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VACUUM TECHNOLOGY

VACUUM TECHNOLOGY 7.5

to obtain this kind of information. Most commercial PPAs have three functional components—an ion-
izer, a mass selector, and a detector. In the ionizer, neutral gas molecules that randomly enter the detec-
tor inlet are bombarded with energetic electrons that have sufficient energy to ionize the gas molecules.
In addition to ionizing, the energetic electrons may break the gas molecules into fragments consisting of
one or more of the atoms in the original molecule. These fragments are often also ionized. All the gas-
phase ions created in the ionizer are electrostatically attracted to the mass selector by an applied electric
field. Based on the mass and charge (single or multiple) of the ion, its trajectory through the magnetic
field of the mass selector will determine if the ion completes its journey all the way to the charged par-
ticle detector to be counted. Ions that reach the detector extract one or more electrons at the detector thus
creating a current that is a function of the number and charge (single or multiple) of the ions impacting
the detector. In practice, the control system for the PPA creates a set of operating conditions (accelerat-
ing voltage for the ions entering the mass selector and magnetic field strength of the mass selector) such
that at any one time only one mass-to-charge ratio (M/z) is allowed to pass through the mass selector
and continue to the detector to be counted. Ions having an M/z other than the value prescribed by the
mass selector are neutralized or otherwise prevented from being counted by the detector. After a signal
for a specified M/z has been collected, the controller will adjust the operating parameters of the PPA for
another M/z and will collect data for ions having this new M/z. In this manner the PPA scans through a
range of M/z and records data for each ionized gas species. Data from the PPA are typically plotted as
detector current as a function M/z.

7.3 METHODS FOR CREATING A VACUUM

7.3.1 Primary Vacuum Pumps

These pumps are used for reducing pressure in a vacuum vessel from an initial state of atmospheric
pressure (760 torr) to a reduced pressure (typically in the range of 10 mtorr). The principles of oper-
ation and operating ranges for a variety of commercially available pumps are covered in the follow-
ing subsections.

Oil Sealed Rotary Vane Mechanical Pumps. These pumps move gases by isolating a small volume
of gas from the vacuum system, compressing this isolated volume to atmospheric pressure, and then
exhausting this gas to the atmosphere. The mechanisms of the oil sealed rotary vane pump include a
rotor with sliding vanes, a stator, inlet and exhaust ports, a means for rotating the stator, and the pump
oil. In operation, the rotor, which is smaller in diameter than the bore of the stator and offsets concen-
trically from the stator bore, is caused to rotate by the drive mechanism. As the rotor spins within the
stator, the sliding vanes maintain intimate contact with the inside surface of the rotor. As a sliding vane
passes by the pump’s inlet orifice that connects to the bore of the stator, a crescent-shaped volume of
increasing size is created by the surfaces of the rotor, stator, and sliding vane. The oil helps to create a
seal between the rotor, stator, and sliding vane. As the rotor continues to rotate, the next sliding vane
passes by the pump’s inlet port. As this happens, the gas, which has expanded into the crescent-shaped
volume, is isolated from the vacuum system. Continued rotation of the rotor causes the crescent-shaped
volume to reduce in size, thus compressing the isolated gas. Compression continues until the pressure
is just above atmospheric pressure, and the compressed gas is released to the atmosphere through an
exhaust valve and port. Each rotation of the rotor continues to isolate, compress and exhaust gas in this
manner to reduce pressure at the inlet of the pump. The oil in pumps of this type, in addition to form-
ing a tight seal, serves to remove the heat generated by compression of gas to lubricate the sliding sur-
faces of the pump and to help remove wear particles and other debris during maintenance cycles.

Dry Pumps
Diaphragm Pumps. In these pumps, a diaphragm (usually made from thin stainless steel or
polymer sheet) is flexed by the rocking motion of an eccentric connecting rod and rotating shaft.

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