Particle accelerator – Definition and Explanations

The particle accelerators are instruments which use electric and / or magnetic fields for supplying electrically charged particles to high speeds. In other words, they communicate energy to the particles …

There are two main categories: linear accelerators and circular accelerators.

In 2004, there were more than 15,000 accelerators worldwide . Only a hundred are very large installations, national or supranational (CERN). Industrial type electrostatic machines make up more than 80% of the global fleet of industrial electron accelerators. Numerous small linear accelerators are used in medicine (anti-tumor radiotherapy).

Historical

In 1919, the physicist Ernest Rutherford (1871-1938) transformed atoms of nitrogen isotope of atom of oxygen in bombarding with particles generated by an alpha isotope natural radioactive. But the study of the atom and especially of its nucleus requires very high energies. The particles coming from natural radio-elements are too few and not very energetic to penetrate the potential barrier of the core of the heaviest elements. The potential at the nuclear surface increases from one million volts for ordinary hydrogen to 16 million foruranium . The astroparticle (cosmic rays) enabled major discoveries but their energy is very variable and must pick them up in altitude where they are less rare and more energy. In the 1920s, it became obvious that a more detailed study of the structure of matter would require more energetic and more controlled beams of particles. The source of the charged particles was varied. Discharges in gases produce ions, while for electrons it was possible to use emission by a heated wire or other systems. The energy (E) of a particle in an electric fieldcorresponds to the product of its charge (q) multiplied by the voltage (U) of the field: E = qU Thus, a first possible solution was essentially to accelerate the particles in a vacuum tube subjected to a very high voltage . The race to the million volts had begun. Several systems were proposed.

The Cockcroft-Walton generator was a voltage multiplier made of capacitors and rectifiers. It was part of an accelerator. Built in 1937 by Philips in Eindhoven. Exhibited at the London Science Museum

In England , John Cockcroft and Ernest Walton, who in 1932 accomplished the first successful decay of the nucleus by electrically accelerated particles, used a voltage multiplier with the help of a complicated assembly of rectifiers and capacitors (Greinacher assembly, 1919). Undoubtedly, one of the best ideas was developed by Robert Jemison Van de Graaff, who chose to develop a machine from the antique electrostatic . Finally, the others (such as Ernest Orlando Lawrence with his cyclotron) chose a completely different route : renouncing to obtain all at once the 10 or 20 MeVnecessary to penetrate all the nuclei Ernest Orlando Lawrence thought of reaching these energies by successive alternating electrical impulses . Periodic pulses suppose the maintenance of a certain synchronism with the accelerated particle which naturally describes a straight line at a very high speed . By using a powerful electromagnet in the air gap of which the particles are confined by the magnetic field itself, EO Lawrence simultaneously solved both problems.

The main components necessary to accelerate particles are the electric and magnetic fields and a good quality vacuum; the electric and magnetic fields are used to accelerate and direct the particles and the high vacuum allows the accelerated particles not to be slowed down following collisions with other particles present in the cylindrical tube within which the beam circulates.

The classification of particle accelerators can follow the history of the technologies used: for example, the electrostatic accelerator, “tandem” machines, linear microwave accelerators, cyclotrons (including the isochronous cyclotron and betatron), synchrotrons ( including synchrocyclotron, proton and electron synchrotrons), collision rings (electron-positron rings, proton collision rings ). Of course, each machine can be associated with the historical discoveries that they have allowed.

Accelerators can be classified according to energy:

  • low energies: from 10 to 100 MeV
  • mediumenergies: from 100 to 1000 MeV
  • high energies: more than 1 GeVand beyond TeV (Tera electronvolt = 10 12  eV ).

Other classifications are possible depending on the applications of the accelerator: industry, medicine, basic research , exploration and understanding of the elementary components of matter, energy and space and time.

Put simply, these very large machines the XX th and XXI th  centuries can be classified according to the geometry of the trajectories of acceleration: linear or circular. The fundamental character of many modern accelerators is the presence of a magnetic field winding the trajectories in the form of circles or spirals . They can be called “circular”. Others accelerate in a straight line, they are called “rectilinear or linear”.

