The CERN Accelerator Complex

Oct 10, 2011
Kitty Grey

Inside the CERN LHC Tunnel - Juhanson

Though the LHC is the most massive machine on earth, it is but one component in a system of particle accelerators at the CERN Accelerator Complex.

The Large Hadron Collider is a particle accelerator 27 kilometres in circumference that sits 100 metres underground between Switzerland and France near Geneva. The specific type of particle that the LHC deals with is a hadron, more specifically protons or lead ions. A particle accelerator’s function is to increase the speed and energy of particles for collision and research. The LHC directs and collides particles by using magnetic fields to steer them and electrical fields to accelerate or add energy to them. Though the Large Hadron Collider does perform this function, it is but one final stage in a series of accelerators known as the CERN Accelerator Complex.

Hydrogen Atoms and the LINAC2

Before any acceleration can begin, the protons have to be harvested from condensed hydrogen atoms. These atoms are fed at a precisely controlled rate into the first stage: CERN’s linear accelerator the LINAC2. These atoms, while contained in the LINAC2’s source chamber, are stripped of their electrons to leave hydrogen nuclei. The protons are ready to be accelerated by use of an electrical field.

Lead Ions, the LINAC3 and the LEIR

As an alternative to proton packets, lead ions, generated from vaporised lead, are fed into the source chamber of another linear accelerator, the LINAC3. From there, the lead ions travel to the Low Energy Ion Ring (LEIR). From this ring, the ions follow the same path as their proton packet counterparts to acceleration and eventual injection into the Large Hadron Collider.

The Booster

Once the packet exits the LINAC2, the protons have an energy of 50 MeV and are traveling at one third the speed of light. The LINAC2 feeds the proton packet directly into a four ring stage called the Booster, 157 metres in circumference. The single packet is broken into four separate packets and sent along four vacuum rings to achieve maximum energy and speed. Accelerating the protons in a circular fashion is far more logical at this point than linear acceleration. During the Booster stage, an electrical field is pulsed at a specific point during their travel around the circle. At the same time, powerful magnets exert force at a right angle to the particles’ trajectory, bending the packet along the inside of the rings. This stage accelerates the particles to 91.6% the speed of light and increases their energy to 1.5 GeV.

The Proton Synchrotron

After the packets have completed the Booster stage, they move on to the 628 metre-in-circumference Proton Synchrotron. The protons travel for 1.2 seconds in this stage, during which they reach 99.9% the speed of light. A very curious quantum physics occurrence happens in the Proton Synchrotron: the protons reach the point of transition. This accelerator continues to subject the packets to electrical fields. Since their speed is so close to the speed of light, they are incapable of traveling any faster. Instead, the particles become heavier, translating most of the energy into mass. Upon leaving the Proton Synchrotron, the particles have an energy of 25 GeV.

The Super Proton Synchrotron

The only stage remaining between the packets and the LHC itself: the 7 kilometre in circumference Super Proton Synchrotron—similar to the Proton Synchrotron, the Super Proton Synchrotron increases the packets to an energy of 450 GeV. The packet is ready to be injected into the LHC’s two vacuum pipes.

The Large Hadron Collider

In the LHC, the packets move in the two tubes in opposite directions. There are two kickers, highly sophisticated machinery that synchronises incoming packets into the LHC’s tubes. Four detector caverns exist inside of the machine where the tubes cross and the beams of protons can be forced to collide. The packets spend 20 minutes being accelerated from 99.9997828% the speed of light and 450 GeV to 99.9999991% the speed of light and 7000 GeV or 7 TeV.

CMS (Compact Muon Solenoid) Detector

The CMS detector is one of the two general purpose detectors in the LHC. It sits in one of the four detector caverns and is comprised of a massive coil of superconducting cable that takes the form of a solenoid magnet. This magnet generates a field of four teslas. Most of the detector’s 25000 tonne weight is attributed to the steel yoke that contains the magnetic field. This detector is used for studies of the Higg’s boson, extra dimensions and dark matter.

