Electron Gun
At the western end of the two mile tunnel that houses the beam line is the electron gun, which produces the electrons to be accelerated. Any filament that is heated by an electrical current flowing through it releases a few electrons into the space around it. When a strong electric field is applied, more electrons are pulled out of the hot filament. The electric field accelerates the electrons towards the beginning of the accelerator structure. This is the way your TV or computer monitor produces it's electron beams.
Polarized Electron Gun
In the polarized electron gun, polarized laser light knocks electrons off the surface of a semiconductor and an electric field accelerates them toward the end of the accelerator pipe.
The polarized electron gun is kept at an even lower level of vacuum than the accelerator, down to 10-12 Torr.
