What's an Insertion Device?
The insertion device (ID) is an instrument to generate synchrotron radiation (SR) much more intense than that from bending magnets (BMs), and is "inserted" in the straight sections between adjacent BMs. The principle of ID is shown in the figure below.
The electrons follow a periodic trajectory determined by the periodic magnetic field in the ID, and the SR emitted at each period is superimposed coherently or incoherently. In order to change the wavelength of SR, the magnetic field strength is varied by changing the gap between top and bottom magnet arrays.
Two types of IDs exist. One is called an undulator and the other is called a wiggler. Especially, the beam size and angular spread of SR from the undulator is much lower than that from the BM and wiggler, which makes the undulator well suited as a light source in SR facilities that have a low-emittance electron beam. The storage ring of the SPring-8 is optimized to take advantage of the undulators.