Coherent Radiation in Insertion Devices
  We calculate the coherent radiation in an undulator/wiggler with a vacuum chamber of arbitrary cross section. The backward radiation is a coherent and it has wavelengths about twice the period of the undulator/wiggler. Mostly of coherent radiation is going with the wavelengths approximately the bunch length at small angles however. 
A.A.Mikhailichenko (Cornell University), E.G.Bessonov (Lebedev FIAN)
physics.acc-ph