Ernest Orlando Lawrence’s development of the first successful magnetic resonance accelerator for protons marked the beginning of the modern era of particle accelerators.
The invention of the cyclotron by Ernest Orlando Lawrence was a first step toward the modern era of high-energy physics. Although the energies have increased steadily, the principles incorporated in the cyclotron have been fundamental to succeeding generations of accelerators, many of which were also developed in Lawrence’s laboratory. The care and support of such machines have also given rise to “big science”: the massing of scientists, money, and machines in support of experiments to discover the nature of the atom and its constituents.
Ernest Orlando Lawrence.
Lawrence received his undergraduate degree in physics at the University of South Dakota and studied under W. F. G. Swann, an expert in electromagnetic theory and experiment at the Universities of Minnesota and Chicago and at Yale University, where he received his Ph.D. in 1925. Lawrence was an assistant professor until 1928, when he was lured to the University of California by an associate professorship, the opportunity to teach more graduate courses, and the university’s new physics research laboratory.
At the University of California, Lawrence took an interest in the new physics of the atomic nucleus,
In order to explore the nucleus of the atom, however, suitable probes were required. Rutherford had used alpha particles ejected from radioactive substances to make his early studies, but these were not energetic enough to penetrate the nuclei of most atoms. An artificial means of accelerating ions to high energies was needed. During the late 1920’s, a variety of means were tried to accelerate alpha particles, protons (hydrogen ions), and electrons, but none had been successful in causing a nuclear transformation when Lawrence entered the field. The high voltages required stressed the resources available to physicists. It was believed that more than one million volts would be required to accelerate an ion to sufficient energies to penetrate even the lightest atomic nuclei. At such voltages, insulators broke down, releasing sparks across great distances. European researchers even attempted to harness lightning to the task, with fatal results.
Early in April, 1929, Lawrence chanced on an article in Archiv für Electrotechnik by Rolf Wideroe,
Lawrence knew that a magnetic field would cause the ions to be deflected and form a curved path. If the electrodes were placed across the diameter of the circle formed by the ions’ path, they should spiral out as they were accelerated, staying in phase with the accelerating charge until they reached the periphery of the magnetic field. This, he reasoned, would afford a means of producing indefinitely high voltages without using high voltages by recycling the accelerated ions through the same electrodes. Many doubted that such a method would be effective. No mechanism was known that would keep the circulating ions in sufficiently tight orbits to avoid collisions with the walls of the accelerating chamber. Others tried to use resonance acceleration without success. Lawrence waited nearly a year before encouragement from Otto Stern, a visitor in the University of California physics department from the University of Hamburg, persuaded him to try the method.
In the spring of 1930, Lawrence put one of his graduate students, Niels Edlefsen, to work on reducing his idea to practice. Edlefsen used glass flasks coated with silver or copper with a diametrical gap to serve as their electrodes. A filament introduced through one aperture in the flask produced protons from hydrogen introduced through another. When the flask was placed between the poles of a 10-centimeter (3.9-inch) electromagnet and radio frequency current was applied to the metallic electrodes, Edlefsen thought he saw evidence of particle acceleration. Unfortunately, he left Berkeley before this could be confirmed.
Another graduate student, M. Stanley Livingston, took up the project. He decided quickly that resonance could not be achieved with Edlefsen’s apparatus. For his dissertation project, he used a brass cylinder 10 centimeters in diameter sealed with wax to hold a vacuum, a half pillbox of copper mounted on an insulated stem to serve as the electrode, and a Hartley radio frequency oscillator producing 10 watts. The shape resembled the letter D. The box itself constituted the other electrode: A brass bar was placed across it parallel to the straight side of the D with slots corresponding to those in the D. The hydrogen molecular ions were produced by a thermionic cathode mounted near the center of the apparatus from hydrogen gas admitted through an aperture in the side of the cylinder after a vacuum had been produced by a pump. Once formed, the oscillating electrical field drew them out and accelerated them as they passed through the gap between the bar and the D. The accelerated ions spiraled out in a magnetic field produced by a 10-centimeter electromagnet to a collector. By November, 1930, Livingston observed peaks in the collector current as he tuned the magnetic field through the value calculated to produce acceleration.
