Definitions

# H alfven

This article concerns the physical phenomenon of synchrotron radiation. For details on the production of this radiation in laboratories, see synchrotron. For applications, see synchrotron light.

Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of ultrarelativistic (i.e., moving near the speed of light) charged particles through magnetic fields. This may be achieved artificially by storage rings in a synchrotron, or naturally by fast moving electrons moving through magnetic fields in space. The radiation typically includes radio waves, infrared light, visible light, ultraviolet light, and x-rays.

The radiation was named after its discovery in a General Electric synchrotron accelerator built in 1946 and announced in May 1947 by Frank Elder, Anatole Gurewitsch, Robert Langmuir, and Herb Pollock in a letter entitled "Radiation from Electrons in a Synchrotron. Pollock recounts:

"On April 24, Langmuir and I were running the machine and as usual were trying to push the electron gun and its associated pulse transformer to the limit. Some intermittent sparking had occurred and we asked the technician to observe with a mirror around the protective concrete wall. He immediately signaled to turn off the synchrotron as "he saw an arc in the 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 Cherenkov radiation, but it soon became clearer that we were seeing Ivanenko and Pomeranchuk radiation.

## Synchrotron radiation from storage rings

• High brightness and high intensity, many orders of magnitude more than with X-rays produced in conventional X-ray tubes
• High brilliance, exceeding other natural and artificial light sources by many orders of magnitude: 3rd generation sources typically have a brilliance larger than 1018 photons/s/mm2/mrad2/0.1%BW, where 0.1%BW denotes a bandwidth 10-3w centered around the frequency w.
• High collimation, i.e. small angular divergence of the beam
• Low emittance, i.e. the product of source cross section and solid angle of emission is small
• Widely tunable in energy/wavelength by monochromatization (sub eV up to the MeV range)
• High level of polarization (linear or elliptical)
• Pulsed light emission (pulse durations at or below one nanosecond, or a billionth of a second);

Electrons are accelerated to high speeds in several stages to achieve a final energy that is typically in the GeV range. The electrons are stored in an ultrahigh vacuum ring on a closed loop and thus circle the ring a vast number of times. The electrons are forced to travel in a closed loop by strong magnetic fields. The magnets also need to repeatedly recompress the Coulomb-exploding space charge electron bunches. The change of direction is a form of acceleration and thus the electrons emit radiation at GeV frequencies. This is similar to a radio antenna, but with the difference that the relativistic speed changes the observed frequency due to the Doppler effect by a factor $gamma$. Relativistic Lorentz contraction bumps the frequency by another factor of $gamma$, thus multiplying the GeV frequency of the resonant cavity that accelerates the electrons into the X-ray range. Another dramatic effect of relativity is that the radiation pattern is also distorted from the isotropic dipole pattern expected from non-relativistic theory into an extremely forward-pointing cone of radiation. This makes synchrotron radiation sources the brightest known sources of X-rays. The planar acceleration geometry makes the radiation linearly polarized when observed in the orbital plane, and circularly polarized when observed at a small angle to that plane.

The advantages of using synchrotron radiation for spectroscopy and diffraction have been realized by an ever-growing scientific community, beginning in the 1960s and 1970s. In the beginning, storage rings were built for particle physics and synchrotron radiation was used in "parasitic mode" when bending magnet radiation had to be extracted by drilling extra holes.

As the application of synchrotron radiation became more intense and promising, devices that enhanced the intensity of synchrotron radiation were built into existing rings. Third-generation synchrotron radiation sources were conceived and optimized from the outset to produce bright X-rays.

Nowadays, fourth-generation sources that will include different concepts for producing ultrabright, pulsed time-structured X-rays for extremely demanding and also probably yet-to-be-conceived experiments are under consideration.

As mentioned above, bending electromagnets are usually used to generate the radiation, but to generate stronger radiation, another kind of device, called an insertion device, is sometimes employed. Current third-generation synchrotron radiation sources are typically heavily based upon these insertion devices, when straight sections in the storage ring are used for inserting periodic magnetic structures (composed of many magnets that have a special repeating row of N and S poles) that force the electrons into a sinusoidal path or helical path. Thus, instead of a single bend, many tens or hundreds of "wiggles" at precisely calculated positions add up or multiply the total intensity that is seen at the end of the straight section. Thus these devices are called wigglers or undulators. The main difference between an undulator and a wiggler is the intensity of their magnetic field and the amplitude of the deviation from the straight line path of the electrons.

There are openings in the storage ring to let the radiation exit and follow a beam line into the experimenters' vacuum chamber. A great number of such beamlines can emerge from modern third-generation synchrotron radiation sources.

• Synchrotron radiation of an electron beam circulating at high energy in a magnetic field leads to radiative self-polarization of electrons in the beam (Sokolov-Ternov effect). This effect is used for producing highly polarised electron beams for use in various experiments.
• Synchrotron radiation is used in particle accelerators in radiation damping, a method of reducing beam emittance.

Synchrotron radiation is also generated by astronomical structures and motions, typically where relativistic electrons spiral (and hence change velocity) through magnetic fields. Two of its characteristics include (1) Non-thermal radiation (2) Polarization.

### History

It was first detected, in a jet emitted by M87, in 1956 by Geoffrey R. Burbidge , who saw it as confirmation of a prediction by Iosif S. Shklovskii in 1953, but it had been predicted several years earlier by Hannes Alfvén and Nicolai Herlofson in 1950.

T. K. Breus noted that questions of priority on the history of astrophysical synchrotron radiation is quite complicated, writing:

"In particular, the Russian physicist V.L. Ginsburg broke his relationships with I.S. Shklovsky and did not speak with him for 18 years. In the West, Thomas Gold and Sir Fred Hoyle were in dispute with H. Alfven and N. Herlofson, while K.O. Kiepenheuer and G. Hutchinson were ignored by them.

Supermassive black holes have been suggested for producing synchrotron radiation, by gravitationally accelerating ions through magnetic fields.