 Solution

For a proton, the fundamental cyclotron resonance condition isA half-orbit takes $\pi m_p/(eB)$, during which the applied voltage reverses sign once. The proton then crosses the gap in the opposite direction and receives another accelerating impulse. The electric force in the gap, rather than the magnetic bending inside a dee, provides the kinetic energy. The injection phase must be chosen so the first crossing accelerates the proton.