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KERST 1940

Kerst, D.W.;
Acceleration of Electrons by Magnetic Induction
Phys. Rev. 58 (1940) 841;

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Reprinted in
The Physical Review - the First Hundred Years, AIP Press (1995) CD-ROM.

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For some time it has been realized that it might be possible to make use of the electromotive force induced by a changing magnetic flux to accelerate charged particles traveling in an orbit around the changing flux. Although previous attempts to accelerate electrons by this means have been unsuccessful, (Wideroe, Archiv f. Elektrotechnik 21 (1938) 400, E. T. S. Walton, Proc. Camb. Phil. Soc. 25 (1929) 469) careful examination showed that it should be possible to get good magnetic focusing by the proper arrangement of a magnetic field to guide the electrons around the changing flux and that if the rate of change of flux within the orbit were sufficiently high it would be possible to capture electrons in usable orbits and that vacuum requirements should not be difficult to satisfy.
It seamed feasible to attempt the experiment with a 600-cycle per second magnetic field, since a sufficiently high rate of change of of flux would be obtained and since it seemed that it would not be necessary to have a vacuum better than 10^-6 millimeter of mercury in the acceleration chamber, in spite of the fact that at this frequency the length of the electron path would be of the order of 10⁷ centimeters.
To hold the electrons in the acceleration chamber for such a long path it is necessary to fulfill the condition that φ = 2π R₀²H, where φ is the flux enclosed by the orbit and H is the magnetic field at the orbit which causes the electrons to travel in a circle of radius R₀. When this condition is satisfied, the electron orbit neither shrinks nor expands, and the electrons can be accelerated by increasing φ and H together.
A laminated electromagnet with pole faces 8 inches in diameter, which satisfied all the necessary conditions, was constructed. The stable orbit was shrunk from R₀ toward the position of a tungsten target by causing saturation of the portion of the magnetic circuit which supplied the flux through the center of the orbit. X-rays produced by the impact of the electrons upon the target showed that the accelerator operated, and a lead collimator in front of a Geiger-Müller counter showed that the only portion of the acceleration chamber from which x-rays came was the target.
By taking the sweep voltage for an oscillograph from a coil surrounding the core of the magnet and putting the pulses from the Geiger-Müller counter circuit on the vertical deflection plates, the phase of the magnetic field at which the electrons struck the target could be determined. It was possible to hold the electrons in the acceleration chamber for one-fourth of a cycle during which the magnetic field changed from a low value to its maximum. Conservative estimates of the magnetic field at the target when the electrons strike it indicated that the energy of the electrons was about 2.2 MeV. This estimate was substantiated by a comparison of the absorption of the X-rays in lead with published data on the absorption of X-rays produced by 2-million-volt electrons. (D. L. Northrup and L. C. Van Atta, Am. J. Röntgenology and Radium Therapy 41 (1939) 633). After filtering the X-rays from the accelerator through about 1.8 cm of lead, their absorption coefficient is 0.57 cm^-1. A correction had to be made for scattering of X-rays from the magnetic yoke. Since the absorption coefficient for X-rays produced by 2.0 MeV electrons is 0.62 cm^-1, the electrons in the new accelerator must have reached about 2.35 MeV energy before striking the target. The absorption measurements were taken with Lauritsen electroscopes, and calibration of the electroscopes showed that the intensity of the radiation was greater than the intensity of the gamma-rays from 10 millicuries of radium.
Of several suggestions which have been made for naming the apparatus, induction accelerator seems to be the shortest descriptive one.
It has been a great help to be able to discuss the theoretical aspects of the accelerator with Professor R. Serber and Professor H. M. Mott-Smith.