Vacuum Technology and Vacuum Design Handbook for Accelerator ...

12. Internal Devices
There are many devices that are commonly used inside of vacuum systems. Not all are represented
in this section however similar reasons are use to assess new and unusually devices for use in
vacuum.
12.1 Lighting
Lighting is necessary in most sample measurement chambers where the operator must be able to see
the sample in order to position it in front of the ion beam. In most cases a standard bare 12Vac
halogen bulb is used with good results. Care must be taken to ensure the bulb can be dimmed as
when running at maximum power the heat load can only be dissipated through the connecting wires.
That is, there is little gas inside a vacuum to conduct head away from the bulb. Bright white LED’s
are also used with mixed success. They have less radial light output as compared with a bare
halogen bulb. If the LED is setup correctly the light can be focussed towards the necessary target.
Preferably, wires must be stranded or solid wire with a loose Teflon sleeve but normal plastic coated
hook up wire works in high vacuums. In some cases where the wires as not likely to come into
contact with other components then they are used uninsulated. All wiring must be restrained so that
it cannot move into the path of the ion beam. As a rule-of-thumb wires should be at least 20 mm
from the beam.
A standard double BNC feedthrough can be used, that is, only the centre core connections are used.
There are many off-the-shelf electrical feedthroughs available that will be more suited for this
specific use.
12.2 Heating
Providing power to a heating stage on a sample (target) stick requires a high current feedthrough. If
a feedthrough isn’t provided from the supplier of the heating stage then many types can be found
from larger vacuum suppliers. Ensure the feedthrough selected is designed to carry the maximum
current that can be supplied and not the maximum current likely to be used. Like in the lighting
section above, care must be taken to ensure wires are insulated and away from the beam path.
Heating stages rely on heat dissipation through the support for the stage which is usually made from
stainless steel, a relatively poor conductor. When opening devices for handling be sure they are cool
to touch first.
12.3 Cooling
Cooling stages come in 2 distinctly different types. The first is the Peltier cooled stage which is all
electric. Powering the stage can be managed as for lighting and heating. The second type is a liquid
nitrogen cooled system. It uses an insulated tube to carry LN2 through a feedthrough into the
vacuum space. A typical design is where LN2 is pumped through a coil attached to a heavy copper
braid. The braid is then connected to the cooling stage where heat is conducted away from the
mounting stage. Thermal isolation of the cooling stage from the holder can be Teflon or sapphire
standoffs or similar materials.
These cooling devices must only be switched on when the vacuum is at the working level otherwise
condensation and possibly ice can form on their surface becoming a large outgassing problem.
Likewise ensure the stage is close to room temperature before opening up the vacuum system to
atmospheric pressure. Always vent with a dry gas.
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