“A Very Bold and Original Device”: Donald Kerst and the Betatron

On July 17, 1940, F. Wheeler Loomis, Head of the Department of Physics at the University of Illinois, received a letter from Donald W. Kerst. The latter, a young physicist who had only begun working at the University of Illinois the prior year, penned to Loomis:

Monday afternoon the electron accelerator started to work. It was its first trial with the new glass doughnut and the new pole pieces. By evening the intensity of the X-rays produced when the electrons strike the target was up to about the effect of 10 millicuries of radium gamma rays (radium at target distance) according to the callibration on the electron-scope.[1]

Soon to be known as the “betatron,” Kerst’s induction electron accelerator was an innovation on which his colleagues had cast a shadow of doubt. Loomis, who was on leave from the university during World War II for government-related work in the Radiations Laboratory at MIT, later admitted to Dean Melvin L. Enger, “This changes his project from an off-chance one to the most promising and original ones that has ever occurred in the department…it is capable of bringing as much renown to our department as the cyclotron did to Berkeley.”[2] A few years later, the betatron would be hailed as “the most important development of a decade.”[3] Indeed, the impact of this “atom smasher” would prove to be far-reaching, holding the attention of the world as it made its appearance on the horizons of medical science and atomic research.

Photo of Donald W. Kerst posing in front of betatron, ca. 1950. From Record Series 39/2/20.

Photo of Donald W. Kerst posing in front of betatron, ca. 1950. From Record Series 39/2/20.

In many ways the birth of the betatron in 1940 marked an important moment in the history of science and technology. Amid increasing international and political tensions as World War II erupted, the world became engulfed by an ensuing arms race that served as a catalyst for many engineers’ and scientists’ research to outpace their counterparts on the opposing side. At the same time, new research into the nature of the Atom’s nucleus influenced the direction of many nuclear physics programs. Loomis recognized the importance of building a pioneering nuclear physics program, especially one that employed both theoreticians and “machine makers.” He thus hired Kerst in 1939 after receiving glowing recommendations from faculty at the University of Wisconsin-Madison, where Kerst had earned his PhD in 1937. One wrote to Loomis: “Kerst is very gifted experimentally…definitely a physicist rather than a gadgeteer, skilled though he is in mechanical work.”[4]

In an oral history interview conducted by University of Illinois Archivist Maynard Brichford in 1965, Loomis noted the lack of theory in physics programs in the United States: “American Physics didn’t come of age until some date, let’s say it really didn’t amount to much until after the First War.”[5] The arrival of such faculty members as Robert Serber and Kerst influenced the direction of the department, bringing with them many of the theoretical leanings Loomis sought. Loomis recalled of Kerst, “He was the one who had ideas. He was very good at seeing the problems that were worth doing and doing them. These are almost two entirely different kinds of animals. The good machine makers and the physicists who use them. Sometimes the same people do both…You need good theory for both.”[6] Kerst’s experimental nature was well known not only at the University of Wisconsin, but also while he worked at the General Electric X-Ray Corporation after graduation. Before arriving at Illinois, Kerst wrote a series of letters to Loomis about an idea he had been working on at GE for “a small electron accelerator which will go to very high voltages,” and would “increase the flux within the orbit of an electron and so induce an E.M.F. along its orbit which causes an acceleration.”[7] Kerst may well have been aware that four years after the University of California, Berkeley, developed the cyclotron in 1932, the University of Illinois had created a second cyclotron that accelerated atomic particles. Built under the direction of P. Gerald Kruger, the cyclotron was the largest of its kind at the time and was “able to produce protons having an energy of 30,000,000 electron volts.”[8] While both the betatron and cyclotron were accelerators of atomic particles, the ways in which they functioned were quite different. The betatron, for instance, produced high-speed electrons which generate X-rays:

Electrons from a small gun are shot into a hollow torroidal vacuum tube which is placed between poles of a powerful alternating current magnet. The magnetic field causes the electron stream to bend into a circular orbit within the torrid, while changing magnetic flux linked with the orbit produces an accelerating electromotive force along the orbit…As the flux builds up within the electron orbit, the energy of the electron continues to increase; and the electron strikes the X-ray target before the flux begins to decrease.[9]

The cyclotron, on the other hand, was a “hollow cylinder composed of two flat semicircular shells resembling hollow “D’s”” in which the particles are “shot into the center…are drawn by alternating charges from one shell to the other while rotating in a circle under the influence of the magnetic field.”[10] Kerst wanted to create an accelerator which used a “fixed equilibrium orbit” so that “electrons, when scattered by residual gas, would oscillate about their equilibrium orbit without decreasing amplitude or would oscillate with increasing amplitude, and hence strike the wall, where they would be lost.”[11] Soon after arriving at the U of I, Kerst rapidly advanced his research. In an annual report recently transferred to the University Archives from the Dean’s Office, Loomis remarked on Kerst’s work:

He has this year invented, designed, and nearly completed the construction of a very bold and original device for accelerating electrons to energies of millions of volts. He and all of us realize that it is a very long shot to hope that this device will be successful since it is so original and so different from anything previously attempted…If it succeeds it will be of really extraordinary importance since one can not see in advance any limit to the voltages that can be obtained with this device. If it fails he will just have to forget about it, but he should not be allowed to continue at it too long as he is obviously an experimentalist of great talent.[12]

Despite only having $500 to spend on materials, Kerst completed the research within a matter of months without having to rework his ideas. The induction electron accelerator, however, worked the first time he turned it on. Loomis was astonished and remarked, “This thing which ought to work according to the principles of theory and actually did.”[13]

Photo of Betatron, ca. 1942. From Record Series 39/2/20.

