Insertion device
Encyclopedia
An insertion device is a component in modern synchrotron light sources. They are periodic magnetic structures that stimulate highly brilliant, forward-directed synchrotron radiation
emission by forcing a stored charged particle beam to perform wiggles, or undulations, as they pass through the device. This motion is caused by the Lorentz force
, and it is from this oscillatory motion that we get the names for the two classes of device, which are known as wiggler
s and undulator
s.
in the USSR. However it was Motz and his team who in 1953 installed the first undulator in a linac at Stanford, using it to generate millimetre wave radiation through to visible light.
It was not until the 1970s that undulators were installed in electron storage rings to produce synchrotron radiation. The first institutions to take these devices were the Lebedev Physical Institute
in Moscow
, and the Tomsk Polytechnic University
. These installations allowed a fuller characterisation of the behaviour of undulators.
Undulators only became practical devices for insertion in synchrotron light sources in 1981, when teams at the Lawrence Berkeley
SSRL
and at BINP in Russia developed permanent magnetic arrays, known as Halbach array
s, which allowed short repeating periods unachievable with either electromagnetic coils or superconducting coil
s.
Despite their similar function, wigglers were used in storage ring
s for over a decade before they were used to generate synchrotron radiation for beamlines. Wigglers have a damping
effect on storage rings, which is the function to which they first put at the Cambridge Electron Accelerator in Massachusetts in 1966. The first wiggler used for generation of synchrotron radiation was a 7 pole wiggler installed in the SSRL in 1979.
Since these first insertions the number of undulators and wigglers in synchrotron radiation facilities throughout the world have proliferated and they are one of the driving technologies behind the next generation of light sources, free electron laser
s.
s, pass through the ID the alternating magnetic field experienced by the particles causes their trajectery to undergo a transverse oscillation. The acceleration associated with this movement stimulates the emission of synchrotron radiation.
There is very little mechanical difference between wigglers and undulators and the criterion normally used to distinguish between them is the K-Factor. The K-factor is a dimensionless constant defined as:
where q is the charge of the particle passing through the ID, B is the peak magnetic field of the ID,
is the period of the ID, 
relates to the speed, or energy of the particle, m is the mass of the accelerated particle, and c is the speed of light
.
Wigglers are deemed to have K>>1 and undulators to have K<1.
The K-Factor determines the energy of radiation produced, and in situations where a range of energy is required the K-number can be modified by varying the strength of the magnetic field of the device. In permanent magnet devices this is usually done by increasing the gap between the magnet arrays. In electromagnetic devices the magnetic field is changed by varying the current in the magnet coils.
In a wiggler
the period and the strength of the magnetic field is not tuned to the frequency of radiation produced by the electrons. Thus every electron in the electron bunch radiates independently, and the resulting radiation spectrum is broad. A wiggler can be considered to be series of bending magnets concatenated together, and its radiation intensity scales as the number of magnetic poles in the wiggler.
In an undulator
source the radiation produced by the oscillating electrons interferes constructively with the motion of other electrons, causing the radiation spectrum to have a relatively narrow bandwidth. The intensity of radiation scales as
, where 
is the number of poles in the magnet array. Undulators are the brightest sources of X-ray
s and extreme ultraviolet
radiation mankind has yet invented.
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
emission by forcing a stored charged particle beam to perform wiggles, or undulations, as they pass through the device. This motion is caused by the Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
, and it is from this oscillatory motion that we get the names for the two classes of device, which are known as wiggler
Wiggler (synchrotron)
A wiggler is an insertion device in a synchrotron. It is a series of magnets designed to periodically laterally deflect a beam of charged particles inside a storage ring of a synchrotron...
s and undulator
Undulator
An undulator is an insertion device from high-energy physics and usually part of a largerinstallation, a synchrotron storage ring. It consists of a periodic structure of dipole magnets . The static magnetic field is alternating along the length of the undulator with a wavelength \lambda_u...
s.
History
The theory behind undulators was developed by Vitaly GinzburgVitaly Ginzburg
Vitaly Lazarevich Ginzburg ForMemRS was a Soviet theoretical physicist, astrophysicist, Nobel laureate, a member of the Russian Academy of Sciences and one of the fathers of Soviet hydrogen bomb...
in the USSR. However it was Motz and his team who in 1953 installed the first undulator in a linac at Stanford, using it to generate millimetre wave radiation through to visible light.
It was not until the 1970s that undulators were installed in electron storage rings to produce synchrotron radiation. The first institutions to take these devices were the Lebedev Physical Institute
Lebedev Physical Institute
The Lebedev Physics Institute of the Russian Academy of Sciences , situated in Moscow, is one of the leading Russian research institutes specializing in physics. It is also one of the oldest research institutions in Russia: its history dates back to a collection of physics equipment established by...
in Moscow
Moscow
Moscow is the capital, the most populous city, and the most populous federal subject of Russia. The city is a major political, economic, cultural, scientific, religious, financial, educational, and transportation centre of Russia and the continent...
