Three bunches of particles are accelerated during each machine cycle. The first two bunches are the positrons and electrons (respectively) which are to be brought into collision. The third bunch is known as the ``scavenger'' electron bunch. This bunch is diverted from the linear accelerator at approximately 2/3 the way down its length and steered into a tungsten positron production target. The resulting electromagnetic showers produce both electrons and positrons. Positrons in the energy range 2-20 MeV are captured and returned to the front end of the LINAC where they are accelerated and stored in the damping ring. Typical yields from the positron target are one positron per incident electron.
The bunches are accelerated down approximately 3 kilometers of linear accelerator (LINAC). The beams gain energy from radiofrequency cavities which are pumped with energy from klystrons. The accelerating gradient is 17 MeV/m. In typical running conditions, ~3x10^10 particles per bunch are accelerated down the linear accelerator.
After being accelerated to ~0.9 GeV above collision energy in the linac, the electrons and positrons are steered through two opposing arcs of 1km in length. The beams ``coast'' through the arcs, losing energy by synchrotron radiation. The arcs do not lie in the horizontal plane, so the beam transport is complicated by motion in both dimensions perpendicular to the momentum of the electrons.
In flat beam mode, the electron arc is utilized to flip the spin vector from transverse to longitudinal by way of ``spin bumps''[37], which utilize a strong resonance between the vertical betatron tune and the spin tune. Spin bumps are vertical orbit distortions which a separated by a fixed spin rotation around the vertical axis. The net effect of the spin bumps in the arc is to bring the electron spin from transverse entering the arcs to longitudinal exiting the arcs (entering the final focus).
To make up for the relatively low repetition rate of a linear collider, the beams must be focused very strongly at the interaction point. This is achieved at the SLC via three superconducting quadrupole magnets. The final quadrupole is approximately 1.5 meters from the interaction point (IP). At the point of collision, the dimensions of the beams (in flat-beam mode) are ~3um in the horizontal, ~0.8um in the vertical and ~1 mm along the beam axis. The size and location of the IP is very stable over time (see Section 6.6.3).