page_title: Cyclotron
section_title: Principle of operation
hierarchical_section_title: Cyclotron / Principle of operation
caption_attribution_description: English: Diagram of a cyclotron, a particle accelerator invented by Earnest O. Lawrence in 1932 and widely used from the 1930s to the 1950s. It consists of a pair of "D" shaped sheet metal electrodes called "Dees" placed face to face inside a vacuum chamber, between the poles of an electromagnet. An oscillating radio frequency voltage of several thousand volts is applied to the dees. Atomic particles to be accelerated, such as protons are released in the center. The magnetic field causes them to travel in a spiral path from the center to the rim of the dees, being accelerated each time they pass from one electrode to the other. When the particles reach the rim they pass out of the dees through a small gap and strike a target. In this diagram the electromagnet pole pieces are not shown full size; they must be at least as big as the dees to create a uniform field. Caption: How the cyclotron works. Size of the magnets has been kept down to show the path of the electron
context_page_description: A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. A cyclotron accelerates charged particles outwards from the center of a flat cylindrical vacuum chamber along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying electric field. Lawrence was awarded the 1939 Nobel Prize in Physics for this invention.
  Cyclotrons were the most powerful particle accelerator technology until the 1950s when they were superseded by the synchrotron, and are still used to produce particle beams in physics and nuclear medicine. The largest single-magnet cyclotron was the 4.67 m synchrocyclotron built between 1940 and 1946 by Lawrence at the University of California, Berkeley, which could accelerate protons to 730 mega electron volts. The largest cyclotron is the 17.1 m multimagnet TRIUMF accelerator at the University of British Columbia in Vancouver, British Columbia, which can produce 500 MeV protons.
  Over 1200 cyclotrons are used in nuclear medicine worldwide for the production of radionuclides.
context_section_description: A cyclotron accelerates a charged particle beam using a high frequency alternating voltage which is applied between two hollow "D"-shaped sheet metal electrodes called "dees" inside a vacuum chamber. The dees are placed face to face with a narrow gap between them, creating a cylindrical space within them for the particles to move. The particles are injected into the center of this space. The dees are located between the poles of a large electromagnet which applies a static magnetic field B perpendicular to the electrode plane. The magnetic field causes the particles' path to bend in a circle due to the Lorentz force perpendicular to their direction of motion.
  If the particles' speeds were constant, they would travel in a circular path within the dees under the influence of the magnetic field. However a radio frequency (RF) alternating voltage of several thousand volts is applied between the dees. The voltage creates an oscillating electric field in the gap between the dees that accelerates the particles.  The frequency is set so that the particles make one circuit during a single cycle of the voltage. To achieve this, the frequency must match the particle's cyclotron resonance frequency
  ,
  where B is the magnetic field strength, q is the electric charge of the particle and m is the relativistic mass of the charged particle. Each time after the particles pass to the other dee electrode the polarity of the RF voltage reverses. Therefore, each time the particles cross the gap from one dee electrode to the other, the electric field is in the correct direction to accelerate them. The particles' increasing speed due to these pushes causes them to move in a larger radius circle with each rotation, so the particles move in a spiral path outward from the center to the rim of the dees. When they reach the rim a small voltage on a metal plate deflects the beam so it exits the dees through a small gap between them, and hits a target located at the exit point at the rim of the chamber, or leaves the cyclotron through an evacuated beam tube to hit a remote target. Various materials may be used for the target, and the nuclear reactions due to the collisions will create secondary particles which may be guided outside of the cyclotron and into instruments for analysis.
  The cyclotron was the first "cyclical" accelerator. The advantage of the cyclotron design over the existing electrostatic accelerators of the time such as the Cockcroft-Walton accelerator and Van de Graaff generator, was that in these machines the particles were only accelerated once by the voltage, so the particles' energy was equal to the accelerating voltage on the machine, which was limited by air breakdown to a few million volts. In the cyclotron, in contrast, the particles encounter the accelerating voltage many times during their spiral path, and so are accelerated many times, so the output energy can be many times the accelerating voltage.
caption: Diagram showing how a cyclotron works. The magnet's pole pieces are shown smaller than in reality; they must actually be as wide as the dees to create a uniform field.
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