, and the Tomsk Polytechnic University
Tomsk Polytechnic University
Tomsk Polytechnic University in Tomsk, Russia, is the oldest technical university in Russia east of the Urals. The university was founded in 1896 and opened in 1900 as the Tomsk Technological Institute. In 1923, the school was renamed the Siberian Technological Institute and in 1930, the institute...
. These installations allowed a fuller characterisation of the behaviour of undulators.
Undulators only became practical devices for insertion in synchrotron light sources in 1981, when teams at the Lawrence Berkeley
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
SSRL
Stanford Synchrotron Radiation Laboratory
The Stanford Synchrotron Radiation Lightsource , a division of SLAC National Accelerator Laboratory, is operated by Stanford University for the Department of Energy...
and at BINP in Russia developed permanent magnetic arrays, known as Halbach array
Halbach array
A Halbach array is a special arrangement of permanent magnets that augments the magnetic field on one side of the array while cancelling the field to near zero on the other side...
s, which allowed short repeating periods unachievable with either electromagnetic coils or superconducting coil
Superconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields...
s.
Despite their similar function, wigglers were used in storage ring
Storage ring
A storage ring is a type of circular particle accelerator in which a continuous or pulsed particle beam may be kept circulating for a long period of time, up to many hours. Storage of a particular particle depends upon the mass, energy and usually charge of the particle being stored...
s for over a decade before they were used to generate synchrotron radiation for beamlines. Wigglers have a damping
Radiation damping
Radiation damping in accelerator physics is a way of reducing the beam emittance of a high-velocity beam of charged particles.There are two main ways of using radiation damping to reduce the emittance of a particle beam—damping rings and undulators—and both rely on the same principle...
effect on storage rings, which is the function to which they first put at the Cambridge Electron Accelerator in Massachusetts in 1966. The first wiggler used for generation of synchrotron radiation was a 7 pole wiggler installed in the SSRL in 1979.
Since these first insertions the number of undulators and wigglers in synchrotron radiation facilities throughout the world have proliferated and they are one of the driving technologies behind the next generation of light sources, free electron laser
Free electron laser
A free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam...
s.
Operation
Insertion devices are traditionally inserted into straight sections of storage rings (hence their name). As the stored particle beam, usually electronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, pass through the ID the alternating magnetic field experienced by the particles causes their trajectery to undergo a transverse oscillation. The acceleration associated with this movement stimulates the emission of synchrotron radiation.
There is very little mechanical difference between wigglers and undulators and the criterion normally used to distinguish between them is the K-Factor. The K-factor is a dimensionless constant defined as:
where q is the charge of the particle passing through the ID, B is the peak magnetic field of the ID,
is the period of the ID, relates to the speed, or energy of the particle, m is the mass of the accelerated particle, and c is the speed of lightSpeed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
.
Wigglers are deemed to have K>>1 and undulators to have K<1.
The K-Factor determines the energy of radiation produced, and in situations where a range of energy is required the K-number can be modified by varying the strength of the magnetic field of the device. In permanent magnet devices this is usually done by increasing the gap between the magnet arrays. In electromagnetic devices the magnetic field is changed by varying the current in the magnet coils.
In a wiggler
Wiggler (synchrotron)
A wiggler is an insertion device in a synchrotron. It is a series of magnets designed to periodically laterally deflect a beam of charged particles inside a storage ring of a synchrotron...
the period and the strength of the magnetic field is not tuned to the frequency of radiation produced by the electrons. Thus every electron in the electron bunch radiates independently, and the resulting radiation spectrum is broad. A wiggler can be considered to be series of bending magnets concatenated together, and its radiation intensity scales as the number of magnetic poles in the wiggler.
In an undulator
Undulator
An undulator is an insertion device from high-energy physics and usually part of a largerinstallation, a synchrotron storage ring. It consists of a periodic structure of dipole magnets . The static magnetic field is alternating along the length of the undulator with a wavelength \lambda_u...
source the radiation produced by the oscillating electrons interferes constructively with the motion of other electrons, causing the radiation spectrum to have a relatively narrow bandwidth. The intensity of radiation scales as
, where is the number of poles in the magnet array. Undulators are the brightest sources of X-rayX-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s and extreme ultraviolet
Extreme ultraviolet
Extreme Ultraviolet radiation is high-energy ultraviolet radiation, generally defined to be electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 120 nm down to 10 nm, and therefore having photons with energies from 10 eV up to 124 eV...
radiation mankind has yet invented.