Synchrotron radiation refers to relativistic high-speed motioncharged particleOutmagnetic fieldWhen moving along a curve under the action oftangentDirectionalelectromagnetic radiation。According to electrodynamics, charged particles will emit electromagnetic radiation when moving at a variable speed, including bremsstrahlung before moving due to the change of speed in linear motion, and synchrotron radiation in tangential motion due to the change of motion direction in curved motion.Synchrotron radiation was initiallyElectron synchrotronIt is named after being observed on the.Synchrotron radiation light source has many advantages that conventional light sources do not have, such as wide spectrum range, high spectral brightness, high photon flux, high collimation, high polarization, and pulse time structure. It makes some experiments that conventional light sources cannot do possible.[1-2]
The earliest theoretical research on synchrotron radiation can be traced back to the late 19th century. This unnamed radiation first attracted widespread attention whenrutherfordAfter putting forward the nuclear structure model of the atom.In classical electromagnetic theory, the electronCoulomb forceRotating around the atomic nucleus, high-speed electrons must lose energy due to synchrotron radiation, so their orbits will shrink until they collide with the atomic nucleus, unable to maintain the stability of the atomic structure.This question remained until modern timesquantum mechanicsThe birth was explained.
In experiments, synchrotron radiation was not discovered as a by-product of particle physics experiments until the 1940s and 1950s.In 1947, a 70 MeVSynchronous cyclotronSynchrotron radiation was observed for the first time on.At that time, synchrotron radiation was only accepted by particle physics researchers as an inevitable negative product: in order to produce new particles and observe new phenomena in the micro world, the progress of particle physics science needed to build larger accelerators to accelerate particles to higher energy, but synchrotron radiation would inevitably lead to the loss of particle energy,And when the particle energy is higher, the energy loss caused by synchrotron radiation is greater (proportional to the fourth power of relativistic moving particle energy).In that year, researchers related to accelerators calculated that the energy limit that particles can reach after considering synchrotron radiation is 500 MeV.Fortunately, accelerator physicists soon proposed a new principle of synchrotron, breaking this energy limit.
The first generation synchrotron radiation light source
Contrary to the need of particle physics researchers to reduce synchrotron radiation to increase particle energy, other researchers hope to optimize synchrotron radiation light sources to use this radiation to promote non nuclear physics research.The application research of synchrotron radiation originated from the absorption spectrum. Since the 1960s, researchers have carried out a lot of research on electron synchrotron around the worldVacuum ultraviolet(VUV) toSoft X-rayThe research of (SX) band absorption spectroscopy experiment has obtained many exciting experimental results, which has started the first wave of upsurge of synchrotron radiation application research.Until today, synchrotron radiation is still the strongest continuous light source from vacuum ultraviolet to soft X-ray.
At this stage, synchrotron radiation research is the application of the negative products of high-energy physics experiments. Researchers have no equipment of their own, and can only "parasitize" on the accelerator of particle physicists for research.This kind of dual-use light source based on high-energy physics experiments is called the first generation synchrotron radiation light source, which can run on the storage ring or synchrotron. "dual-use" includes simultaneous (synchrotron radiation parasitic operation) or time-sharing (with synchrotron radiation dedicated time).Most of them were built in 1965-1975, and many of them were storage rings that were already in operation or under construction and were originally used as positron colliders. Due to the development of the situation, the function of providing synchrotron radiation was added.Because the requirements of synchrotron radiation experiment are not consistent with those of high-energy physics experiment, the performance and time of the first generation synchrotron radiation light source applied to synchrotron radiation research are limited.[2]
ChineseBeijing Synchrotron Radiation Facility(Beijing Synchrotron Radiation Facility, BSRF)Beijing Electron Positron ColliderThe first generation synchrotron radiation light source, running at 2.5 GeV, has 14 beam lines and 15 experimental stations, covering a wide range of wavebands, from vacuum ultraviolet to hard X-ray.Because it is a dual-use light source, there are only about 2000 hours of synchrotron radiation dedicated machine hours per year.[4]
Schematic Diagram of Beam Line and Experimental Station of Beijing Synchrotron Radiation Facility (BSRF)[4]
Second generation synchrotron radiation light source
In the 1960s, in order to improveIncident particleandTarget particleThe energy of interaction is used to generate new particles and explore new phenomena in the micro world. High energy physics researchers proposed the idea of replacing incident electrons with high-energy incident particle beams and target particle beams to bombard fixed targets. The feasibility of this idea was confirmed with the completion of the storage ring.[3]
