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VACUUM TECHNOLOGY

VACUUM TECHNOLOGY 7.7

the inner surface of the water-cooled pump body. Gas molecules that randomly enter the diffusion
pump inlet are intercepted by the high-speed oil vapor jet. This collision with a high-speed and rel-
atively high-molecular-weight oil vapor molecule produces a net downward trajectory for the affect-
ed gas molecule. Through the use of multiple stages of jets, gas molecules are compressed and forced
to the foreline port of the diffusion pump. At the foreline, sufficient pumping speed applied by the
primary roughing pump (in this case functioning as the foreline pump) causes the gas ejected from
the diffusion pump foreline to be compressed and expelled to the atmosphere. Often, a liquid nitro-
gen cold trap is installed at the inlet of the diffusion pump to reduce the amount of pump oil that can
migrate from the diffusion pump into the vacuum vessel. The operating range of most commercial
−4
diffusion pumps is from 10 to 10 −10 torr.
Turbomolecular Pumps. In turbomolecular vacuum pumps, the random trajectory of gas mole-
cules entering the pump inlet is influenced by the high-velocity surfaces of the rotating turbine
blades. Gas molecules impacted by the blades stay on the surface of the blades for a brief period
called the residence time. During this period, gas molecules lose any directional identity they had
and assume the trajectory of the blade (not unlike a bug interacting with the windshield of a car trav-
eling at 60 mi/h!). At the end of the residence time, gas molecules leave the surface of the turbine
blade following the Cosine Law. According to this law, gas molecules are most likely to leave a sur-
face in a direction normal to the plane of the surface and are least likely to follow a trajectory paral-
lel to the surface. The combination of these two effects cause gas molecules to be forced from the
inlet of the turbomolecular pump to the exhaust. Most modern turbomolecular pumps are of the axial
flow design, which bears similarity to the engines on commercial jet aircraft. The rotors of most tur-
bomolecular vacuum pumps are precision machined and carefully balanced for operation at rota-
tional velocities ranging from 30,000 to 90,000 RPM depending on the pump size and model. Rotors
of turbo pumps often are designed with several “stages.” At the inlet of the pump, the rotor blades
are wide, have a steep pitch, and form a relatively open blade structure. This design is optimized for
high pumping speed at the pump inlet with relatively modest compression. At the middle of the rotor,
blades are often more closely spaced, and the pitch of the blades is reduced compared to blades at
the inlet. At the output or exhaust of the turbo pump, the blades are very closely spaced and the pitch
of the blades is optimized to achieve high compression of gas and relatively modest pumping speed.
The critical foreline pressure of many turbomolecular pumps is approximately 100 mtorr. A suitable
foreline pump is required for proper operation of a turbo pump. Some turbo pumps have an inte-
grated molecular drag stage that further compresses the gas before exhausting it to the foreline of the
pump. These hybrid pumps are often called turbo-drag pumps. The critical foreline pressure of many
commercial turbo-drag pumps can be as high as 1 torr, allowing for evacuation of the foreline by a
diaphragm pump. In either design, care should be taken to prevent objects from entering the inlet of
a turbo pump during operation. Were this to occur, almost certainly the pump would be damaged,
and in some cases a hazard to personnel would be created.
Gas Capture Pumps. Unlike the momentum transfer pumps discussed previously, gas capture
pumps reduce pressure in a vacuum vessel by removing gas molecules from the gas phase. In cryo-
genic pumps, gas molecules are cryocondensed or cryosorbed, while in sputter-ion pumps, gas mol-
ecules are chemically reacted to form solid by-products or are otherwise immobilized.
Cryogenic Vacuum Pumps. There are generally two types of mechanisms used in cryogenic vac-
uum pumps—direct cooling via the use of cryogenic liquids or cooling by a mechanical compressor.
One of the oldest cryogenic pump designs is the Meissner coil. In this design a helical coiled tube
(usually copper) is placed inside the vacuum vessel, both ends of the helical coil pass through and
are tightly sealed to a flange attached to the vacuum vessel. After primary evacuation of the vacuum
vessel to a pressure of approximately 100 mtorr, liquid nitrogen is flowed through the inside of the
copper coil. Gas molecules randomly striking the cooled surface of the Meissner coil will be cry-
ocondensed if the gas species has a boiling point above that of liquid nitrogen. Gas species with boil-
ing points below that of liquid nitrogen will be removed from the gas phase for a residence time that
is a function of the temperature of the Meissner coil and the molecular weight of the gas species.

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