An insertion device is a part of a synchrotron which produces highly-brilliant, forward-directed and quasi-monochromatic synchrotron radiation. The name comes from the fact that these are devices which are inserted into a straight section of a synchrotron or a storage ring. They are not essential to the operation of the storage ring itself, their only function is to generate synchrotron radiation.

Insertion devices typically consist of arrays of magnets which are used to generate a spatially periodic magnetic field at the electron beam path. The magnetic fields cause transverse acceleration to the relativistic electrons, causing them to emit synchrotron radiation.

There are two types of insertion devices in wide usage at synchrotron radiation facilities. In a wiggler the period and the strength of the magnetic field is not tuned to the frequency of radiation produced by the electrons. Thus every electron in the electron bunch radiates independently, and the resulting radiation spectrum is broad. A wiggler can be considered to be series of bending magnets concatenated together, and its radiation intensity scales as the number of magnetic poles in the wiggler.

In an undulator source the radiation produced by the oscillating electrons interferes constructively with the motion of other electrons, causing the radiation spectrum to have a relatively narrow bandwidth. The intensity of radiation scales as $N^2$, where $N$ is the number of poles in the magnet array. Undulators are the brightest sources of X-rays and extreme ultraviolet radiation mankind has yet invented.

Insertion devices are also the radiation producing element in free electron lasers.

References

Lecture notes on Insertion Devices to open SESAME