Vacuum Technology and Vacuum Design Handbook for Accelerator ...

Final clean. It is recommended that methanol is used for the final clean or another low residue
solvent. The final clean must use lint free wipes dampened with methanol and lightly rubbed over
the entire surface. An alternative final clean is to use the solvent bath and suspend the equipment in
the hot vapour for a few minutes. Care must be taken with the bath as the hot vapour will degrease
skin very quickly.
If a shiny (healthy looking) clean is desired then a metal polish such as Brasso can be used.
Whenever Brasso is used the residue must be cleaned off with ethanol. Care must be taken not to get
metal polish pastes into gaps that cannot be thoroughly cleaned out. Brasso is an enemy of vacuum,
it traps water.
6.4 leak detection
In the accelerator area, a leak is characterised by a flow of gas molecules into a space that is below
atmospheric pressure. The pressure of the gas entering the space will restrict the vacuum system
from reaching its design pressure which is typically in the high vacuum range. As a benchmark, if a
vacuum is 1 x 10 -5 Pa or more then disregarding the effects from outgassing, there may be a leak. In
system designed for ultrahigh vacuum the benchmark will be in the order of 1 x 10 -7 Pa.
Leak detection is usually only necessary for new chambers or equipment. It can be done offline or
after the part has been mounted into a beam line. The most common method of leak detection is a
helium leak test. Helium atoms being so small will find their way through openings that are too
small for most other molecules. Generally, the smaller the leak, the less helium that can make its
way along a leak path. The leak detector is usually a mass spectrometer tuned to have highest
sensitivity for helium. The detector is connected directly to the device being tested. Any helium that
makes its way through the leak will eventually be measured in the mass spectrometer. Residual Gas
Analysers (RGA), although not specifically leak detectors, can be used for just that. Working on the
same principle, mass spectrometry, the RGA can detect helium as well as other gases.
In some cases leaks can be too large for a helium leak test, that is, the amount of gas in the device
being tested is too high diluting the helium to an ineffective percentage. Also, if the pressure is too
high the leak detector cannot be opened to the device as the pressure gradient will be too high. Leak
detectors work best from the higher end of the low vacuum range.
Often there are multiple leaks in systems. It may mean repairing large leaks in order to find small
leaks. Where a leak is too large for the helium leak detector a few other methods can be used to
locate leaks.
• Solvents have a very low viscosity and vaporise very quickly in air. Wetting surfaces with
small amounts of acetone or ethanol can show small changes on Pirani and Penning gauges.
• Isolation of seals can, by process of elimination, restrict the testing area allowing large leaks
to be isolated. For example, if a chamber with many ports and devices attached has a leak. It
may be prudent to close all valves leading away from the chamber except for the leak
detector valve then sequentially blank off the ports one at a time (using a sound flange and
new seal) and test for a leak.
7. Soldering wires vs mechanical joints
It is preferable that wires in a vacuum space are joined together with a mechanical clamping type
joint. The clamp must allow for gas to escape. Where a direct joint is necessary then soldering is
permitted. Standard lead/zinc with resin core is okay provided that ALL flux is removed with
ethanol. This may require some soaking in solvent with gentle agitation. Solder without resin core
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