The diffusion pump is used to produce and maintain low pressures in the electron column, spectrometers, and sample chamber. An oil diffusion pump requires a forevacuum of about 50 millitorr (5 to 10 Pa) to operate. Therefore, it is isolated from the column and sample chamber by a "gate" valve until the mechanical pump has produced a sufficiently good vacuum in the microprobe (Figure 3.4.2a). Diffusion pumps are so named because rather than actively "pull" air molecules out of a volume, they wait for the molecules to diffuse into the active part of the pump where they are then trapped and removed.
A diffusion pump operates by boiling oil at the base of the pump that rises as a hot vapor. As it rises, it is directed into a funnel-shaped set of baffles (known as a chimney stack.) and jetted as a supersonic vapor towards the sides, which are kept cool by cold water tubing that surrounds the upper part of the pump. Air molecules that diffuse into this portion of the pump collide with the vapor molecules and are trapped. The oil vapor cools and condenses, dragging air molecules down as it sinks to the bottom of the pump (Figure 3.4.2b). At the bottom, the heater reboils the oil, releasing the air molecules, which are pumped away by the mechanical pump. The diffusion pump produces a working pressure in the sample chamber of about 1 x 10-5 Torr. There is a water-cooled baffle located above the pump to prevent oil vapor from entering the electron column.
Figure 3.4.2a. Diffusion pump (blue) at the base of the electron column. The gate valve, which separates the pump from the electron column, is housed in the blue-gray metallic box at the top of the pump (with light circle). The door through which samples are inserted into the microprobe is at the top of the picture. The horizontal silver tube level with the sample change door is the SEM photomultiplier tube. Large coil springs used to isolate the microprobe from outside vibration are also visible.
|Figure 3.4.2b. Schematic diagram of an oil diffusion pump. Red arrows indicate the movement of boiling oil vapor (after Potts 1987).||
Copyright 1997-2003, James H. Wittke
Last update: 01/18/2006 01:47 PM.