Transition undulator radiation as bright infrared source

Undulator radiation contains, in addition to the usual component with narrow spectral features, a broadband component in the low frequency region emitted in the near forward direction, peaked at an angle 1/gamma, with gamma the relativistic factor. This component is caused by a sudden change in the electron's longitudinal velocity as it enters and leaves the undulator and is referred to as the transition undulator radiation, as its characteristics are very similar to the well-known transition radiation. The transition undulator radiation is potentially a bright, intense, infrared radiation source. Its radiation angular cone is much narrower than the infrared radiation from bending magnets or usual undulator harmonics.