Ion Pumps: What is Ultra High Vacuum?

UHV (Ultra High Vacuum), is a vital element of all manners of scientific applications used in modern research
and development today. There are many uses for ion pumps within multiple scientific fields. 

Through the examination of the difference (at the molecular level) between the various vacuum levels, you can
begin to appreciate the challenges associated with working with and achieving high vacuum, ultra high vacuum
and extreme high vacuum. 

What are the definitions of high, ultra high and extreme high vacuum?
The pressure range of extreme high vacuum is usually defined as 10-12 mbar and lower, while ultra high vacuum
is between 10-7 and 10-12 mbar, and high vacuum is between 10-7 and 10-3 mbar.
Extreme high vacuum is associated with the levels that are found in outer-space in the form of geo-stationary
orbiting satellites; ultra high vacuum with nuclear research and high-energy physics, such as that being
conducted by the scientists are CERN. High vacuum is commonly associated with industrial and research

Key Considerations
There are several key considerations when working with high vacuum, ultra high vacuum and extreme high
vacuum conditions; these are associated with the system design, and the materials being employed. 

Along with these considerations, the condition of the system/chamber surface is also an important factor and
can be optimised by:
  • Reducing the number of seals, feed-throughs
  • Employing metallic seals
  • Using materials with low desorption/outgassing rates
  • Only welding from the inside
  • Minimising the chamber’s internal surface area
  • Making sure that there are no internal gaps or trapped volumes

Pre-treatment of the system is important, this includes heating to a high-temperature (this process is known as
“baking”), careful handling using powder-free latex gloves - this is to avoid fingerprint greasing, and a thorough
cleaning to remove fillings, hydrocarbons and other contaminants (these can be both chemical and physical).

What Pump Types can be used to Generate High Vacuum, Ultra High Vacuum and Extreme High Vacuum?
Obtaining any of the three vacuum levels can only be efficiently and effectively obtained through the use of a
fore pump, which then charges the main pump. 

Fore pumps, (sometimes referred to as “backing pumps”) reduce the pressure to a level where HV, UHV and
XHV pumps can take over in order to operate in a safe, effective and efficient manner. However, the pairing of
different types of vacuum pumps for optimum performance is not a straightforward exercise. There are no
off-the-shelf pumping systems which have the capabilities to cover all applications, eventualities and
requirements simultaneously, as there are a lot of critical factors and impacts that should be taken into account. 

Pump selection (both fore and main), depends on a number of factors which include; noise/vibration, tolerance to contamination, maintenance schedules, cost (initial and ongoing), footprint, resilience to shock etc. However, there is no single ideal HV, UHV or XHV pump; each type has its own set of advantages and disadvantages.

The main fore pumps include:
  • Screw pumps
  • Scroll pumps
  • Multi-stage roots pumps
  • Diaphragm pumps

The main pumps capable of delivering HV, UHV and XHV, and which can do so in a rapid draw-down time
  • Cryo pumps
  • Diffusion pumps
  • Turbomolecular pumps
  • Titanium sublimator pumps
  • Non-evaporable getters

The commonality of these HV, UHV and XHV pumps are their ability to produce vacuum conditions by either
rapidly evacuating gas molecules or by capturing/tying them up.