Applications

The AGLAE particle accelerator used for the non-destructive analysis of museum pieces.

Common features

All particle accelerators consist of several successive subsets, fulfilling various functions, from source to target and in a high vacuum:

  • Production and emission of charged particles (for example thanks to a cathode): ions (proton) or electrons in general, antiparticles like antiproton and positron.
  • the injectioninto the cylindrical tube empty of air where the particles will be accelerated.
  • the actual acceleration(possibly by several successive sections), using various technical processes: continuous or alternating electric fields at high frequency .
  • guiding the beam along the accelerator using electrostatic or magnetic deflectors.
  • focusing the beam to prevent its divergence (electrostatic or magnetic lenses).
  • finally the preparation of the particle beam for its use:
    • deflectors which move the beam in the desired direction.
    • collimation system (also for medical applications).
    • particle detectors.
    • target(thick or thin), metallic intended to produce high energy X-rays (in particular for medical applications). The target can be another beam.
    • connection to another accelerator (research in particle physics).

Disciplined

The study and design of particle accelerators is an extremely rich discipline because at the confluence of many advanced physics and technologies:

  • Particle source  Atomic, ionic, plasma physics, particle-matter interaction.
  • Vacuum and ultra-vacuum techniques accelerator chamber .
  • Accelerating and transport structuresVacuum techniques, Mechanics , thermics , electromagnetism , superconductivity , HF electronics.
  • IT control , automation , BF electronics.
  • DiagnosisParticle-matter interaction, signal
  • High voltages Electrical engineering.
  • Radiation protection Nuclear physics .
  • Beamdynamics , relativity, kinematics , Hamiltonian dynamics, transport, statistics , numerical

In addition to its own physics, the astonishing variety of accelerator applications allows its physicists to rub shoulders with many communities of researchers (see previous paragraph)

The discipline, because of the gigantic projects it generates, has an international dimension . It is represented and run in France by a division at the French Physical Society and in Europe by a group at the European Physical Society. These entities, in collaboration with other foreign learned societies (USA, Russia, Japan, China, …), organize numerous conferences and workshops (conferences).

In France, the physics and technology of accelerators is taught, from master level 2, by some European universities or organizations. Let us cite, for example, a master’s degree from Paris-sud 11 university , the Joint Universities Accelerator School, or the different sessions of the CERN accelerator school.

Circular accelerators

It is the circular accelerators that hold the energy record. It is easy to see why. The energy received per meter of trajectory , that is to say the intensity of the accelerating electric field , is limited by physical and technical factors. By “winding” the trajectory, we obtain the equivalent of a straight accelerator having, not kilometers , but thousands of kilometers in length .

Among the “circulars” we first distinguish those which use a fixed magnetic field (and a massive magnet ) and where, consequently, the trajectories are spirals: these are the cyclotron (E. Lawrence, 1929) and the synchrocyclotron (designed at Berkeley in 1946). On the contrary, in synchrotrons (E. Mc Millan and V. Veksler), the magnetic field varies during acceleration, so that it takes place on an invariable circle and that the electromagnet(annular) is, for the same energy, considerably reduced. Synchrotrons are therefore, for economic reasons, the accelerators making it possible to have orbits of very large radius.

There are thus two types of circular accelerators:

  • cyclotrons:

The trajectories of the particles are spirals , consist of a single curvature magnet whose diameter can reach several meters. Historically, the cyclotron has allowed the discovery of several fundamental particles. They can accelerate charged particles, heavy ions but not electrons. In France, the GANIL (Large National Heavy Ion Accelerator) located in Caen is made up of two isochronous cyclotrons.

