Synchrotron Design

1. The Linac consists of an electron gun to send electrons into a vacuum and then uses  radio-frequency electromagnetic waves to accelerate the electrons to high speed.

Electrons escape from the electron gun as they are boiled off the heated solid. They are then accelerated across a potential difference of about 100kev and exit the gun at a speed of about 50% the speed of light. The electron beam travels through an ultra-high vacuum within the linac to prevent energy loss through collisions with air particles. Then radio-frequency electromagnetic waves accelerate the electrons, energising them to a kinetic energy of 100MeV.

2. The Booster Ring: within the circular booster ring, bending magnets provide a force at right angles to the motion of the electrons in order to bend them in a circular path. The electrons' energy increases to 3000MeV (3 GeV or 3 gigaelectronvolts). The energy boost is supplied by a radio-frequency chamber through which the electrons travel on each orbit of the ring.

Rather than gaining extra speed, the electrons are only getting slightly faster as they are already near the speed of light. They have increased in mass by a factor of 6000 times their rest mass.

The radius of curvature  of their bending path is given by
                                                  r = mv/(eB) 
such that as the radio-frequency booster gives the electrons more energy and their mass increases, then the beam radius increases which would result in the electrons hitting the wall of the evacuated tube. To prevent this happening, the strength of the magnetic field (B) is increased at the same rate as the energy (and hence mass) increases.

3. The Storage Ring:  The booster ring transfers electrons into the storage ring, which has a radius of 34 m and circumference of 216 m. In one second the electrons make over one million revolutions of the storage ring and continue to circulate for between 5 and 50 hours.

As the electron path bends, synchrotron radiation is given off tangentially to the path. The energy lost to the electrons in giving off the radiation is replenished by a radio-frequency boost each revolution, so that the electrons maintain the same energy. Unused high energy X-rays given off by the storage ring are continually absorbed by concrete radiation shielding. This tunnel completely encloses the storage ring, except for narrow openings providing the beamlines through which the synchrotron X-ray radiation is guided.

4. A beamline is the path that the synchrotron light travels from the storage ring to its target experimental work in the optics room. A beamline is a stainless steel tube, 15-35 m in length and around 4 cm in diameter.

If scientists only want a specific range of wavelengths for their experiments, a device called a monochromator (a crystal or a grating) is used as a wavelength selector. As the beam hits the monochromator, a specific light frequency can be selected from the broad band of frequencies in the incident beam. Devices called attenuators can be used to reduce the intensity of the beam if required.

Scientists monitor their experiments at a work station using a computer in an external control room, in which they are protected from the intense electromagnetic radiation being used in the experiments.

A great benefit of synchrotron light sources is that virtually any radiation wavelength can be used in experiments requiring radiation wavelengths from infrared to X-ray.


5.  Insertion Devices: Dipole Magnets, Wigglers and Undulators can be inserted in straight sections of the storage ring.
The dipole magnets in the storage ring which are used to bend the beam are a source of synchrotron light.
Stronger sources of synchrotron light are wigglers and undulators. They are not essential to the operation of the storage ring, their only function is to create more intense synchrotron light.

A wiggler consists of two rows of small alternating magnetic poles, which force the elctrons into a wavy path (the electrons "wiggle" along the path).

An undulator consists of less powerful magnets than that of the wiggler, producing gentler deflections of the electron beam. The emitted synchrotron light overlaps to produce a beam that is collimated to a narrow beam.

The undulator insertion device results in interference effects that produce a spectrum of synchrotron light that is enhanced at  specific wavelengths.  These wavelengths are determined by the spacing between magnetic poles of the undulator, and can be mechanically tuned to select wavelengths. For the wavelengths produced, the brightness can be one million times that produced by the ending dipole magnets.

Synchrotron Light compared to X-ray tubes and Lasers

The X-ray light produced by a Synchrotron is much brighter and intense than X-rays produced by an X-ray tube (100 million times brighters).

Synchrotron light compared to Laser light. Lasers produce light of one wavelength only, so that you need many different individual lasers for different wavelengths. The power of a laser is very weak compared to that of the synchrotron. Lasers are limited to infrared and visible wavelengths. Some lasers can produce ultraviolet light. Bothy the synchrotron and Lasers can produce coherent (in phase) light. Normal lasers cannot produce X-ray light. Synchrotrons can produce laser light in the X-ray region using an undulator.