The storage ring is a special round accelerator, which can store particles and maintain a near light speed cycle for several hours.The storage ring mainly includes electronic sourcelinear accelerator（linac）、synchrotron(booster synchrotron), main ring and beamline and experience.The electron is emitted by the electron source, and passes throughlinear acceleratorAcceleration andsynchrotronAfter acceleration, it is transferred to the main ring.On the main ring, these particles are accelerated to close to the speed of light. In order to keep the electrons in the closed orbit in the storage ring, powerful bending and focusing magnets are installed on the path of the ring. When the electrons pass through these bending magnets, they can produce synchrotron radiation.[5]
Schematic diagram of storage ring[6]
Storage ringIt provides fairly stable electron beam current, adjustable and controllable synchronization of various frequency bandsSpectral distributionAnd the working environment of ultra-high vacuum makes the construction of special synchrotron radiation light source possible.In addition, due to the rapid growth of synchrotron radiation user groups in various disciplines, the first generation "parasitic" synchrotron radiation light source is not enough to meet their demand for machine time.In 1968, two electronic storage rings dedicated to synchrotron radiation research were put into operation, one was "Tantalus" of the University of Wisconsin in the United States, and the other was the Synchrotron Orbital Radiation ring of the University of Tokyo in Japan, which symbolized thatSynchrotron radiation light sourceThe end of the initial stage and the beginning of the great development stage.This special synchrotron radiation facility based on the storage ring is called the second generation synchrotron radiation light source. Their optimization goal in design is to make full use of synchrotron radiation. Generally, they have smaller beam cross-section, higher current intensity, and can install more beam lines and experimental stations. Most of them were built in 1975-1990.[3]
Hefei, ChinaNational Synchrotron Radiation Laboratory(National Synchrotron Radiation Laboratory, NSRL) is the second generation synchrotron radiation light source, which operates at 800 MeV. Its superior energy range is vacuum ultraviolet and soft X-ray bands. Its annual operation time is more than 7000 hours, and the power on rate is better than 99%, providing users at home and abroad with more than 40000 hours of high-quality machine hours.[7]
The third generation synchrotron radiation light source
In the second generation synchrotron radiation light source, synchrotron radiation occurs when charged particles pass through the bent iron on the main ring. It extends from the tangent of the electron track outward in a narrow radiation cone, and the angle is about the ratio of the static energy of the electron to its moving energy, that is
The radiation spectrum of bent iron is very wide.The third generation of synchrotron radiation light source obtains synchrotron light with very low emittance, long-term stable beam, and excellent polarization and coherence through the use of inserts.
Insertion devices are a kind of magnetic parts, which are composed of periodic magnet arrays with alternating polarity. When electrons pass through the swing device, they will change their tracks at each magnet, so that they can vibrate.In each oscillation cycle, the electron emits radiation when it deflects in the direction of motion. After the radiation superposition of each cycle, the total radiation emits along the direction of the oscillating device, thus increasing the intensity of radiation.Wigglers and Undulators are the two main insertion devices used. They are only different in field strength and have the same working principle.Among them, the wiggler has a higher magnetic field, which will get synchrotron radiation with larger angle and lower brightness, while the undulator has a smaller magnetic field, which will get quasi monochromatic radiation with smaller angle and high brightness.[5]
Schematic diagram of bending iron, wiggler and undulator[5]
The emergence of the third generation of light sources set off the second wave of building "new light sources", which started in the 1990s and extended to the early 21st century.At the beginning of the second wave of boom, the synchrotron radiation community recognized that the electronic energy
The synchrotron radiation light source can be divided into two categories,
GeV's high-energy light source (hard X-ray source) and
GeV's medium and low energy light sources (vacuum ultraviolet and soft X-ray light sources) have their own division of labor and complement each other to ensure the effective and reasonable use of synchrotron radiation research capabilities.[3]
X-ray brightness increases with the construction of synchronous light sources, about three orders of magnitude every ten years[3]
Taiwan Light Source Beamlines (TLS), Taiwan Photon Source (TPS) andShanghai Synchrotron Radiation Light Source(Shanghai Synchrotron Radiation Facility, SSRF) are the third-generation synchrotron radiation light sources.Among them, Shanghai Light Source, which was built in 2009, is a 3.5 GeV intermediate energy third-generation synchrotron radiation device with a wide wavelength range, high intensity, high brightness, high collimation, high polarization and quasi coherence, accurate calculation, high stability and a series of excellent characteristics.[8]Taiwan Light Source TLS was put into operation in 1994, with an electron beam energy of 1.5 GeV, and TPS was built in 2015. It is a more advanced 3 GeV third-generation synchrotron radiation light source, with a brightness of 1021 phs/mm2/mrad2/s/0.1% BW, known as the "brightest 3 GeV synchrotron radiation light source in Asia".[2]
