Synchrotron

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The synchrotron (from synchronous , "simultaneously") is a type of particle accelerator and belongs to the ring accelerators . Charged elementary particles or ions can be accelerated to very high ( relativistic ) velocities, which gives them very high kinetic energies . Synchrotrons were developed to go beyond the energies that can be achieved with cyclotrons .

The storage ring is a special form of the synchrotron . The synchrotron itself can also be operated as a storage ring after the particles have been accelerated to a desired energy. A complete system consisting of a storage ring and a separate synchrotron for its filling is sometimes simply referred to as a synchrotron.

Electron synchrotron in Clayton near Melbourne (Australia). The storage ring and on the right in the foreground a beamline for using the synchrotron radiation are visible

Principle and structure

According to its basic construction plan, a synchrotron consists of several deflecting magnets and straight acceleration sections arranged in between, thus combining the principles of the ring accelerator and the linear accelerator. In contrast to the cyclotron or betatron , in which the particle paths are spiral-shaped, in the synchrotron they run as a closed ring from the beginning to the end of the acceleration process. The field of the deflection magnets cannot therefore remain constant over time, as in the cyclotron or betatron, but must be increased during the acceleration of each particle packet proportionally to its current particle impulse, which increases from run to run . High-frequency alternating electric fields in cavity resonators are used to accelerate the straight acceleration sections . So that the particles are not lost through collisions with gas molecules, an ultra-high vacuum (UHV) must also prevail in the ring tube in which they move - as with other accelerators .

A synchrotron does not accelerate the particles “from zero”, but is always fed by a pre-accelerator (injector). This brings them to an energy - for example 20 or 50 MeV - which in the case of electrons is already well above the rest energy of the particle, in the case of ions, however, far below it. Accordingly, electrons enter the synchrotron at almost the speed of light. There, as described by relativistic mechanics, their energy and momentum increase, but practically no longer their speed; the frequency with which the current of the magnets is modulated and the phase position of the acceleration sections relative to one another can therefore be constant. In the case of protons and even heavier particles, however, the speed in the synchrotron itself also increases considerably. During the acceleration of each particle packet, not only the magnetic field but also the phase of the high-frequency voltages of the individual resonators must be continuously adjusted .

Because of these significant technical differences, a synchrotron is always special

  • either for electrons / positrons
  • or built for protons (and possibly even heavier ions).

history

The basic concepts for the synchrotron were developed independently in Russia by Wladimir Iossifowitsch Weksler (1944 at the Lebedew Institute ) and by Edwin McMillan (during World War II in Los Alamos ). The first electron synchrotron was built in 1945 by McMillan, the first proton synchrotron in 1952 by Mark Oliphant . The discovery of the principle of strong focusing by Ernest Courant , M. Stanley Livingston and Hartland Snyder in the USA (and independently before that by Nicholas Christofilos ) led to the construction of synchrotrons around 1960 that opened up the GeV energy range: the proton was created at CERN Synchrotron and in Brookhaven the Alternating Gradient Synchrotron , both for protons in the 30 GeV range, and at about the same time at MIT and DESY electron synchrotrons with around 6 GeV.

Today (2016) the largest synchrotron facility Large Hadron Collider accelerated protons up to 6.5 TeV, so that colliding beam experiments with 13 TeV are possible. In contrast, electron synchrotrons are no longer used for particle physics, but as sources for synchrotron radiation.

Uses

Ions accelerated in synchrotrons are usually used for collision or target experiments in basic research in particle physics , in some cases also for therapeutic purposes. In contrast, electron storage rings have been used mainly as sources of synchrotron radiation since the 1980s ; Most of the synchrotron systems that exist today serve this purpose.

Accessible energies

The particle energy that can be achieved in a certain synchrotron depends on the maximum magnetic flux density B , on the radius r of the ring (here assumed to be a circle for simplicity) and on the particle properties. For high energies the following applies approximately:

.

Here q is the electric charge of the accelerated particle and c is the speed of light . No dependence on the mass of the particle can be seen in the formula ; however, the emission of synchrotron radiation was not taken into account. Lighter particles are faster than heavier particles with the same energy and therefore radiate more strongly. The loss of energy due to this radiation must be compensated for by the electrical acceleration.

