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What is Synchrotron Radiation ?

Synchrotron Radiation - A Form of Light
" Light" has always been indispensable to man's exploration of nature. All wavelengths of the electromagnetic spectrum can be referred to as "light". "Light" of different wavelengths is used for different purposes. The longest wavelengths, radio waves, are used for observing the expansive universe, and microwaves are used to detect planes, ships, and typhoons. Infrared light is an ideal light source for night vision systems and the detection of missiles through tracking their heat sources. Visible light is the only wavelength humans can see with their naked eyes. Ultra-violet light is used to examine the structure of gas molecules and condensed matter. X-rays are the best source for researching crystal structures; and gamma rays, with the shortest wavelength, allow researchers to explore the inner world of atoms.

" 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

Synchrotron accelerators are important tools for researchers of high-energy physics to study elementary particles. Scientists started to use synchrotron radiation for various experiments after its discovery at synchrotron accelerators. Partially dedicated to synchrotron-radiation research, these high-energy accelerators are called the first-generation synchrotron radiation sources.

During the 1970's, scientists gradually discovered the many useful characteristics of synchrotron light, and specialized accelerators were developed as dedicated light sources. These are the second-generation synchrotron radiation source.

During the 1980's, scientists invented a method of creating even brighter synchrotron light. Specially-designed magnets, called wigglers and undulators, were inserted in storage rings to deflect the electron beam many times over a short distance. An increase in brightness of over one thousand times could be achieved through the accumulation of emitted synchrotron light. This advanced type of synchrotron source is called the third-generation synchrotron light source. During the 90's, a number of third-generation light sources were built, including the Taiwan Light Source which began operation in 1993.

The Properties of Synchrotron Radiation

High Intensity

Continuous Spectrum

Excellent Collimation

Low Emittance

Pulsed-time Structure

Polarization

As an example, the intensity of synchrotron X-rays is more than a million times higher that of X-rays from a conventional X-ray tube. Experiments that took a month to complete can now be done in only a few minutes. With synchrotron radiation, molecular structures that once baffled researchers can now be analyzed precisely, and this progress has opened up many new research fields over the last few years.

How a Synchrotron Light Source Works
Injector | Transport Line | Storage Ring | Insertion Devices | Beamlines | Experimental Station
How is synchrotron radiation produced? Whenever electrons moving close to the speed of light are deflected by a magnetic field, they radiate a thin beam of radiation tangentially from their path. This beam is called "synchrotron radiation". Taking the NSRRC's synchrotron light source as an example, the electrons are first accelerated in the linear accelerator (LINAC) and the booster ring (1). They are then sent through the transport line (2) and into the storage ring (3), where they circulate in vacuum pipes for several hours, emitting synchrotron radiation. The emitted light is channeled through beamlines (4) to the experimental stations (5) where experiments are conducted.
(1) Injector (LINAC and Booster Ring)

The injector consists of an electron gun, a LINAC that accelerates the electrons to an energy of 50 MeV, and a booster (72-meter circumference) that accelerates the electrons to an energy of 1.5 GeV. The electrons travel at 99.999995% of the speed of light.
Parameters of Injector
Injection energy: 1.5 GeV Injection cycle rate : 10 Hz

Linac energy:

50 MeV
Booster circumference: 72 m Booster RF frequency : 499.654 MHz    
(2) Transport Line

After reaching the target energy, the electrons are transferred from the booster ring to the storage ring through a 70-meter-long transport line.

(3) Storage Ring

The storage ring is roughly hexagonal in shape and has a circumference of 120 meters. Electrons with an energy of 1.5 GeV circulate in an ultra-high-vacuum chamber. A series of magnets situated around the ring steer the electrons along circular arcs, and synchrotron radiation is continuously emitted tangentially from the arcs.
 
Parameters of Storage Ring
Maximum energy : 1.5 GeV
Maximum beam current, multibunch: 300 mA
Maximum beam current, single bunch: 25 mA
Beam lifetime (1.5 GeV, 200 mA): > 9 h
Circumference: 120 m
Orbital period: 400 ns
Number of superperiods: 6
Lattice type: Triple bend achromat
Natural horizontal beam emittance: 2.5 x 10 -8 m-rad
RF frequency: 499.654 MHz
Harmonic number : 200
Bending radius: 3.495 m
Critical photon energy: 2.14 keV
Bunch length ( 1σ ): 25 ps
Maximum length of insertion devices: 4.5 m

(4) Insertion Devices (wigglers/undulators)

Insertion devices comprise of rows of magnets with alternating polarity, and produce brighter synchrotron radiation by causing the beam to oscillate.

Wigglers :
cause multiple direction changes (or wiggles) in the electron beam that generate extremely bright white light with short wavelengths.

Undulators :
cause periodic changes in the electron beam's direction that produce ultra-brilliant, single-wavelength radiation from the resulting interference patterns.

(5) Beamlines

The beamlines house many specially-designed optical components which provide focused synchrotron radiation of the desired energy into users' experimental end stations.

(6)Experimental Station

Experiments using synchrotron radiation attempt to analyze electrons, photons, and other particles that are emitted when synchrotron radiation strikes matter. The resulting data are then used to deduce the matter's chemistry, geometry, electronic structure, or magnetic properties.