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The Large Hadron Collider (LHC) is the most powerful particle accelerator ever built.
The LHC allows scientists to reproduce the conditions that existed within a billionth of a second after the Big Bang. This is the moment, around 14 billion years ago, when the Universe is believed to have started with an explosion of energy and matter. During this first moment of time the particles and forces that shaped our Universe came into existence.
Scientists recreate these conditions by colliding beams of high-energy protons or ions at almost the speed of light. This takes place inside the LHC’s 27 km circular accelerator 100 m below the ground.
In 2012, before the long shutdown, two of the four experiments on the LHC, ATLAS and CMS, announced that they had detected the Higgs boson, a particle physicists had been looking for since it was predicted 50 years ago. When the LHC comes back online at higher energies, scientists will learn more about the Higgs boson and continue to explore the frontiers of high energy physics.
The LHC is scheduled to restart early in 2015, with the experimental physics programme resuming in spring.
The Super Proton Synchrotron is the second-largest machine in CERN’s accelerator complex. Measuring nearly 7 km in circumference, it takes particles from the Proton Synchrotron and accelerates them to provide beams for the Large Hadron Collider, the NA61/SHINE and NA62 experiments, and the COMPASS experiment.
The SPS was switched on in 1976, and research using SPS beams has probed the inner structure of protons, investigated nature’s preference for matter over antimatter, looked for matter as it might have been in the first instants of the Universe and searched for exotic forms of matter. A major highlight came in 1983 with the Nobel prize-winning discoveries of the W and Z particles, with the SPS running as a proton-antiproton collider. The accelerator has handled many different kinds of particles: sulphur and oxygen nuclei, electrons, positrons, protons and antiprotons.
Powering tests took place on the SPS in July 2014, and the machine should be ready for physics in October 2014.
The Proton Synchrotron is a key component in CERN’s accelerator complex, where it usually accelerates either protons delivered by the Proton Synchrotron Booster or heavy ions from the Low Energy Ion Ring (LEIR).
The PS was CERN’s first synchrotron, and when it came online in 1959, it was the world’s highest energy particle accelerator. When new accelerators were built in the 1970s, the PS’s principal role became to supply particles to the new machines. Over the years, it has undergone many modifications and the intensity of its proton beam has increased a thousandfold. The accelerator operates at up to 25 GeV. In addition to protons, it has accelerated alpha particles (helium nuclei), oxygen and sulphur nuclei, electrons, positrons and antiprotons.
During the long shutdown, the Proton Synchrotron received upgrades that are essential to meet the future demands of the LHC. As the PS wasn’t built in an underground tunnel, it has received extra shielding so that it can continue to operate safely – which meant reinforcing the walls to take the extra weight. An ageing ventilation system was also replaced, and changes have been made to the radiofrequency (RF) system (used to accelerate particles) to make it even more versatile. A new beam extraction system means that beams can be delivered more cleanly.
Commissioning began on the Proton Synchrotron Booster in June 2014, after the implementation of a new low level RF system, and was followed by the restart of the Proton Synchrotron.
The Antiproton Decelerator provides low-energy antiprotons, mainly for studies of antimatter. A beam of protons from the Proton Synchrotron is fired into a block of metal, and the energy from the collisions is enough to create a new proton-antiproton pair about once in every million collisions. The antiprotons produced travel at almost the speed of light and have too much energy to be useful for making antiatoms; they also have a range of energies and move randomly in all directions. The job of the Antiproton Decelerator is to tame these unruly particles into a useful low-energy beam.
A ring of bending and focusing magnets keeps the antiprotons on the same track, while strong electric fields slow them down. Passing the antiprotons through clouds of electrons – a technique known as ‘cooling’ – reduces the sideways motion and the spread in energies. Finally, when the antiprotons have slowed down to about 10% of the speed of light, they are ready to be ejected. Each ‘deceleration cycle’ lasts around a minute.
In 2002 the AD made headlines around the world when the ATHENA and ATRAP experiments successfully made large numbers of antiatoms for the first time. Currently it serves four experiments that are studying antimatter: AEGIS, ALPHA, ASACUSA and ATRAP. The ACE experiment also uses antiprotons, to assess their suitability for cancer therapy.
The antimatter physics programme restarted at the AD in August 2014.
The CLIC Test Facility (CTF3) is being used to test new concepts in accelerator technology. The Compact Linear Collider (CLIC) study is an international collaboration working on a concept for a machine to collide electrons and positrons (antielectrons) head-on at energies up to several teraelectronvolts (TeV). This energy range is similar to the LHC’s, but using electrons and their antiparticles rather than protons, physicists will gain a different perspective on the underlying physics.
Probably the most innovative element of the CLIC design is that it has two beams – a drive beam and a main beam. Two accelerators would sit side by side; one, the main linear accelerator or “linac”, getting the beams of particles from source to collision, and the other, the “drive beam”, passing as much power as possible on to the main beams.
