I have read this:
This states that electrons are free.
Now there is a debate on this site over whether a truly free electron in vacuum can or cannot emit a photon.
Based on the answers, I believe that a free electron in a vacuum cannot emit a photon. There needs to be a third party in the equation, to whom the electron relatively accelerates.
Electrons can only emit a photon when they are :
either bound to an atomic system
or are accelerating in a metal conductor, but these electrons are not free, they are loosely bound to the atoms
Now based on the wiki article, this is not true. Free electron lasers emit photons, and the electrons are here said to be free in a vacuum. So they are not bound to any atomic system.
The electron beam must be maintained in a vacuum, which requires the use of numerous vacuum pumps along the beam path.
Now how do these truly free electrons in a vacuum emit the photons of the laser?
are these electrons in the free electron laser the same as accelerating electrons in a metal conductor? So are they loosely bound to the atoms? But there are no atoms in the vacuum so how are they emitting photons?