The synchrocyclotron electromagnet at the Orsay proton therapy center

  • synchrotrons:

Unlike the cyclotron, the magnetic field is not applied to the entire circular surface , but only to the circumference. In this type of accelerator, the particles circulate on the same almost circular trajectory inside a series of curvature magnets. Acceleration is achieved by a resonant electric field. The alternating current is applied only over the interval and not over the entire path of the particles. The more the energy increases, the more the frequency of the alternating signal applied to the interval must increase, to keep the acceleration constant. In order to keep the particles on the same trajectory, the magnetic field increases asand as the energy of the particles increases. These machines have made it possible to discover many elementary particles. One of the first synchrotrons, the Bévatron (Berkeley, 1954) was used to demonstrate the existence of the antiproton. Synchrotrons made it possible to obtain experimental proofs of fundamental elements such as quarks. They are used in current colliders. There are those which accelerate the electrons (like LEP) and those which accelerate the protons (like SPS). Today a synchrotron (even a third generation) is a very large, commonplace, shared, accessible, formative and multidisciplinary instrument . The synchrotron light (synchrotron radiation) is the object request oftimefast growing access in all countries of the world , in particular in France.

 

Accelerators have applications as varied as:

  • the nuclear physics(producing neutrons) for basic research on elementary particles in high energy;
  • the medical field, for the treatment of cancers by radiotherapy;
  • the military field, in particular for the simulation of nuclear weapons.

In fundamental physics , they are used to accelerate beams of charged particles (electrons, positrons, protons, antiprotons, ions, etc.) to make them collide and study the elementary particles generated during this collision. The energy of the particles thus accelerated is measured in electron volts (eV) but the units are often the million ( 1 MeV = 10 6  eV ), the billion electronvolts ( 1 GeV = 10 9  eV ). High energy physics (or subnuclear or elementary particles) is precisely defined from GeV and beyond.

General applications of particle accelerators
Field Methods Goals sought
Search physics Energy beams of particles Exploration of the material (seefollowing table )
Medicine Radioisotope production Imaging , scintigraphy, plotters
Medicine Irradiations: X-rays, gamma, protons, electrons, heavy ions Anti tumor radiotherapy
Electronic Electron beams Engraving of integrated circuits
Food Safety Food irradiation Sterilization
Archeology Accelerator mass spectrometry Dating

 

Application of accelerators for research
Research Methods Accelerators
Particle physics Collisions Synchrotrons, proton or electron colliders
Nuclear physics Core-core collisions Heavy ion accelerators: synchrotron , cyclotron , Tandem, Linac
Atomic physics Atomic collisions Heavy ion accelerators: synchrotron, cyclotron, Tandem, Linac
Condensed and physical matter of surfaces (structure of matter, magnetic, chemical and electronic properties of materials) Diffraction , imaging, absorption spectroscopies, circular magnetic dichroism, photoemission spectroscopies, Synchrotron radiation (IR, UV, soft X, hard X)
Condensed matter (structure and magnetic properties) Dissemination of neutrons Proton linac
Biology , chemistry Crystallography of proteins, viruses , activation , chemical and biochemical kinetics Synchrotron radiation ,free electron laser
Materials physics Activation analysis, mass spectrometry Van de Graaff Tandem

Livingston’s diagram

Stanley Livingston, a physicist specializing in particle accelerators, established this diagram in the 1960s. It shows the exponential growth of the energy of accelerated beams.
This classic diagram has been modified  : the horizontal axis has been extended to the years 2010. The vertical axis has been extended to 100,000 TeV . To compare the different accelerators, the energy of the colliders, which is expressed in the center of mass, has been recalculated as if the energy of the particles observed was the result of a collision with a proton at rest. The cost per eV of beam energy is reduced by a factor of 1000 per 7-year period.
In the past , we gained a factor of 10 every 7-8 years in the energy of the collisions made. If the trend had been maintained, we would have reached 60 TeV by 2005. The LHC ( Large Hadron Collider , 7 TeV + 7 TeV , CERN, 2008) therefore does not follow the extrapolation. There is a marked decline in performance which perhaps indicates a first sign of fatigue in the discipline.

Common features

All particle accelerators consist of several successive subsets, fulfilling various functions, from source to target and in a high vacuum:

  • Production and emission of charged particles (for example thanks to a cathode): ions (proton) or electrons in general, antiparticles like antiproton and positron.
  • the injectioninto the cylindrical tube empty of air where the particles will be accelerated.
  • the actual acceleration(possibly by several successive sections), using various technical processes: continuous or alternating electric fields at high frequency .
  • guiding the beam along the accelerator using electrostatic or magnetic deflectors.
  • focusing the beam to prevent its divergence (electrostatic or magnetic lenses).
  • finally the preparation of the particle beam for its use:
    • deflectors which move the beam in the desired direction.
    • collimation system (also for medical applications).
    • particle detectors.
    • target(thick or thin), metallic intended to produce high energy X-rays (in particular for medical applications). The target can be another beam.
    • connection to another accelerator (research in particle physics).

