial beamlines and experimental stations. These 7 beamlines are used for macromolecular crystallography, XAFS, hard X-ray microfocus, X-ray imaging and biomedical application, soft X-ray spectromicroscopy, diffraction and small angle X-ray scattering respectively. The former five beamlines are based on insertion devices, and other two are based on bending magnets.
SSRF is an extremely complex project with many subsystems, most of them are dealt with advanced technology, such as superconductive RF and cryogenic, ultra-high vacuum, ultra-high precision digital power supply, high performance magnets and mechanical collimation, beam diagnosis, advanced control system, and advanced beamline, etc. The difficulty of the system development and integration is very high, especially how to keep the fault rate very low on the premise of ensuring all system¡¯s performance, in order to achieve the targets which can store beam for several tens of hours and provide synchrotron radiation light for more than 5000 hours per year.
Small emittance is necessary for the demand of high brilliance. The horizontal emittance of SSRF is about 4 nmrad, the beam size at the light source point is only about 150¦Ìm horizontally and 10¦Ìm vertically. But low emittance requirement will make the ring¡¯s dynamic aperture very small, then several beam instability will be introduced in, and the beam lifetime will be shorten too. So it must be studied how to optimize the dynamic property of the light source.
To make the beam stable, the vertical orbit stability must be less than 1¦Ìm, this is one of the difficulties of SSRF. Lots of methods are used to satisfy this demand, such as to control the sedimentation of the groundsill, distortion of the tunnel and floor strictly, to restrict the temperature variety of air and water, to monitor and control all vibration source, to optimize the mechanical structure of the device, to close off and damp the vibration, to increase the stability of power supply, and to use orbit feedback system, etc.