A charged particle beam including charged particles (e.g., electrons) is generated from a charged particle source (e.g., a cathode or scanning electron beam). As the beam is projected, it passes between plural alternating electric fields. The attraction of the charged particles to their oppositely charged fields accelerates the charged particles, thereby increasing their velocities in the corresponding (positive or negative) direction. The charged particles therefore follow an oscillating trajectory. When the electric fields are selected to produce oscillating trajectories having the same (or nearly the same) frequency as the emitted radiation, the resulting photons can be made to constructively interfere with each other to produce a coherent radiation source.