Photo of Betatron, ca. 1942. From Record Series 39/2/20.

Kerst continued working on the induction electron accelerator following his initial success, including creating a more powerful versions. The first produced 2 1/2-million volts of energy and a 1941 version was capable of 24-million volts. The third generation betatron yielded 80-million volts, though a 340-million volt betatron surfaced only three years later.[14] Despite the success, the device remained unnamed until the Fall of 1941. In September, several colleagues suggested names, including “inductron,” “peritron,” “rheotron,” “Illinitron,” and even the German “Ausserordentlichhochgeschwindigkeitelektronenentwickelndenschwerarbeitsbeigollitron. [15] It was not until December that Kerst decided upon “betatron,” the Greek letter “beta” representing the symbol for “electron” and “tron” as “instrument for.”[16]

Despite having decided upon its name, nicknames for the betatron proliferated, as its myriad uses became apparent. Called “one of the world’s most potent merry-go-rounds,” and the “Industry’s New Seeing Eye,” the betatron’s potential for use in the war effort also became a possibility.[17] In 1942, Kerst began exploring ways to develop the betatron during World War II, including acquiring support from the National Defense Research Committee. The betatron also began to be utilized in the medical industry, not only as an X-ray machine, but also as a radiation treatment for cancer.

The University Archives holds several records series that document the evolution of the betatron, especially the Betatron Correspondence, the Betatron Project Records, and the Research Laboratory Betatron Files. Although these records trace both its technological development as well as the harnessing of its capabilities by various industries, the impact of the betatron as a significant innovation pervades the records of the College of Engineering. Even in his monumental history of the College of Engineering, Ira O. Baker noted that the betatron “is opening up an unlimited range of study in the field of nuclear phenomena, and no doubt, is the most outstanding development of physics made within recent years.”[18] In no small way, Kerst’s “outstanding development” yielded a new means to study atomic particles, contributing to advancements in particle accelerators in the era of Big Science.


[1] Kerst to Loomis, July 17, 1940, Betatron Correspondence, 1938-1970, Record Series 11/10/11, University of Illinois Archives.

[2] Ira O. Baker and Everett E. King, A History of the College of Engineering of the University of Illinois, 1868-1945 (Urbana: University of Illinois Libraries, 1947), 373; Ibid, Loomis to Enger, August 30, 1940.

[3] A. R. Wildhagen, “The Betatron,” Scientific American (May 1943): 207, Record Series 35/3/65, University of Illinois Archives.

[4] Gregory Breit to Loomis, March 28, 1938, Betatron Correspondence, 1938-1970, Record Series 11/10/11, University of Illinois Archives.

[5] Oral history transcript, 1965, F. Wheeler Loomis Papers, 1920-1976, Record Series 11/10/22, University of Illinois Archives.

[6] Ibid.

[7] Kerst to Loomis, Nov. 28, 1938, Betatron Correspondence, 1938-1970, Record Series 11/10/11, University of Illinois Archives.

[8] Ira O. Baker and Everett E. King, A History of the College of Engineering of the University of Illinois, 1868-1945 (Urbana: University of Illinois Libraries, 1947), 366.

[9] Kerst to G. R. Harrison, Nov. 4, 1941, Betatron Correspondence, 1938-1970, Record Series 11/10/11, University of Illinois Archives.

[10] L. Schaffer, “No More Privacy Than an Atom,” Technograph, (May 1940), Record Series 35/3/65, University of Illinois Archives.

[11] D. W. Kerst,” Historical Development of the Betatron,” Nature 157 (January 26, 1946): 92, Record Series 35/3/65, University of Illinois Archives.

[12] Annual Reports of Heads of Departments, Physics, 1938-1939, Record Series 11/1/3, University Archives; also cited in R. A. Kingery, R. D. Berg, and E. H. Schillinger, Men and Ideas in Engineering (Urbana: University of Illinois Press, 1967), 68.

[13] Oral history transcript, 1965, F. Wheeler Loomis Papers, 1920-1976, Record Series 11/10/22, University of Illinois Archives.

[14] “The Betatron: Its Uses, Development, and Basic Operation,” Engineering Experiment Station publication, n.d., Record Series 35/3/65, University of Illinois Archives.

[15] Kerst to Kruger, September 27, 1941; November, 26, 1941, Betatron Correspondence, 1938-1970, Record Series 11/10/11, University of Illinois Archives.

[16] Ibid, Kerst to Loomis, December 19, 1941.

[17] “Electron Whirligig,” The Technograph (April 7, 1942): 24; J. P. Girard, “Betatron: Industry’s New Seeing-Eye, Allis-Chalmers Electrical Review (1946), Record Series 35/3/65, University of Illinois Archives.

[18] Ira O. Baker and Everett E. King, A History of the College of Engineering of the University of Illinois, 1868-1945 (Urbana: University of Illinois Libraries, 1947), 1966.

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