Vacuum Technology and Vacuum Design Handbook for Accelerator ...

services, can be regenerated without removing them from service, have a high pumping speed for
most gases and can maintain high vacuums for long periods in a static vacuum application.
Turbomolecular pumps
Where a vacuum system is cycled such as at a measurement chamber, then turbo pumps are used.
Turbo pumps are transfer type pumps meaning, gas is transferred from the vacuum space into the
atmosphere. A turbo pump can routinely recover a vacuum from low vacuum after a measurement
chamber has been roughed out without needing regeneration as for a cryopump.
Turbo pumps are also more effective at pumping lighter molecules such as helium and hydrogen as
compared with cryopumps. Using turbo pumps near systems that use these gases will reduce the
effect of loading on cryopumps.
Most new turbo pumps can be serviced in-house. Some models purchased require special balancing
of the bearings after they had been changed. In this case they are sent back to the manufacturer at
great cost. It is important that models chosen can be maintained in-house.
Other high vacuum pumps
There have been few applications where titanium sublimation or ion pumps have been used in the
accelerator area. Most have been stand alone UHV systems or cryostats for specialised applications.
The practicality of cleaning and regenerating these gas trapping type vacuum pumps on systems
requiring relatively frequent cycling is prohibitive.
Getter materials once used widely in electronics in tubes are becoming more used within larger
static volumes such as beam lines and associated equipment. They can be coated onto surfaces to
assist in maintaining uniform high vacuums in spaces that have poor pumping speeds.
13.3 Low Vacuum, Roughing or backing pumps
Oil free pumps
These pumps are categorised by either being an oil type pump or dry (oil free) pump. Oil free
vacuum systems are of choice for accelerator systems now that the technology has been proven to
be reliable and cost effective. The majority of roughing and backing pumps now in use are scroll
pumps as well. Piston pumps have been trialled with little success due to the short life of the seals.
Some applications have used diaphragm pumps and multistage roots pumps but scroll pumps remain
the most cost effective and easiest to maintain option. The Edwards type scroll was chosen over
other brands as this type of scroll pump is designed with the bearings mounted outside of the
vacuum which in other brands often caused the bearings to fail from lubricant loss.
Oil vane pumps
Low Vacuum, Roughing or backing pumps are all the same type of pump. Oil vane type pumps
were widely used in the accelerator area until 2006 when a systematic change began on the STAR
accelerator to replace oil vane pumps with oil free scroll pumps. Some oil vane pumps remain in use
where some back streaming oil vapour is not considered a problem. If an oil vane pump is to be
used for roughing out purposes, it is high recommended that a liquid nitrogen cold trap is used to
trap vapours. It is the ultimate aim to have oil free pumping on all accelerators in the future.
14. Monitoring
14.1 Vacuum Gauges
The standard type of high vacuum gauge in use is the cold cathode penning type gauge.
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