Disciplined

The study and design of particle accelerators is an extremely rich discipline because at the confluence of many advanced physics and technologies:

  • Particle source  Atomic, ionic, plasma physics, particle-matter interaction.
  • Vacuum and ultra-vacuum techniques accelerator chamber .
  • Accelerating and transport structuresVacuum techniques, Mechanics , thermics , electromagnetism , superconductivity , HF electronics.
  • IT control , automation , BF electronics.
  • DiagnosisParticle-matter interaction, signal
  • High voltages Electrical engineering.
  • Radiation protection Nuclear physics .
  • Beamdynamics , relativity, kinematics , Hamiltonian dynamics, transport, statistics , numerical

In addition to its own physics, the astonishing variety of accelerator applications allows its physicists to rub shoulders with many communities of researchers (see previous paragraph)

The discipline, because of the gigantic projects it generates, has an international dimension . It is represented and run in France by a division at the French Physical Society and in Europe by a group at the European Physical Society. These entities, in collaboration with other foreign learned societies (USA, Russia, Japan, China, …), organize numerous conferences and workshops (conferences).

In France, the physics and technology of accelerators is taught, from master level 2, by some European universities or organizations. Let us cite, for example, a master’s degree from Paris-sud 11 university , the Joint Universities Accelerator School, or the different sessions of the CERN accelerator school.

Circular accelerators

It is the circular accelerators that hold the energy record. It is easy to see why. The energy received per meter of trajectory , that is to say the intensity of the accelerating electric field , is limited by physical and technical factors. By “winding” the trajectory, we obtain the equivalent of a straight accelerator having, not kilometers , but thousands of kilometers in length .

Among the “circulars” we first distinguish those which use a fixed magnetic field (and a massive magnet ) and where, consequently, the trajectories are spirals: these are the cyclotron (E. Lawrence, 1929) and the synchrocyclotron (designed at Berkeley in 1946). On the contrary, in synchrotrons (E. Mc Millan and V. Veksler), the magnetic field varies during acceleration, so that it takes place on an invariable circle and that the electromagnet(annular) is, for the same energy, considerably reduced. Synchrotrons are therefore, for economic reasons, the accelerators making it possible to have orbits of very large radius.

There are thus two types of circular accelerators:

  • cyclotrons:

The trajectories of the particles are spirals , consist of a single curvature magnet whose diameter can reach several meters. Historically, the cyclotron has allowed the discovery of several fundamental particles. They can accelerate charged particles, heavy ions but not electrons. In France, the GANIL (Large National Heavy Ion Accelerator) located in Caen is made up of two isochronous cyclotrons.

The synchrocyclotron electromagnet at the Orsay proton therapy center

  • synchrotrons:

Unlike the cyclotron, the magnetic field is not applied to the entire circular surface , but only to the circumference. In this type of accelerator, the particles circulate on the same almost circular trajectory inside a series of curvature magnets. Acceleration is achieved by a resonant electric field. The alternating current is applied only over the interval and not over the entire path of the particles. The more the energy increases, the more the frequency of the alternating signal applied to the interval must increase, to keep the acceleration constant. In order to keep the particles on the same trajectory, the magnetic field increases asand as the energy of the particles increases. These machines have made it possible to discover many elementary particles. One of the first synchrotrons, the Bévatron (Berkeley, 1954) was used to demonstrate the existence of the antiproton. Synchrotrons made it possible to obtain experimental proofs of fundamental elements such as quarks. They are used in current colliders. There are those which accelerate the electrons (like LEP) and those which accelerate the protons (like SPS). Today a synchrotron (even a third generation) is a very large, commonplace, shared, accessible, formative and multidisciplinary instrument . The synchrotron light (synchrotron radiation) is the object request oftimefast growing access in all countries of the world , in particular in Fra

 

 

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