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).