ATLAS Detector

The second general purpose detector in the LHC is the ATLAS detector, also used for investigation into theoretical quantum physics. This detector uses six different subsystems to measure energy and momentum. It also contains a magnet to alter particle trajectory for extremely precise momentum measurements.

LHCb (Large Hadron Collider beauty) Detector

The specific function of this detector is to study beauty quarks, also known as b quarks. This machine functions with a series of sub-detectors placed over a span of 20 metres; the first sub-detector is placed very close to the collision point with subsequent sub-detectors placed behind it.

ALICE (A Large Ion Collider Experiment) Detector

Using the lead ions collected from the LEIR and accelerated through the Proton Synchrotron and other stages, the LHC monitors lead collisions that mimic conditions similar to right after the big bang. This process heats the particles to a temperature 100,000 times the centre of the sun, melting the quarks and freeing them of their bonds to the gluons. The remaining quark-gluon plasma is hypothesized to have existed just after the big bang, and physicists monitor its cooling and expansion as it shifts into new particles.

The Antiproton Decelerator

When proton packets leave the Proton Synchrotron, they can also enter another machine in the complex called the Antiproton Decelerator, tasked with creating, taming and delivering antiprotons, the antimatter equivalent of protons. This decelerator accepts beams of protons and fires them into a block of metal. One in every million collisions will result in a proton-antiproton pair. The antiprotons are traveling at near the speed of light, and their energy levels are quite chaotic; they are unfit for use in experimentation. The decelerator uses magnetic fields to direct the antiprotons around the vacuum ring and completes a process called “cooling” that reduces their speed to 10% the speed of light. One deceleration cycle takes approximately one minute. The Antiproton Decelerator manufactures and delivers antiprotons to four current experiments: ALPHA, ASACUSA, ATRAP and ACE.

ALPHA (Antihydrogen Laser Physics Apparatus)

The ALPHA experiment delves into the creation of antihydrogen with the antiparticles antiprotons and positrons. Since antihydrogen atoms have no charge, they cannot be trapped in devices used to manipulate electrical charges. The atoms drift against the chamber walls; when they contact ordinary matter, they annihilate within milliseconds of their creation. The ALPHA project utilises a newer trapping method than its predecessor ATHENA and is capable of retaining the atoms for longer periods of time. Thus, it is easier for physicists to analyse the antihydrogen and compare antimatter with ordinary matter.

ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons)

The ASACUSA experiment focuses on manufacturing hybrid atoms called antiproton helium atoms for the observation of antiprotons. A helium atom consists of merely two electrons orbiting a nucleus, and this project constructs hybrid atoms by replacing one electron with an antiproton, the antimatter counterpart of the proton. Physicists use the Antiproton Decelerator to fire a beam of antiprotons into cold helium gas. Most of the antiprotons annihilate when they encounter the ordinary matter; however, a very small amount of antiprotons combine with the already present helium atoms to form the hybrid atoms consisting of both matter and antimatter. Laster beams are used to excite the particles so that they may be observed by physicists for a better comparison between antiprotons and protons.

ATRAP (Antrihydrogen TRAP)

ATRAP is a successor of the original TRAP, and its main function is to produce antihydrogen atoms, which contain one antiproton and one positron (the antimatter equivalent of the electron). Since antiprotons contain such high energies and travel near lightspeed at their creation, ATRAP utilises cold positrons to cool the antiprotons. When they have reached around the same temperature, a small number of the positrons will orbit an antiproton, creating antihydrogen atoms that can be sustained long enough for accurate measurements.

ACE (Antiproton Cell Experiment)

In current particle-beam therapy, protons with a carefully controlled energy level are sent through the healthy tissue of a patient and decelerate and stop at the depth of cancerous cells. While the protons cause no significant damage to heath tissue on their way through, repeated usage of this treatment can cause injury over time. ACE employs antiprotons to target atoms in the nuclei of cancerous cells. The antiprotons then annihilate and destroy surrounding matter. This experiment has yielded results that four times fewer antiprotons than protons are needed, reducing damage to surrounding healthy tissue.