The fourth generation synchrotron radiation light source is characterized by extremely high peak brightness and high coherence, and haspulseExtremely short, high average power, etc. Possible options include: short wavelengthFree electron laser(FEL), the diffraction limited storage ring (DLSR), energy recovery linacs (ERL) and infrared coherent light sources.Its excellent brightness, energy and coherence enable a series of new experimental methods such as X-ray coherent imaging to be realized, which provides important support for cutting-edge research in various disciplines.[9]Three fourth generation synchrotron radiation light sources have been built in the world, and nearly 10 more are expected to be built in the next five years.[9]
HEPS is the first high-energy synchrotron radiation light source in China. The electron beam energy in its storage ring accelerator is 6 GeV, which is the first high-energy synchrotron radiation device in China.By reasonably optimizing the design of the insert, it can produce synchrotron radiation with a brightness higher than 1 × 1022 phs/(mm2 · mrad2 · 0.1% BW), the world's highest brightness, and an energy up to 300 keV, which is more than 100 times higher than the brightness of SSRF.After a series of precision optical systems, such as high-precision bending, monochromator, focusing mirror, etc. on the beam line, such as beam splitting, collimation, focusing, and other reprocessing, HEPS can provide synchronous light with nm spatial resolution, ps temporal resolution, and meV energy resolution.[2]
Today, synchrotron radiation has become the largest number of online large-scale devices. There are more than 50 light sources under construction or operation in the world, distributed in 23 countries and regions.[2]
Synchrotron radiation light sources in the world today[10]
For the sake of simplicity, we will only discuss the electromagnetic field in vacuum.In classical electrodynamics, the vector potential is definedAAnd scalar potential φ to describe the electromagnetic field, and vector potential is introduced from the passivity of the magnetic fieldA：
Substitute it intoLorentz equationGroup, the expression of the changing electric field is obtained. Different from the electrostatic field, the magnetic induction term is added to the electric field:
Because in the definition, onlyAThe curl ofAThere is no constraint on the divergence of, and only the definition of curl is not enough to determine the vector field. Therefore, we use the standard conditions to determineAThe divergence of.When dealing with radiation problems, the Lorentz specification is generally used:
Substituting it into Lorenz equations, we can get the d'Alembert equation:
This is a non-homogeneous wave equation, and its free terms are current density and charge density, indicating that the charge will produce scalar potential fluctuations, while the current will produce vector potential fluctuations.After leaving the distribution area of current and charge, vectors and scalars will propagate in space in the form of waves, and the electric and magnetic fields derived from them will also propagate in space in the form of waves.It should be noted that the distribution of electric and magnetic fields is independent of the selection of specifications.
Substitute the distribution of charge and current into the d'Alembert equation to solve the electromagnetic field, and obtain the general time varying charge distribution
The excited scalar potential is:
Where r is the luminous point
To observation point
Distance.Similarly, the vector potential distribution excited by the changing current is:
Describe a point in space
The electromagnetic field at time t changes from the earlier time
At position
The distribution of the charge current at the location determines that the physical action generated by the charge cannot be transmitted to the field point immediately, and the electromagnetic action requires a certain propagation time.This vector potential and scalar potential are called retarded potential.
Schematic Diagram of Delay Potential[11]
Replacing the calculated retarded potential back to its definition, the distribution of electric and magnetic fields generated by moving charges can be obtained.In the simplest case, the electromagnetic field generated by a single electron movement can be calculated as:
Among them,nIs the unit vector of the luminous point pointing to the direction of the observation point,βIs the velocity of charged particles normalized by the speed of light c, γ is the Lorentz change factor and is the retardation factor
, subscript t*Indicates that the relevant quantity is in t*The time is the value of the delay time.
The electromagnetic field can be divided into two parts: the first part is the near field, which is only related to the speed of the charge movement, because its size is proportional to the square of the distance r from the light-emitting point to the observation point, and this term does not radiate to the distance;The second part is the far field, which is related to the acceleration of charges and is the source of electromagnetic radiation.When the charged particle moves in a uniform straight line, the second part is zero, and its electromagnetic field does not produce radiation. The wave Hinting vector of this electromagnetic field wraps the charge and then flies forward, not emitting energy to the distance;Bremsstrahlung is emitted when charged particles are decelerated by external force;When the particles move back and forth at low speed, they emit the radiation of oscillating charges;Synchrotron radiation is emitted when the electron moves at near light speed, the acceleration direction is basically perpendicular to the speed direction, and the electron speed direction changes but the size is basically unchanged.
