Varian VHS-400 Diffusion Pump

The picture below shows the inside of one of the VHS-400 diffusion pumps. There are no moving parts in a diffusion pump, they are really little more than a large (mainly empty) cylinder. The diffusion pump oil is heated to boiling by powerful heater elements (the upper surface of the heater elements can be seen in the bottom of the pump). The oil vapor streams up and then squirts out through gaps in the side of the jet assembly (bottom picture). The oil vapor molecules collide with any molecules which find their way into the pump body and force them away from the vacuum chamber. This is the mechanism by which molecules are removed from the chamber. The gas is then pumped away by the roughing pump which is connected by the PVC foreline to the outlet of the diffusion pumps. The whole diffusion pump body is surrounded by a coil of copper tubing which carries cooling water (which needs to be flowing whenever the diffusion pump is running). Oil vapor hitting the walls of the pump is condensed back to a liquid and runs back into the base of the pump. Truly incredible pumping speeds may be attained using diffusion pumps (the VHS-400 pumps pictured here are rated to pump 8000 liters of air per second) and this pumping speed is necessary to maintain the low pressure in the chamber that is essential to observe weakly bound complexes. (Note: a diffusion pump on a much smaller scale is also used to provide the high vacuum for sample preparation).

Below is a picture of the (not particularly clean) Tin-Man-like jet assembly from one of the two pumps sitting in the lab at the University of Michigan before they made their trip to EIU. This assembly sits in the center of the body of the pump and directs the flow of oil vapor that is responsible for the action of the diffusion pump. Diffusion pumps can easily reach a temperature of 200 degrees Celsius at their base during operation so can cause nasty burns. (Various pieces of gate valve and foreline assembly can also be seen scattered around the lab after their disassembly).