BETATRON

device that accelerates electrons (beta particles) to high speeds in a circular orbit. The first successful betatron was completed in 1940 at the University of Illinois, Champaign-Urbana, under the direction of the American physicist Donald W. Kerst, who had deduced the detailed principles that govern the operation of such a device. The betatron consists of an evacuated tube formed into a circular loop and embedded in an electromagnet in which the windings are parallel to the loop. Alternating electric current in these windings produces a varying magnetic field that periodically reverses in direction. During one quarter of the current cycle, the direction and strength of the magnetic field at the orbit, as well as the rate of change of the field inside the orbit, all assume the values needed to accelerate electrons in one direction. The acceleration is brought about by two forces, one acting in the direction of the electrons' motion and the other at right angles to that direction. The force in the direction of the motion is exerted by the electric field induced along the circle by the strengthening of the magnetic field within the circle. The perpendicular force arises as the electrons move through the magnetic field at the orbit. At the beginning of the proper quarter-cycle, electrons are injected into the betatron, where they traverse hundreds of thousands of orbits, gaining energy all the while. At the end of the quarter-cycle, the electrons are deflected to a target to produce X rays or other phenomena. Large betatrons have produced electron beams with energies greater than 340 MeV (million electron volts) for use in research. Weight problems become severe with high-energy betatrons; the electromagnet of a 340-MeV unit weighs about 330 tons. X rays generated by betatron beams of about 20 MeV are widely used in industrial radiography.