Damping Rings
After the first ten feet of the linac,
the electrons are traveling in bunches with an energy of approximately
10 MeV. This means the electrons have reached 99.9% the speed
of light. These bunches have a tendency to spread out in the
directions perpendicular to their travel.
Because a spread out beam gives fewer collisions than a narrowly focused one, the electron and positron bunches are sent into damping rings (electrons to north, positrons to south).
These are small storage rings located on either side of the main accelerator. As the bunches circulate in the damping ring, they lose energy by synchrotron radiation and are re-accelerated each time they pass through a cavity fed with electric and magnetic fields. The synchrotron radiation decreases the motion in any direction, while the cavity re-accelerates only those in the desired direction. Thus, the bunch of electrons or positrons becomes more and more parallel in motion as the radiation "damps out" motion in the unwanted directions.
The bunches are then returned to the accelerator to gain more energy as they travel along it.
