Vacuum Technology and Vacuum Design Handbook for Accelerator ...

• Feedthroughs can be purchased is almost any flange style but are usually restricted to small
size flanges.
• Insulation must be used on all wires carrying voltage. It is highly recommended to use single
or stranded wires (preferably nickel coated) with loose fitting PTFE (Teflon) spaghetti.
b) Signal
Signal feedthroughs other than coaxial such as BNC, SMC, Microdot, etc are from one to multiple
pins. It is recommended that connections to multi-pin feedthroughs are by the use of slide on pins.
Many are either sliver of gold plated for good electrical contact. Avoid soldering unless care is used
to remove all traces of flux and no pin-holes in the solder have been created
On coaxial feedthroughs the coaxial connector will be either both sides of the feedthrough and
hermetically sealed or with the coaxial connector on the atmospheric side and a plain pin on the
vacuum side, also hermetically sealed. For either type, where coaxial cable must continue into the
vacuum space it is recommended to remove the outer plastic insulation and replace it with loose
fitting PTFE spaghetti. This is also recommended for all single or stranded wires carrying signals.
c) High Voltage, High Current
There is a wide range of high voltage and high current feedthroughs available in most flange types.
High voltage feedthroughs usually have a larger insulator in ceramic and sometimes glass. These
must be kept clean on the atmospheric side. Long term breakdown on high voltage feedthroughs can
damage the insulator causing vacuum leaks.
For high current applications such as heaters, dedicated high current feedthroughs must be used.
These are characterised by larger cross section wires to carry the higher currents. Like HV
feedthroughs they must be kept clean. It is recommended to clean the insulators on signal
feedthroughs periodically to remove any accumulation of dust.
d) Power
There are few feedthroughs designed for powering devices at mains voltages. It is highly
recommended that experiments are designed not to have mains voltages in vacuum vessels or
equipment due to safety concerns.
11.2 Mechanical Feedthroughs
a) Rotary
Rotary feedthroughs allow the transition of rotary movement into the vacuum space to, for example,
operate shutters or position samples or detectors. They are widely available in many of the common
flange styles. There are three common types both of which are very good in high vacuums.
The first type is a continuous shaft between the atmospheric side and vacuum side using rubber
vacuum seals in the form of either supported ‘o’ rings, square or odd shaped rings (like common
shaft seals), and flat rubber sheet seals. These are best for general rotary actuation rather than
continually rotating. Okay in high vacuum but care must be taken to keep the seals clean and
periodically replaced or whenever wear is noticed.
The second type is a ferro-fluidic type which also uses a continuous shaft between the atmospheric
side and vacuum but the seal is made by encapsulating a dense fluid around the shaft. These types
usually have bearings to keep the shaft centred and free moving. They are best for high revolutions
in high vacuum applications.
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