The conventional cyclotron, pioneered by Lawrence in the 1930's, is limited in energy by contradictory requirements for the magnetic field. To provide beam stability in the direction of the magnetic field, the field must decrease with radius. But to keep the beam in phase with a constant‐frequency dee voltage, the field must increase with radius as the particle mass increases. In a conventional machine, the first requirement is satisfied and phase loss of the beam has limited the energy of protons from this first‐generation cyclotron to about 15 MeV, but it has been pushed as high as 22 MeV by the Oak Ridge group.