The Synchrotron

The Diamond Synchrotron in the UK

In 1945, the synchrotron was proposed as the latest accelerator for high-energy physics, designed to push particles, in this case electrons, to higher energies than a cyclotron was capable of, it was the particle accelerator of the day!

It is basically lots of linacs in a circle with a magnetic field to ensure the beam stays on track!

An accelerator takes stationary charged particles, such as electrons, and drives them to velocities near the speed of light. In being forced by magnets to travel around a circular storage ring, charged particles tangentially emit electromagnetic radiation and, consequently, lose energy. This energy is emitted in the form of light and is known as synchrotron radiation.

The General Electric (GE) Laboratory in Schenectady built the world's second synchrotron, and it was with this machine in 1947 that synchrotron radiation was first observed.

For high-energy physicists performing experiments at an electron accelerator, synchrotron radiation is a nuisance as it causes a loss of particle energy. But condensed-matter physicists realised that this was exactly what was needed to investigate electrons surrounding the atomic nucleus and the position of atoms in molecules. So, in the early days, the two branches of physics worked together in so-called "parasitic" operation, where synchrotron light illuminated the condensed-matter physicists' experiments while particle physicists used the electron beam.

Many of the synchrotron systems in commercial operation around the world were no built to produce the high energy particles but to produce 'synchrotron light'.

The part of the electromagnetic spectrum that the human eye can see is called visible light. In order of decreasing wavelength and increasing frequency. The region with wavelengths shorter than violet is the ultraviolet and, overlapping and going beyond it, the X-ray region. Meanwhile, on the other side of red, with longer wavelengths, is the infrared region. The shorter the wavelength, the higher the frequency and the more "energetic" the light. While it cannot be seen by the human eye, when used in certain ways and viewed by appropriate detectors, this light can reveal structures and features of individual atoms, molecules, crystals, cells and more, especially when the wavelength and corresponding energy of the light are matched to the size and energy of the sample being viewed. Because synchrotron light is very intense and well collimated, it is preferred to light produced by conventional laboratory sources.

How does it work?

There is a direct relationship between the speed of charged particles and their radius of curvature of the path.

We will then get an acceleration of the particles from an electric field, according to:

F = qE


F is the force vector,

q is the charge and

E is the electric field intensity vector.

Now, the magnetic force is equal to the centripetal force of the circular movement (it is what makes it move in a circle!), so we get the cyclotron equation from equating the two:

Centripital force = mv2/r

Magnetic force = Bvq

\ mv2/r = Bvq

Cancelling a ' v ' and rearranging we have

r = mv/qB

where r is the radius of the particle orbit.

So for very large velocities we would need a very large circular path.

This would be impossible with a cyclotron.

A sychrotron is capable of producing such speeds because it is constructed as a large circular tunnel. The acceleration in the ring is achieved by small accelerating sections like in the LINAC but a magnetic field is also present to produce the curved path.

The particles are already travelling at very high speeds before they enter the storage ring. This is a circular (or near circular) structure in which either high energy electrons and/or positrons, or protons and/or antiprotons can be circulated many times and thus "stored" to produce synchrotron light. They can then also be released and used to achieve high energy collisions. Because of the very different masses of protons and electrons a storage ring must be designed for one or the other type and cannot work for both.

Synchrotron radiation 

Whenever a charged particle undergoes accelerated motion it radiates electromagnetic energy. A common example is the emission of radio waves when electrons move back and forth in a radio antenna. A charged particle traveling in the arc of a circle is also undergoing acceleration, due to its change in direction it cannot be travelling at constant velocity. The radiation emitted by such particles is called synchrotron radiation and it is particularly intense and very directional when electrons travelling at close to the speed of light are bent in magnetic fields.

So when high energy electrons are deflected by strong magnetic fields, they emit electro-magnetic waves covering the whole spectral range from microwaves to hard X-rays. The electromagnetic energy (or synchrotron light) produced by the storage ring of a Synchrotron comes in the form of a fine and very intense beam, similar to that from a laser.

The X-ray beams produced are about a trillion times brighter than those of conventional X-ray sources used in laboratories and hospitals.

This makes the synchrotron a powerful tool for scientists, helping to deepen our present understanding of physics, materials and life sciences as well as improving industrial processes.

The Linac (green), a 16 m long linear accelerator, brings electrons to an energy of 200 MeV.

The Booster Synchrotron (red), 300 m in circumference, repeatedly accelerates the electron bunches emitted from the Linac. Once the electron beam reaches the operating energy of 6 GeV, it is injected in the storage ring.

In the Storage Ring (magenta), 844 m in circumference, the electron beam is maintained at the operating energy of 6 GeV. Here, electrons travelling with nearly the speed of light emit synchrotron radiation.

In its normal mode of operation, the storage ring provides a current of 200 mA for a lifetime of about 50 h. Energy losses of the electron beam are compensated by 6 accelerating cavities, operating at a frequency of 352 MHz.

In the storage ring, the needle thin electron beam is travelling with nearly the speed of light in ultra high vacuum.

When we look inside the tunnel, we can see the functional elements of the synchrotron X-ray source: bending magnets and insertion devices - furthermore, focusing magnets collimate the electron beam, while 'front-ends' carry the X-ray beam to the experimental hall.



A level standard question