What is a synchrotron?

A synchrotron is an extremely powerful source of X-rays. These are produced by highly energetic electrons moving in a large circle in the synchrotron.

The experimental hall building of the ESRF, June 2005 (Credit: ESRF/P. Ginter)The entire world of synchrotron science depends on one physical phenomenon: When a moving electron changes direction, it emits energy. When the electron is moving fast enough, the emitted energy is at X-ray wavelength.

A synchrotron machine exists to accelerate electrons to extremely high energy and then make them change direction periodically. The resulting X-rays are emitted as dozens of thin beams, each directed toward a beamline next to the accelerator. The machine operates day and night, with periodic short and long shutdowns for maintenance.


Principal structures



Storage Ring

The storage ring is a tube 844 metres in circumference where the electrons circle for hours close to the speed of light. The tube is maintained at very low pressure (around 10-9 mbar). As the electrons travel round the ring, they pass through different types of magnets (see below) and in the process produce X-rays. Units called RF cavities resupply the energy electrons have emitted as X-rays.

Booster synchrotron

Radiofrequency cavity accelerating electrons in the ESRF booster synchrotron tunnel, January 2005 (Credit: ESRF/P. Ginter).This is a 300-metre-long pre-accelerator where the electrons are accelerated to an energy of 6 billion electron-volts (6 GeV) before being injected into the storage ring. The booster synchrotron only works a few times a day for a few minutes, when the storage ring is refilled. Every 50 milliseconds, it can send a bunch of 6 GeV electrons into to the storage ring.


Here, the electrons for the storage ring are produced in an electron gun, a device similar to the cathode ray tubes found in older televisions or computer screens. These electrons are packed in “bunches” and then accelerated to 200 million electron-volts, enough for injection into the booster synchrotron.


View of the first hutch of ESRF beamline ID23 devoted to protein crystallography, January 2005 (Credit: ESRF/P. Ginter).The X-ray beams emitted by the electrons are directed toward "beamlines" that surround the storage ring in the experimental hall. Each beamline is designed for use with a specific technique or for a specific type of research. Experiments run throughout the day and night.


Magnets in the storage ring

The storage ring includes 32 straight and 32 curved sections in alternating order. In each curved section, two large bending magnets force the path of the electrons into a racetrack-shaped orbit 844 metres in circumference. In each straight section, several focusing magnets ensure that the electrons remain close to their ideal orbital path. The straight sections also host the undulators, where the intense beams of X-rays are produced.

Inside the storage ring


Undulator (or Insertion Device)


Installation of an undulator in the ESRF electron storage ring, January 2005 (Credit: ESRF/P. Ginter).These magnetic structures, made up of a complex array of small magnets, force the electrons to follow an undulating, or wavy, trajectory. The radiation emitted at each consecutive bend overlaps and interferes with that from other bends. This generates a much more focused, or brilliant, beam of radiation than that generated by a single magnet. Also, the photons emitted are concentrated at certain energies (called the fundamental and harmonics). The gap between the rows of magnets can be changed to fine-tune the wavelength of the X-rays in the beam.


Bending magnets

bending magnet

The main function of bending magnets is to bend the electrons into their racetrack orbit. However, as the electrons are deflected from their straight path when passing through these magnets, they emit a spray of X-rays tangentially to the plane of the electron beam. The synchrotron light from a bending magnet covers a wide and continuous spectrum, from microwaves to hard X-rays, and it is much less focused, or brilliant, than the fine beam of X-rays from an insertion device.




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