Strong focus

The particles inevitably carry out vibrations (so-called betatron vibrations ) around their target path during the revolution . The amplitude of these oscillations determines the “thickness” of the beam, thus the required width of the magnetic pole pieces and thus the overall size and construction costs. Synchrotrons for higher energies therefore use the principle of strong focusing : the deflection magnets have alternating pole pieces that are bevelled on both sides, so that the magnetic fields transverse to the direction of flight of the particles have gradients with alternating directions. This results in a stabilization (focusing) of the particle trajectories. In relation to the deflection of a particle in the transverse direction, it clearly corresponds to the arrangement of converging and diverging lenses for light one behind the other, with focusing as a net effect.

In addition to changing gradients of the deflecting magnetic fields, strong focusing can also be achieved outside of the deflecting magnets with quadrupole lenses .

Electron synchrotron

The SOLEIL electron synchrotron in France
Overview scheme from SOLEIL. Inside the ring of the pre-accelerator; The synchrotron radiation is observed and used in the outer tangential arms

Because the radiation loss increases with the fourth power of the energy at relativistic speeds , electrons in the synchrotron can only be accelerated economically up to approx. 10  GeV . As an exception, electrons were brought to over 100 GeV in an experiment at the LEP facility in 1999 . It is cheaper to get electrons with more than a few GeV with linear accelerators . With the almost exclusive use of electron synchrotrons as a radiation source today, electron energies of up to about 6 GeV are used.

In the electron synchrotron, an electron gun with a hot cathode generates free electrons, which are then fed into a linear accelerator, a microtron or even a first synchrotron acceleration ring via a DC acceleration section . In this the electrons are accelerated to their final energy and then - in the case of a storage ring system - stored in a storage ring that can be up to a few hundred meters in circumference. The electrons circulate there until they are lost through collisions with any gas molecules that are still present. With modern electron synchrotrons such as BESSY or ESRF , the lifetime of the electron stream in the storage ring is a few days; however, electrons are supplied at regular intervals in order to provide a permanently sufficient ring current.

Synchrotron radiation

The intense and broadband electromagnetic radiation in the spectral range of X-ray and ultraviolet radiation was detected for the first time on electron synchrotrons, which is generated due to the deflection of very quickly charged particles and thus withdraws kinetic energy from the particles. At first, it appeared disturbing at electron synchrotrons for research in particle physics; their excellent suitability for investigations in other areas of physics and other natural sciences, but also for industrial and medical applications, was only gradually recognized. It is therefore now being produced specifically. For this purpose, the dipole magnets required to guide the particle beam are no longer used, but rather additionally built-in devices, the undulators .

Some electron synchrotron facilities

Synchrotron systems for ions

View into the tunnel of the currently (2017) most energetic synchrotron LHC at CERN near Geneva (Switzerland)

The energy that can be achieved for ions in modern synchrotrons is mainly given by the radius and magnetic flux density according to the above formula. Since the flux density that can be achieved in large magnets is limited to a few Tesla , synchrotrons must inevitably have large radii for very high energies. In the Large Hadron Collider with a radius of about 4.2 km, protons have been accelerated to 6.5 TeV (tera-electron volts), i.e. 6500 GeV.

Some ion synchrotron facilities

literature

  • F. Hinterberger: Physics of Particle Accelerators and Ion Optics . 2nd edition, Springer 2008, ISBN 978-3-540-75281-3
  • H. Wiedemann: Particle Accelerator Physics . 3rd edition, Springer 2007, ISBN 3540490434
  • Ralph Burmester: The four lives of a machine. The 500 MeV electron synchrotron at the University of Bonn. Wallstein-Verlag, Göttingen 2010, ISBN 978-3-8353-0595-3 .

Web links

Commons : Synchrotron  - collection of images, videos and audio files

Individual evidence

  1. ↑ The rest energy of an electron is 0.511 MeV, that of a proton is 938 MeV.
  2. ↑ Hinterberger (see list of literature) p. 62
  3. ↑ Hinterberger (see list of literature) p. 47