More information about the roughing pump is on the vacuum_pump page of the wiki.

The cold trap is cooled using liquid nitrogen. It should be cold before opening the Hi-Vac valve. Cold traps are used to prevent back streaming of the vapor from the diffusion pump into the chamber. The cold trap is just above the throat of the diffusion pump so that oil vapors passing this point are condensed and returned to the pump. The chamber will then reach a lower pressure than would otherwise be possible.