Cyclotron

[3][4] A cyclotron accelerates charged particles outwards from the center of a flat cylindrical vacuum chamber along a spiral path.

[10] In 1927, while a student at Kiel, German physicist Max Steenbeck was the first to formulate the concept of the cyclotron, but he was discouraged from pursuing the idea further.

[11] In late 1928 and early 1929, Hungarian physicist Leo Szilárd filed patent applications in Germany for the linear accelerator, cyclotron, and betatron.

[12] In these applications, Szilárd became the first person to discuss the resonance condition (what is now called the cyclotron frequency) for a circular accelerating apparatus.

[13] Several months later, in the early summer of 1929, Ernest Lawrence independently conceived the cyclotron concept after reading a paper by Rolf Widerøe describing a drift tube accelerator.

[18][19] To construct the first such device, Lawrence used large electromagnets recycled from obsolete arc converters provided by the Federal Telegraph Company.

It was a small design based a prototype by Lawrence, with a 28 cm diameter capable of achieving 530 keV proton energies.

This instrument was first proposed in 1932 by George Gamow and Lev Mysovskii [ru] and was installed and became operative in March 1937 at 100 cm (39 in) diameter and 3.2 MeV proton energies.

[24][25][26] The first Asian cyclotron was constructed at the Riken laboratory in Tokyo, by a team including Yoshio Nishina, Sukeo Watanabe, Tameichi Yasaki, and Ryokichi Sagane.

This was the first production of HEU in history, and was shipped to Los Alamos and used in the Little Boy bomb dropped on Hiroshima, and its precursor Water Boiler and Dragon test reactors.

However, it is likely that Joliot, a member of French Communist Party and in fact president of the National Front resistance movement, sabotaged the cyclotron to prevent its use to the Nazi German nuclear program.

[32][33] In Nazi Germany, one cyclotron was built in Heidelberg, under the supervision of Walther Bothe and Wolfgang Gentner, with support from the Heereswaffenamt.

At the end of 1938, Gentner was sent to Berkeley Radiation Laboratory and worked most closely with Emilio Segrè and Donald Cooksey, returning before the start of the war.

[34][35][36] In Japan, the large Riken cyclotron was used to bombard uranium processed in their Clusius tube gaseous diffusion device.

[37] Following the occupation of Japan, American forces, fearing continuation of the Japanese nuclear weapons program, dissembled the Riken laboratory's cyclotron and dumped it in Tokyo Bay.

[27] By the late 1930s it had become clear that there was a practical limit on the beam energy that could be achieved with the traditional cyclotron design, due to the effects of special relativity.

[38] As particles reach relativistic speeds, their effective mass increases, which causes the resonant frequency for a given magnetic field to change.

To address this issue and reach higher beam energies using cyclotrons, two primary approaches were taken, synchrocyclotrons (which hold the magnetic field constant, but decrease the accelerating frequency) and isochronous cyclotrons (which hold the accelerating frequency constant, but alter the magnetic field).

[44] A cyclotron, by contrast, uses a magnetic field to bend the particle trajectories into a spiral, thus allowing the same gap to be used many times to accelerate a single bunch.

[45] However, given the typically high number of revolutions, it is usually simpler to estimate the energy by combining the equation for frequency in circular motion:

[46] While the trajectory followed by a particle in the cyclotron is conventionally referred to as a "spiral", it is more accurately described as a series of arcs of constant radius.

[47]: ch.2.1.3  Failure of the particle to be injected with phase difference within about ±20° from the optimum may make its acceleration too slow and its stay in the cyclotron too long.

As a consequence, half-way through the process the phase difference escapes the 0–180° range, the acceleration turns into deceleration, and the particle fails to reach the target energy.

In contrast to this approximation, as particles approach the speed of light, the cyclotron frequency decreases due to the change in relativistic mass.

Keeping the frequency constant allows isochronous cyclotrons to operate in a continuous mode, which makes them capable of producing much greater beam current than synchrocyclotrons.

[63] The first suggestion that energetic protons could be an effective treatment method was made by Robert R. Wilson in a paper published in 1946[64] while he was involved in the design of the Harvard Cyclotron Laboratory.

As of 2020, there were approximately 80 facilities worldwide for radiotherapy using beams of protons and heavy ions, consisting of a mixture of cyclotrons and synchrotrons.

If the particles become fast enough that relativistic effects become important, the beam becomes out of phase with the oscillating electric field, and cannot receive any additional acceleration.

[40]: 6 The spiraling of electrons in a cylindrical vacuum chamber within a transverse magnetic field is also employed in the magnetron, a device for producing high frequency radio waves (microwaves).

In the magnetron, electrons are bent into a circular path by a magnetic field, and their motion is used to excite resonant cavities, producing electromagnetic radiation.

Lawrence's 60-inch (152 cm) cyclotron, c. 1939 , showing the beam of accelerated ions (likely protons or deuterons ) exiting the machine and ionizing the surrounding air causing a blue glow
Lawrence's original 4.5-inch (11 cm) cyclotron
Lawrence's 60-inch (150 cm) cyclotron at Lawrence Radiation Laboratory , University of California , Berkeley, California, constructed in 1939. The magnet is on the left, with the vacuum chamber between its pole pieces, and the beamline which analyzed the particles is on the right.
Diagram of a cyclotron. The magnet's pole pieces are shown smaller than in reality; they must actually be at least as wide as the accelerating electrodes ("dees") to create a uniform field.
Diagram of cyclotron operation from Lawrence's 1934 patent. The hollow, open-faced D-shaped electrodes (left), known as dees, are enclosed in a flat vacuum chamber which is installed in a narrow gap between the two poles of a large magnet (right).
Vacuum chamber of Lawrence 69 cm (27 in) 1932 cyclotron with cover removed, showing the dees. The 13,000 V RF accelerating potential at about 27 MHz is applied to the dees by the two feedlines visible at top right. The beam emerges from the dees and strikes the target in the chamber at bottom.
The trajectory followed by a particle in the cyclotron approximated with a Fermat's spiral
In isochronous cyclotrons, the magnetic field strength B as a function of the radius r has the same shape as the Lorentz factor γ as a function of the speed v .
A French cyclotron, produced in Zürich , Switzerland in 1937. The vacuum chamber containing the dees (at left) has been removed from the magnet (red, at right) .
A modern cyclotron used for radiation therapy . The magnet is painted yellow.
M. Stanley Livingston and Ernest O. Lawrence (right) in front of Lawrence's 69 cm (27 in) cyclotron at the Lawrence Radiation Laboratory. The curving metal frame is the magnet's core, the large cylindrical boxes contain the coils of wire that generate the magnetic field. The vacuum chamber containing the "dee" electrodes is in the center between the magnet's poles.