Cyclotron Info

History of the Cyclotron

The cyclotron went from a sketch to the most important instrument in nuclear physics in under a decade, hit a wall relativity built, escaped it twice, and ended up in hospital basements around the world. Dates and figures below follow the standard histories cited at the end.

1929–1931: the idea and the 4-inch

In early 1929, Ernest Lawrence — a 27-year-old professor at Berkeley — was browsing the journals when he came across Rolf Wideröe's 1928 paper demonstrating resonance acceleration in a linear machine. Lawrence couldn't read the German easily, but the diagrams were enough: repeated small pushes could replace one impossible voltage. His addition was the magnetic field that bends the path into a spiral so a single gap can do all the pushing — and the crucial realization that the orbit period is independent of speed, so one fixed frequency serves every turn. He announced the concept, with a preliminary experiment by N. E. Edlefsen, in Science in September 1930.

The proof fell to his graduate student M. Stanley Livingston, whose thesis apparatus — a brass-and-sealing-wax chamber about 4 inches across, one dee, a borrowed magnet — produced 80 keV hydrogen molecular ions on 2 January 1931 with less than 1 kV on the electrode: eighty-fold voltage multiplication, exactly as the resonance principle promised (Lawrence's Nobel lecture recounts the numbers).

1932: the MeV barrier — and a race lost

The next machine, with 11-inch poles, made history: in January 1932 it delivered 1.22 MeV protons — the first artificially accelerated particles past a million volts. The paper describing it (Lawrence & Livingston, Phys. Rev. 40, 19, 1932) is still a remarkably readable account of the whole principle, including a correct analysis of the electric and magnetic focusing that keeps the beam alive. But Berkeley was slow to put a target in the beam, and in April 1932 Cockcroft and Walton at the Cavendish split the lithium nucleus first — with a 600 keV DC cascade generator. The lesson, taught early: the accelerator is only half the experiment.

1933–1939: bigger poles, new elements, first patients

Lawrence's pattern was set — find a bigger magnet. A surplus 80-ton Federal Telegraph magnet became the 27-inch cyclotron in 1933, producing 20 µA of 6.3 MeV deuterons by 1936; re-poled to 37 inches in 1937–38 it reached 8.5 MeV at 100 µA and became the workhorse of 1930s nuclear science. After the Joliot-Curies discovered artificial radioactivity in 1934, the cyclotron became a radioisotope factory: tracers for chemistry, botany, and medicine, with Lawrence's physician brother John leading the first therapeutic uses. By 1938 more isotopes had been created artificially than were known in nature (Craddock 2010).

The 60-inch cyclotron at the Crocker Laboratory, explicitly built for medical and isotope work, came online in June 1939 delivering 16 MeV deuterons (its initial performance is reported in Phys. Rev. 56, 124, 1939). It went on to produce the first transuranic elements — neptunium and plutonium among six discovered there — and supported an early neutron-therapy program that treated roughly 200 patients. The design spread fast: by 1940 there were 24 cyclotrons operating in the United States, 3 in Japan, 2 each in Britain and the USSR, and one each in Denmark, France, Germany, and Sweden (Craddock 2010). In November 1939 Lawrence received the Nobel Prize in Physics for the invention and development of the cyclotron.

1940–1946: the wall, the war, and the synchrocyclotron

Bethe and Rose had shown in 1937 that relativity caps a fixed-frequency cyclotron around a few tens of MeV. Lawrence pressed ahead anyway: in 1940 the Rockefeller Foundation granted $1.15 million for a monster 184-inch machine, on the hope that brute dee voltage could outrun the phase slip. The war intervened — the huge magnet spent the war developing electromagnetic uranium separation — and by the time physics resumed, the problem had been solved outright: Veksler (1944) and McMillan (1945) independently discovered phase stability, and a 1946 test by MacKenzie and colleagues on the old 37-inch confirmed that sweeping the RF frequency could carry ions past the relativistic limit.

Rebuilt as a synchrocyclotron, the 184-inch — its magnet roughly 4,000 tonnes — produced its first beam on 1 November 1946: 190 MeV deuterons, then 380 MeV alphas and later 350 MeV protons, an order-of-magnitude leap in energy. In 1948 it made the first artificially produced pions. Ten more large synchrocyclotrons followed worldwide between 1946 and 1954 — Harvard, Columbia, Chicago, Liverpool, and others, the largest at Dubna ultimately reaching 680 MeV — and carried meson physics until synchrotrons took over the energy frontier. Their weakness was duty cycle: a pulsed trickle of beam, at orders of magnitude less average current than a fixed-frequency machine.

1950s–1970s: the isochronous era

The better escape from the relativistic wall had been published back in 1938, and ignored. L. H. Thomas showed that a field which rises with radius (staying isochronous at any γ) can still focus vertically if it also varies azimuthally — ridged "hill and valley" poles making the orbit scallop through alternating field sectors. The idea waited twelve years; in 1950 Richardson and colleagues at Berkeley quietly built two electron models proving it worked (the results stayed classified until 1956), and Kerst's 1955 spiral-sector suggestion strengthened the focusing further. The first sector-focused ion cyclotron ran at Delft in 1958 — four sectors, 86 cm poles, 12.7 MeV protons (Heyn & Khoe; Craddock 2010) — and the floodgates opened.

Isochronous machines deliver continuous beam at high current with no RF gymnastics, and they became the standard so completely that "cyclotron" now effectively means "AVF cyclotron." The era peaked with the separated-sector giants: TRIUMF's 520 MeV machine in Vancouver — at 18 m diameter still the world's largest cyclotron — produced its first full-energy beam in December 1974, and PSI's 590 MeV ring cyclotron in Switzerland, commissioned the same year, has since been upgraded into the most powerful proton beam of any cyclotron, above a megawatt.

1982–present: superconducting and compact

Superconducting coils rewrote the size equation: several times the field of iron-limited magnets means an order of magnitude less weight for the same beam energy. Michigan State's K500 — the world's first superconducting cyclotron — extracted first beam on 31 August 1982, followed by the K1200 and superconducting machines at Texas A&M, Catania, and Kolkata. At the research frontier, RIKEN's Superconducting Ring Cyclotron (first beam December 2006), the final booster of the RI Beam Factory, holds the record as the world's most powerful heavy-ion cyclotron.

The quieter revolution happened in hospitals. Compact isochronous machines of 10–30 MeV, sold by numerous vendors as turnkey isotope factories, number well over a thousand worldwide today (the IAEA maintains a database of them), producing the short-lived PET and SPECT isotopes that modern imaging runs on. Superconducting compactness reached the clinic too: 230–250 MeV proton-therapy cyclotrons and synchrocyclotrons — some small enough to mount on a rotating gantry — now do work that in 1946 took a 4,000-tonne magnet. Ninety-odd years on, Lawrence's spiral is a mature industrial product; the physics on the previous page is unchanged since 1932, which is precisely why people still build them at home.

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