  In an FEL the electrons traveling through the undulator are surrounded in
their own reference frame by Unruh radiation at a temperature of order 8,000
Kelvin. When these virtual photons scatter from the beam electrons they become
real and can be detected. Because of the microbunching of the FEL electron beam
this process proceeds coherently for a fraction of the electrons in the
microbunch. This enhances the Unruh radiation which is still dominated by the
copious spontaneous radiation in the same energy range. We discuss the
particular case of the Stanford LCLS, as well as the case of extreme
acceleration, when the x-ray beam is brought into collision with the 14 GeV
electron beam.