The Proton Driven Plasma Wakefield Acceleration Experiment is a proof-of-principle experiment investigating the use of plasma wakefields driven by a proton bunch to accelerate charged particles. A plasma wakefield is a type of wave generated by particles travelling through a plasma. AWAKE will send proton beams through plasma cells to generate these fields. By harnessing wakefields, physicists may be able to produce accelerator gradients hundreds of times higher than those achieved in current radiofrequency cavities. This would allow future colliders to achieve higher energies over shorter distances than is possible today – leading to smaller, and cheaper, colliders. The technology also has the potential be used to create table-top accelerators for medical applications, but this is very much in the future.
AWAKE will use proton beams from the Super Proton Synchrotron, and would be the world’s first proton-driven plasma wakefield acceleration experiment. Besides demonstrating how protons can be used to generate wakefields, AWAKE will also develop the necessary technologies for long-term, proton-driven plasma acceleration projects.
The civil engineering for AWAKE began in August 2014. The first phase of experiments is due to start in 2016.
The Isotope mass Separator On-Line facility is a unique source of low-energy beams of radioactive nuclides, those with too many or too few neutrons to be stable. It permits the study of the vast territory of atomic nuclei, including the most exotic species.
The high intensity proton beam from the Proton Synchrotron Booster (PSB) is directed into specially developed thick targets, yielding a large variety of atomic fragments. Different devices are used to ionize, extract and separate nuclei according to their mass, forming a low-energy beam that is delivered to various experimental stations.
An upgrade of the machine, HIE-ISOLDE, is underway that will improve the experimental capabilities of ISOLDE in many aspects. The aim is to have HIE-ISOLDE operating at 5.5 MeV/u by the middle of 2016.
Over the last 50 years the ISOLDE facility has gathered unique expertise in research with radioactive beams. Over 700 isotopes of more than 70 elements have been used in a wide range of research domains, from cutting edge nuclear structure studies, through atomic physics, nuclear astrophysics, fundamental interactions, to solid state and life sciences.
During LS1, three ISOLDE buildings have been demolished, and replaced with a single building with a new control room. Another building has been extended, to house the MEDICIS project. MEDICIS (MEDical Isotopes Collected from ISOLDE) will use ISOLDE’s beams and a separate target to produce radioisotopes for use in medicine.
The Low Energy Ion Ring (LEIR) receives long pulses of lead ions from Linear accelerator 3 (Linac 3) and transforms them into the short, dense bunches suitable for injection to the Large Hadron Collider. LEIR splits each long pulse from Linac 3 into four shorter bunches, then accelerates them to 4.2 - 72 MeV. The ions are passed to the Proton Synchrotron for storage, before being passed from accelerator to accelerator along the CERN complex to end up at their highest energy in the LHC. The LHC uses 592 bunches of ions per beam, so it takes around 10 minutes for LEIR to provide enough for a complete fill.
Linear accelerators use radiofrequency (RF) cavities to charge cylindrical conductors. The ions pass through the conductors, which are alternately charged positive or negative. The conductors behind them push particles and the conductors ahead of them pull, causing the particles to accelerate. Superconducting magnets ensure the protons remain in a tight beam.
The linear accelerators at CERN have swapped and changed roles over the years. Linac 1 was decommissioned in 1991, and Linac 2 supplies protons to the Proton Synchrotron Booster. Linac 2 is due to be replaced at the end of the next operational run, and so hardware changes have been kept to a minimum during the long shutdown, with the most noticeable being the introduction of a new access control system.
Linac 3 started up in 1994, providing ions to the Proton Synchrotron Booster. It now injects lead ions into the Low Energy Ion Ring, which prepares them for injection into the Large Hadron Collider. In early 2015 Linac 3 will be supplying argon ions to the fixed-target NA61 experiment, and so testing for that has taken place during the shutdown.
Linac 4 will be the replacement for Linac 2, and the long shutdown has been used for installation and testing. Linac 4 has been moved to a purpose-built tunnel close to Linac 2, ready for the next installation stage.
The neutron time-of-flight facility is a pulsed neutron source coupled to a 200-metre flight path. It is designed to study neutron-nucleus interactions for neutron energies ranging from a few MeV to several GeV. The wide energy range and high-intensity neutron beams produced at nTOF are used to make precise measurements of neutron-related processes.
To produce neutrons, a pulsed beam of protons from the Proton Synchrotron is directed at a lead target. When the beam hits, every proton yields about 300 neutrons, which are then slowed down and guided through an evacuated beam pipe to an experimental area 185 metres from the target.
Neutron time-of-flight measurements contribute in an important way to understanding nuclear data. Data produced by nTOF are used in astrophysics to study stellar evolution and supernovae. n_TOF also makes a valuable contribution to the UK’s expertise in nuclear science and technology, increasing our understanding of how fission reactions take place under different conditions, which is vital knowledge underpinning the design of next-generation nuclear power stations. n_TOF is helping to build UK expertise in the broad range of skills required to design, build, operate and decommission a nuclear power station safely and efficiently.
HiRadMat (High-Radiation to Materials) is a facility designed to provide high-intensity pulsed beams to an irradiation area where material samples and accelerator component assemblies can be tested.
Experiments in HiRadMat will contribute to the understanding of the behaviour of materials under beam impact, informing the design of the next generation of particle accelerators and spallation target stations.