Unlike most lasers
the free electron laser(FEL) doesn't have a lasing medium
but relies on the emitted light from an electron beam for
a light source. The electron beam is created by an
electron accelerator, accelerated and guided by deflector
coils into the lasing chamber. This consists of a
vacuum chamber with a mirror in each end, forming a
resonator cavity. One of the mirrors is semitransparent
to let some of the light through, forming the laser beam.
The electron beam is led through a series of alternating
magnetic fields, called an undulator or a wiggler.
This makes the electron beam 'wiggle'. This exites
the electrons and they emit light, its frequency and
phase dependent on the electron beam's vibration frequency.
The light 'bounces' between the mirrors, and gets stronger
each time the electron beam emits light.

Basic buildup

       From electron           Back to
       accelerator            accelerator
          v                       ^
        | : |                   | : |
       #| : |#                 #| : |#
       #| : |###################| : |#
M |-----' : `-S---N---S---N---S-' : `-------|    Beam
i |       `       _       _       '  emitted|
r |- - -   \    .' `.   .' `.    /   - - - -:.........
r |- - - -  `._'     `_'     `_.'  - - - - -:.........
o |                                  light  |
r |-----------N---S---N---S---N-------------|
       ###############################


 #  electron beam deflector coils
NS  Undulator polarity
 :  electron beam


The FEL is used in military and applications,
but has also a use in the medical field since the
output frequency can be altered by changing the
frequency of the wiggle (different tissues have
different sensitivity to the same frequency).

One of the downsides of the FEL is the large size, due to the
accelerator. It is also expensive and the accelerated
electrons have a tendency to emit x-rays.

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