Cyclotron

A cyclotron is a type of circular particle accelerator designed to accelerate charged particles along a spiral path using a combination of static magnetic fields and rapidly alternating electric fields. Invented between 1929 and 1930 at the University of California, Berkeley, it became the first practical cyclical accelerator and marked a major advancement in experimental nuclear physics. Unlike early electrostatic accelerators, which allowed particles to cross the accelerating region only once, the cyclotron enabled particles to gain energy repeatedly, producing beams of much higher final energy. This innovation established the cyclotron as the dominant accelerator technology until the mid-twentieth century and earned Ernest O. Lawrence the 1939 Nobel Prize in Physics.

Principles of Operation

The cyclotron consists of a flat, cylindrical vacuum chamber positioned between the poles of a powerful electromagnet. The magnet provides a uniform static magnetic field perpendicular to the plane of the chamber, compelling charged particles into circular orbits. Within this chamber are two hollow, semicircular electrodes known as “dees”, positioned back-to-back with a narrow gap between them. An alternating electric field applied across the dees accelerates the particles each time they cross the gap.
As the particles spiral outward due to increasing kinetic energy, their orbital radius enlarges. They repeatedly pass through the accelerating region, gaining energy on each traversal. This cyclical approach allows for energies far greater than those achievable using a single acceleration step. Eventually, the beam is extracted near the outer edge of the chamber and directed toward experimental targets or medical treatment systems.
By holding the magnetic field constant and maintaining the resonance between orbital frequency and the applied radiofrequency electric field, classical cyclotrons allow continuous acceleration until relativistic effects become significant. At relativistic speeds, particle mass increases, causing the cyclotron resonance condition to fail, an issue later addressed by advanced designs such as synchrocyclotrons and isochronous cyclotrons.

Early Development and Innovation

The concept of a circular accelerator had been considered earlier by several European physicists. Max Steenbeck formulated a cyclotron-like idea in 1927, and Leo Szilárd filed patent applications in 1928–1929 that discussed resonance in circular accelerating devices. However, these early efforts were never published and thus had limited influence on later developments.
Ernest Lawrence independently conceived the cyclotron in 1929 after reading about linear drift-tube accelerators. With the assistance of graduate student M. Stanley Livingston, he constructed the first functioning model, which produced an 80-keV proton beam in early January 1931. His early machines, built from repurposed arc converter magnets, laid the groundwork for larger devices developed throughout the 1930s at the Berkeley Radiation Laboratory. Subsequent cyclotrons at Berkeley achieved significantly higher energies, including 4.8 MeV (1932), 8 MeV (1937), and 16 MeV (1939) accelerators, each setting new records for beam energy at the time.
Cyclotron construction quickly spread internationally. The first European cyclotron was developed in 1934 at the Leningrad Physico-Technical Institute, reaching 530-keV proton energies. A larger 32-MeV machine became operational in Leningrad in 1937. In Asia, the first major cyclotron was built at the Riken Institute in Tokyo, achieving a 29-MeV deuteron beam in 1937.

Applications During the Second World War

During the 1940s, cyclotrons acquired strategic significance. At Berkeley, the 60-inch cyclotron was crucial in discovering neptunium (1940) and plutonium (1941). Lawrence’s invention of the calutron, derived from cyclotron technology, became central to uranium enrichment during the Manhattan Project. These electromagnetic separation devices at the Y-12 plant were responsible for producing the first quantities of highly enriched uranium used in the Little Boy atomic weapon.
Elsewhere, France’s large 7-MeV cyclotron, constructed by Frédéric Joliot-Curie in Paris, was sabotaged following the German occupation to prevent its use in nuclear research. In Germany, a cyclotron under construction in Heidelberg encountered delays due to wartime constraints and was not operational until after the war. In Japan, the large Riken cyclotron was dismantled by American occupation forces and disposed of in Tokyo Bay due to concerns about its potential military use.

Post-War Advances

The classical cyclotron design faced an inherent limitation: as particles approached relativistic speeds, their increasing effective mass altered the orbital frequency, disrupting synchronisation with the fixed-frequency accelerating field. To overcome this, two main approaches were developed.

• Synchrocyclotrons maintain a constant magnetic field while modulating the radiofrequency to match the changing orbital frequency of relativistic particles. Lawrence’s group built one of the earliest such machines in 1946, achieving proton energies up to 350 MeV. However, synchrocyclotrons operate in a pulsed mode and generally produce lower beam intensities.
• Isochronous cyclotrons instead vary the magnetic field strength with radius to maintain a constant orbital frequency. This allows continuous-wave operation at higher energies and significantly greater beam currents, making them suitable for medical and industrial use.

These advances preserved the cyclotron’s relevance for research and applied science even as larger synchrotrons emerged as the leading machines for frontier high-energy physics.

Modern Uses and Significance

Although surpassed by synchrotrons in maximum achievable energy, cyclotrons remain indispensable for numerous scientific, medical, and industrial applications. As of 2020, nearly 1,500 cyclotrons operate worldwide to produce radionuclides used in nuclear medicine, including isotopes such as fluorine-18 for positron emission tomography. Their relatively compact size and ability to generate high-intensity beams make them crucial in the production of radiopharmaceuticals.
Cyclotrons are also essential in particle therapy, where proton or heavy-ion beams are directed at tumours with high precision. Their stable, high-current operation and comparatively small footprint make them suitable for hospital-based treatment facilities.
In research settings, cyclotrons continue to support studies in nuclear structure, material science, and radiation effects. Their versatility, operational efficiency, and long service lifetimes ensure their continued importance in both fundamental physics and practical technology.