The First Cyclotron

The opportunities available in American physics expanded during the 1920s. The center of gravity in American physics had long been in the east. But after World War I, administrators at Caltech and at the University of California at Berkeley decided to foster research in general and physics in particular, and backed up their intentions in budgets.

By the early 1930s their efforts had paid off, and the president of the American Physical Society referred to California as "one of the two foci in the academic ellipse representing American physics." Berkeley and other universities engaged in a bidding war for Lawrence, whose work on photoelectricity and ionization marked him as an up-and-coming experimenter. . Yale held off Berkeley by offering Lawrence an appointment as assistant professor in 1927, bypassing the normal entry level of instructor. A year later, after Swann left Yale, Lawrence accepted an upgraded Berkeley offer of an associate professorship with an ample research budget and a light teaching load.

"I felt out one of the most brilliant experimental young men in the East—a lad whose name is on everyone's lips on account of his recent papers on Ionizing Potentials."

—Leonard Loeb, Berkeley physicist-recruiter's view on Lawrence

Lawrence arrived in Berkeley in the summer of 1928 and adapted easily to his new surroundings. Within a few months, his friend Beams judged him "a ‘Native Son' of California." Continuing his research on the action of light on electrons, Lawrence spent the summers of 1929 and 1930 in Schenectady, New York, where the General Electric Company had a major industrial research lab staffed with physical scientists and engineers. At G.E. he found his host, physicist A.W. Hull, an expert on X-rays, diverted by work on high electric voltages. At that time, many academic physicists were also starting to work on high voltages, although for different reasons than G.E. scientists. High-voltage accelerators promised breakthroughs in the study of radioactive particles and atomic nuclei. An attempt in 1930 by Northwestern University to lure Lawrence away led Berkeley to promote him to full professor at the age of 29.

The leading expert on radioactivity, professor Ernest Rutherford at Cambridge University, had long used atomic particles as projectiles to study the structure of matter. In 1911, by bombarding atoms with alpha particles (helium ions), he found that the atom has inside it a small and heavy nucleus. In 1919 Rutherford managed to bombard the nucleus itself. By absorbing an alpha particle, the nucleus of nitrogen transformed into a nucleus of oxygen and emitted a proton. The old dream of medieval alchemists, the transmutation of chemical elements, had been achieved.

The particles provided by natural radioactive materials, however, were too few and had too little energy for a full range of experiments in nuclear physics. The few million volts of energy in the alpha particles could barely push them past the electrical barrier of the nucleus. In 1927 Rutherford urged physicists to find the means to "a copious supply" of particles more energetic than the alpha and beta particles available from natural radioactive sources. A number of researchers responded to his call.

At Rutherford's Cavendish Laboratory in Cambridge, England, John Cockcroft and E.T.S. Walton produced such particles artificially by accelerating protons, with a high-voltage transformer and then with a voltage multiplier. Robert Van de Graaff, an NRC postdoc at Princeton and then MIT, built an electrostatic high-voltage generator. Merle Tuve, Lawrence's old friend, now working at the Carnegie Institution of Washington, experimented with a Tesla induction coil, then with a Van de Graaff generator and reached a million volts.

Lawrence was acquainted with the problem of high-voltage acceleration through Tuve's work. In spring 1929 he got an idea for a new way of accelerating particles. He was browsing a German electrical engineering journal, not the most common reading for a physicist. A Norwegian engineer, Rolf Wideröe, had published a sketch of a device that allowed one to use the same electrical potential twice, doubling the energy by switching from positive to negative potential in order to push ions and then to pull them.

Lawrence judged the linear scheme impractical for light atomic particles, since it would require a vacuum tube several meters long. But it inspired him to think about how one could use the same potential multiple times instead of just once. Lawrence thought to use a magnetic field to bend charged particles into circular trajectories and thus pass them through the same accelerating region over and over again. The idea required a combination of sophisticated techniques: a high-vacuum chamber with electric fields varying at radio frequencies and with some means to keep the particles in a single horizontal plane.

[You can EXIT to see Lawrence and Livingston's original 1932 paper describing the cyclotron.]

Seeing the difficulties, Lawrence hesitated to build a device and instead set out for his summer sojourn at General Electric. Otto Stern, visiting Berkeley from Germany toward the end of the year, encouraged him to pursue his idea, and Tuve's successes in Washington with the Tesla coil further spurred Lawrence, lest he lose out in the race to high energies.

Lawrence started to construct a cyclotron, as the machine later was named, in early 1930. A graduate student, M. Stanley Livingston, did much of the work of translating the idea into working hardware. In January 1931 Lawrence and Livingston met their first success. A device about 4.5 inches in diameter used a potential of 1,800 volts to accelerate hydrogen ions up to energies of 80,000 electron volts. Lawrence immediately started planning for a bigger machine. In summer 1931 an eleven-inch cyclotron achieved a million volts.

"Dr Livingston has asked me to advise you that he has obtained 1,100,000 volt protons. He also suggested that I add ‘Whoopee'!"

—Telegram to Lawrence, 3 August 1931

Click here to learn more on early particle accelerators and first cyclotrons

Next: The Big Cyclotrons

Also on this page:

Exhibit Contents

© 2002 - American Institute of Physics