A betatron is a cyclic particle accelerator type. It is essentially a transformer with a torus-shaped vacuum tube as its secondary coil. An alternating current in the primary coils accelerates electrons in the vacuum around a circular path. The betatron was the first machine capable of producing electron beams at energies higher than could be achieved with a simple electron gun. The betatron was developed in 1935 by Max Steenbeck in Germany to accelerate electrons, but the concepts ultimately originate from Rolf Widerøe, whose development of an induction accelerator failed due to the lack of transverse focusing. Subsequent development occurred in the United States through Donald Kerst in the 1940s.

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