First betatron of Donald Kerst, 1940
date: label: date made
content: early 1940's
name: label: maker
content: Kerst, Donald
notes: label: Description
content: Object N-08216 is the assembly of the first Kerst Betatron as it appeared in the Atom Smashers exhibition at the NMAH. mounted horizontally, a toroidal vacuum chamber made of white ceramic with two cylindrical ports, each joined to fused-on glass extensions; circular magnet coils mounted above and below plane of vacuum chamber; rectangular laminated iron yoke surrounding the coils on top, bottom and two sides; enclosing the yoke, a rectangular metal frame of four horizontal angle strips joined by four welded vertical straps, two on each side, and by four bolts, two on each end. Between the yoke laminations and each coil is a thin sheet of slightly flexible, warped, black material, like plastic or impregnated cardboard; each is in two pieces to fit around the central axis. Two circular gray plates are positioned at the top and bottom of the vacuum chamber, separating it from the upper and lower coils, respectively.
label: Description
content: It is presumed that the left-hand cylindrical port contains the electron source, and the right-hand port contains the beam of x-rays from the internal target. Wires protrude from both cylindrical ports.
label: Description
content: History and basic principles
label: Description
content: Among the many investigators who attempted to accelerate electrons by magnetic induction, none were successful until Donald Kerst produced 2.3-MeV electrons in a betatron at the University of Illinois in 1940. He later constructed a number of betatrons of successively higher energies, culminating in the 300-MeV betatron at the University of Illinois. Kerst’s success was due to a very careful theoretical analysis of the orbit dynamics in accelerators (including a study of the requirements for injection); to a preliminary analysis of all conceivable effects relevant to the operation of a betatron; and to a careful and detailed design of the magnet structure, vacuum system, and power supply. This was the first new accelerator to be constructed on the basis of a careful scientific analysis and a completely engineered design. Its success represented a turning point in the technology of particle accelerators from cut and try methods to scientifically engineered designs. All later accelerators, including the newest high energy synchrotrons, have been influenced by this early work of Kerst. It is only in the light of these later developments that we see the importance of the betatron not merely as a valuable instrument in itself but as a milestone in the development of particle accelerators generally. For example, the radial and vertical oscillations of the beam in all particle accelerators are now universally called betatron oscillations after the pioneering work of Kerst and Robert Serber, who together in 1941 published the first theoretical analysis of such oscillations as they occur in the betatron.
label: Description
content: The betatron was quickly put to use in industry, medicine, and nuclear physics research. It was the first accelerator to provide gamma rays for photo-nuclear studies. In the late 1940s and early 1950s the betatron was used for much of the experimental research on photo disintegration of the deuteron, on photo-nuclear reactions (including the discovery of the giant dipole resonances), and important early work on nuclear structure from electron scattering. Of great importance was the pioneering use of megavolt electron beams for the production of energetic X rays for the therapeutic treatment of cancer. His fascinating depiction of this treatment included a description of the first use of phantoms and the intense activity precipitated by a student afflicted with brain tumor, heroic efforts that achieved much, but were unable to save the student. Kerst took a one-year leave of absence from the University of Illinois (1940-41), designed a 20-MeV betatron and a 100-MeV betatron working with the engineering staff at General Electric. He oversaw the construction and operation of the 20-MeV betatron, which he brought back to Urbana. During World War II days, Kerst built a 4-MeV portable betatron for inspecting bomb duds in situ and, most importantly, built a 20-MeV betatron at Los Alamos for study of bomb assembly implosions. His work was described in the official history of Los Alamos as: “The technical achievements are amongst the most impressive at Los Alamos.” After World War II Kerst built a 300-MeV betatron at the University of Illinois that was brought into operation in 1950 and provided a facility for studying high energy physics until it was superseded by synchrotrons and then by electron linacs.
label: Description
content: (Above text excerpted from Donald William Kerst, 1911—1993, a Biographical Memoir by Andrew M. Sessler and Keith R. Symon, Copyright 1997, National Academies Press, Washington DC.)
label: Description
content: It is no accident that the magnetic induction accelerator was so late to be realized. The concept had been advanced repeatedly in the preceding twenty years, but the problems of establishing and maintaining a particle beam were far greater than those encountered with several other accelerator types that were reduced to practice in the 1930’s.
label: Description
content: Electrons, due to their relatively small mass, are much more “skittish” than the much more massive protons, and in the betatron must make a thousand times more circuits to reach the same energy as protons accelerated in a cyclotron. Where the cyclotron runs continuously, allowing the protons to find their own orbit, the betatron is pulsed: electrons must be injected at the right velocity to be captured into stable orbits, and held in them by a balance between the magnetic fields responsible for their acceleration and their orbit guidance, respectively. Kerst’s success depended decisively upon close mathematical analysis of these orbits, especially those immediately after injection into the vacuum chamber. This analysis was performed in collaboration with theoretical physicist, Robert Serber.
label: Description
content: The principle of operation of the betatron is similar to that of a high voltage transformer. Alternating currents in the upper coils produce an increasing magnetic field, thus inducing an electromotive force around the electron’s circular orbit. This force, which Kerst calculated to be only 17 volts maximum acts on the electrons during each of their 200,000 circuits of the vacuum chamber, imparting a total energy of over two million electron volts (MeV).
label: Description
content: The electrons are introduced into the vacuum chamber by an injector; they are not brought out of the vacuum chamber, but strike an internal target, converting their energy into x-rays which emerge through a second port.
label: Description
content: Kerst, like E.O. Lawrence at the University of California, Berkeley, saw no intrinsic limits to his device. From the moment Kerst’s first device operated successfully in July 1940, he and his University looked forward to one more powerful (300 MeV) even than the cyclotrons being developed at Berkeley. Begun in 1945, immediately after World War II, with a special appropriation from the Illinois State Legislature, it was soon to be outmoded by newer methods of particle acceleration (e.g., the synchrotron).
label: Location
content: Currently not on view
topic: label: general subject association
content: Science & Scientific Instruments
setName: label: See more items in
content: Medicine and Science: Modern Physics
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content: Energy & Power
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content: Science & Mathematics
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content: Modern Physics
creditLine: label: Credit Line
content: University of Illinois, Champaign-Urbana, IL
dataSource: label: Data Source
content: National Museum of American History
identifier: label: ID Number
content: EM.N-08216
label: accession number
content: 233629
objectType: label: Object Name
content: betatron, Kerst, First Operational Model
physicalDescription: label: Physical Description
content: glass; ceramic; iron (overall material)
label: Measurements
content: overall (approx.): 23 in x 10 1/2 in x 9 in; 58.42 cm x 26.67 cm x 22.86 cm