Characteristics of synchrotron radiation light source
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Wide spectral range
Synchrotron radiation covers a wide range of wavelengths in the electromagnetic spectrum. It varies from synchrotron radiation source, generally including infrared, visible light, ultraviolet and X-ray.Among them, the wavelength of hard X-ray is about 0.1 nm, which is about the atomic size. It is very important for the research of materials and materials on the atomic scale.[2][4]Moreover, the spectrum emitted by synchrotron radiation is characterized by "continuity" and "smoothness", that is, there are neither concave breakpoints nor convex characteristic peaks in the spectrum.As required, monochromatic light with a certain wavelength and bandwidth can be selected by a monochromator for monochromatic light experiment, which is called tunability of synchrotron radiation wavelength, and is particularly suitable for conducting research on light matter interaction at specific wavelength (such as absorption spectrum) and spectral research on scanning by continuously changing wavelength.[3]
High collimation (directivity)
The divergence angle of synchrotron radiation is small, and the light is almost parallel, so it is suitable for long-distance transmission and light experiments that require the consistency of the incident angle of light.[2-3]
High radiation power
Since the instantaneous power emitted by a single electron synchrotron radiation is proportional to the fourth power of Lorentz factor γ, and the γ of relativistic particles is far greater than 1, the synchrotron radiation source has considerable power.[3]
High brightness
The brightness of synchrotron radiation refers to the concentration degree of radiation energy, which is most commonly defined as the peak density of photon in six dimensional phase space: photon number/(time × 0.1% bandwidth × light source area × solid angle).Compared with the X-ray produced by conventional X-ray machines, the brightness of synchrotron radiation is about 4-14 orders of magnitude higher. Higher brightness means that better resolution and higher experimental efficiency can be obtained in space, energy, time and other dimensions.For the material structure probe - X-ray, higher brightness means that the microstructure inside the material can be "seen" more clearly. Therefore, obtaining a higher brightness X-ray source has always been the goal of scientific and technical personnel.[2-3]
Polarization
Synchrotron radiation has naturalPolarization, its electricityvectorThe vibration of is mainly in the direction parallel to the curved track plane. The degree of polarization depends on the intersection angle between the light and the plane, and is also a function of the wavelength.For the whole spectrum, the parallel component accounts for 87.5% of the total radiation power.For monochromatic light, the higher the photon energy, the greater the proportion of parallel components, and the higher the degree of polarization.Polarization of radiation plays an important role in the study of anisotropy of samples.[3]
Pulse time structure
The energy lost by electrons due to synchrotron radiation needs to be supplemented by high-frequency accelerating electric field. The strength of this electric field changes periodically with time, dividing the electron beam into several discontinuous beam clusters. Therefore, the generated synchrotron radiation is also pulsed, the pulse width is equal to the length of a single beam cluster, and the pulse interval is equal to the distance between adjacent beam clusters, with time resolution.The impulsive time structure makes synchrotron radiation particularly suitable for studying some dynamic processes.[2-3]
High vacuum environment (cleanliness)
The electron beam of synchrotron radiation is in an ultrahigh vacuum environment, and the beam does not need to pass through the partition window and the gas, which is of great significance for the vacuum ultraviolet energy band easy to be absorbed by air.[3]
Computability
The luminescence mechanism of synchrotron radiation only involves the movement of high-energy electrons in the magnetic field, which is completely dominated by the basic physical laws. It does not need to consider a series of factors that are difficult to accurately determine, such as the fluctuation of medium density, chemical purity, temperature distribution, etc. The spectral distribution, polarization, angular distribution and other characteristics of its photon flux can be calculated by formulas.This advantage makes the synchrotron radiation source can be used as a standard light source covering a wide frequency band to calibrate other light sources and detectors.[3]
Synchrotron radiation in celestial bodies
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In addition to being generated in the laboratory, synchrotron radiation is also observed in some celestial phenomena. The radiation wavelength ranges from radio waves to gamma rays, which can be generated fromneutron starandPulsar、dwarf star、planet、Galaxy、Interstellar matterandCosmic source。[12]For example“Supernova explosion”The phenomenon is closely related to synchrotron radiation: when a celestial body with enough mass evolves to the decay stage, it will collapse to its center, and at the same time, a large number of high-speed charges will be ejected. The extremely strong magnetic field around will change the direction of motion of high-speed charges and emit extremely strong synchrotron radiation to the distance, which is dazzling.After the supernova explosion, the star will form a dense core (neutron star) and a cloud like diffuse material at the periphery. The diffuse material is huge and constantly expanding, and there is a strong magnetic field inside the system that restricts high-speed charges, becoming a long-term source of synchrotron radiation in space.[3]