Borrowing a stronger magnet and tuning his radio frequency oscillator appropriately, Livingston produced 80,000-electronvolt ions at his collector on January 2, 1931, thus demonstrating the principle of magnetic resonance acceleration.
Demonstration of the principle led to a succession of large cyclotrons, beginning with a 25-centimeter (9.8-inch) cyclotron developed in the spring and summer of 1931, which produced million-electronvolt protons. Lawrence succeeded in winning support for his device from Frederick Cottrell’s Research Corporation and the Chemical Foundation, which were interested in its applications to the production of high-voltage X rays. Lawrence also developed a linear accelerator for heavy ions with the help of David Sloan, another of his graduate students. Sloan built a million-volt X-ray tube as well. With the support of the Research Corporation, Lawrence was able to secure a large electromagnet that had been developed for radio transmission and a disused civil engineering laboratory to house it. This was the Radiation Laboratory.
The 69-centimeter cyclotron was enlarged to 94 centimeters (37 inches) in diameter in 1937. By 1939, Lawrence had built a 152-centimeter (59.8-inch) cyclotron for medical uses, including therapy with neutron beams. In that year, he won the Nobel Prize in Physics for the invention of the cyclotron and the production of radioisotopes.
The cyclotron and the Radiation Laboratory were the center of accelerator physics throughout the 1930’s and well into the postwar era. The invention of the cyclotron not only provided a new tool for probing the nucleus but also gave rise to new forms of organizing scientific work and to applications in nuclear medicine and nuclear chemistry. Cyclotrons were built in many laboratories in the United States, Europe, and Japan, and became standard tools of nuclear physics.
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Childs, Herbert. An American Genius: The Life of Ernest Orlando Lawrence, Father of the Cyclotron. New York: E. P. Dutton, 1968. Details the life of the cyclotron’s inventor in anecdotes, relying heavily on interviews with his colleagues. Includes photographs, bibliography, and index. -
Heilbron, J. L., and Robert W. Seidel. Lawrence and His Laboratory: A History of Lawrence Berkeley Laboratory. Berkeley: University of California Press, 1989. Describes in detail the development of accelerator technology in the 1920’s and 1930’s and the impact of these developments on nuclear physics up to World War II. Includes illustrations, bibliography, and index. -
Lee, S. Y. Accelerator Physics. 2d ed. Hackensack, N.J.: World Scientific Publishing, 2004. Text intended for college physics majors or graduate students begins with a chapter on the history of the development of particle accelerators. -
Livingston, M. Stanley. Particle Accelerators: A Brief History. Cambridge, Mass.: Harvard University Press, 1969. Describes for the general reader the early attempts to produce high voltages for nuclear physics, the invention of the cyclotron and the electrostatic generator, the development of the betatron, and the origin and growth of the principles of synchronous acceleration and alternating gradient focusing. Includes illustrations, references, and index. -
_______, ed. The Development of High-Energy Accelerators. New York: Dover, 1966. Collection of twenty-eight classic articles and an introduction written by the coinventor of the cyclotron. Depicts the evolution of direct voltage, resonance, linear, synchronous, and strong-focusing accelerators. Includes illustrations and index. -
Livingston, M. Stanley, and John P. Blewett. Particle Accelerators. New York: McGraw-Hill, 1962. Classic technical reference on particle accelerators offers a comparative critical analysis of the capabilities of various types of accelerators as well as a brief historical essay on each. Includes illustrations, references, and indexes. -
Mann, Wilfred Basil. The Cyclotron. 4th ed. New York: John Wiley & Sons, 1953. Describes the cyclotron’s history and development. Well illustrated, with sketches of the components of cyclotrons. Features an introduction by Lawrence. Includes references and index. -
Wilson, Edmund. An Introduction to Particle Accelerators. New York: Oxford University Press, 2001. Moderately technical text devotes first chapter to the history of particle accelerators, including brief discussion of the Lawrence’s work on the cyclotron.
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