Synchrotrons and the Large Hadron Collider (LHC)

Here's a good question for your next trivia night. What's the difference between the Australian Synchrotron in Melbourne and the Large Hadron Collider near Geneva?

Stumped? Here's the answer, from Australian Synchrotron accelerator physicist Rohan Dowd.

Well, both are synchrotrons, but built for very different purposes. Strictly speaking, a synchrotron is a circular particle accelerator that uses synchronously ramped magnetic and electric fields to accelerate charged particles. Both the Australian Synchrotron and the LHC use synchrotrons to accelerate charged particles - it's what happens next that makes the big difference.

At the Australian Synchrotron facility, we're interested in using the synchrotron as a light source. We accelerate a single beam of electrons to 3 GeV energy (giga-electron volts) and then place them in a storage ring. The storage ring is designed to keep the electrons tightly focused and circulating at a constant energy. As the electrons circulate they give off synchrotron radiation, which is harnessed for beamline experiments in biological, chemical and materials science.

At the Large Hadron Collider (LHC), physicists are interested in creating extremely high energy particle collisions to study the fundamental forces of the universe. At these high energies electrons lose far too much energy to synchrotron radiation to make an electron-based machine feasible. Protons are used because they have a much higher mass, and therefore emit almost negligible radiation, even through they tend to make far messier collisions to study. Protons are a type of hadron, hence the name of the facility. The LHC storage ring circulates two beams of protons (and occasionally heavy ions for other experiments), moving in opposite directions. The beams are injected into the LHC at 450 GeV energy after passing through several other accelerators (including three other synchrotrons). The LHC storage ring then ramps up to an energy of 7 TeV(tera-electron volts) in each beam before focusing the beams down into four collision points around the ring. Large (building-sized), extremely sensitive particle detectors are positioned around the collision points to record the results of the collisions. The LHC requires very powerful magnetic fields to store the higher energy beams. This necessitates the almost exclusive use of superconducting magnets, cooled by liquid helium to just 1.9 degrees above absolute zero.

Despite the obvious differences in scale and purpose of the two machines, there are many similarities between the Australian Synchrotron and LHC storage rings (and indeed most storage rings around the world). Both use a combination of dipole, quadrupole and sextupole magnets to keep the beams circulating and focused and similar RF (radiofrequency) systems to accelerate and keep the beam circulating. The beam diagnostic instruments and accelerator physics techniques used to control the beams are also nearly identical.

Comparison table

 

Australian Synchrotron

Large Hadron Collider

Energy (per beam)

3 GeV 

7 TeV

Circumference

216 m

26,659 m

Magnet type

warm copper coils ~ 300K 

superconducting 1.9K

Dipole field strength

1.4 T 

8.3 T

Number of dipoles

28 

1232

Number of quadrupoles

84

392

Synchrotron radiation energy loss per turn
(dipoles only) 

938 keV  

7 keV

Number of RF cavities (per beam)  

4

8

RF cavity voltage(total) 

3 MV 

16 MV

Number of bunches (nominal)

300 

2808


Link to LHC page on CERN website: http://public.web.cern.ch/public/en/LHC/LHC-en.html