Vacuum Technology and Vacuum Design Handbook for Accelerator ...

Water vapour and residual solvents are difficult to remove from a vacuum space with normal
vacuum pumping. Most low vacuum backing pumps provide gas ballast which can speed up the
removal of water vapour and solvents that may otherwise condense inside of the pump during the
compression stage. By introducing a small amount of air near the outlet of the compression stage the
small increase in pressure helps carry water vapour and solvents to the exhaust before they can
condense.
Some vacuum systems have been designed to allow chambers and beam lines to be heated. Heating
causes contaminants to be removed from surfaces at lower than normal vacuums. The higher the
temperature the faster the removal will be. Care must be taken to ensure heating doesn’t
inadvertently cause plastics and elastomers used within the vacuum space to outgas. Most bakeable
systems use ceramics and metals only.
It is important that where vacuum systems are permanently connected to high voltage devices for
roughing out, the insulated (plastic) pump out lines between the roughing pump and high voltage
device must be brought up to atmospheric pressure before high voltages are reapplied. Otherwise
the low molecular density of gas in the tube may ionise causing burning of the tube or even spark
damage that can puncture the tube causing a leak.
4.6 Venting
General
Venting can cause damage to vacuum components just like pumping out a system from atmospheric
pressure too fast.
Venting Speed
As discussed earlier the volume of vacuum systems on the accelerators is measured in litres to a
several tens of litres. A catastrophic loss of vacuum will be more of a brief audible event rather than
a dangerous mechanical failure. Little energy is required to change these systems by 100 kPa. There
are a few devices in the vacuum space that may be damaged due to pumping out too fast but none
that are considered a danger to personnel.
Other than venting too fast the second point to consider is the introduction of water vapour if
venting with air. Key vacuum systems on the accelerators are vented with either dry nitrogen or
argon both of which are delivered through a closed reticulation system from gas bottles. The choice
of gas is dependant on molecular contamination within the system for example nitrogen is not
preferred to vent the 846 ion source as nitrogen is an element that is known to bond with other
elements causing molecular ion beams.
Regulation of Flow
There are many low flow regulators and rotameters (tubular flow meters) available that provide
adequate control over venting. A simple in-house gas regulating system which works well is a tee
piece connected between the venting port and the venting gas supply line. The centre port of the tee
piece faces upwards and a ping pong ball sits over the hole with a cage over it so that it cant be
blown away. Under the ball is an ‘o’ ring seal. Before the venting valve is opened the venting gas is
allowed to flow. The pressure of the gas lifts the ping pong ball of its seal. The gas flow is adjusted
depending on the height of the ball above the seal. The ball can only travel a centimetre or so. Once
the flow is set the venting valve is opened and the now regulated gas flows into the vacuum space
and the balls drops onto the seal stopping air from entering the vacuum space. As the pressure
reaches equilibrium the ball begins to rise off the seal until eventually it rises to the original set
height indicating the vacuum space is now at atmospheric pressure.
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