Mu-Metal Chambers
When considering the design and use of chambers where stray magnetic
fields can be an experimental; problem (namely electron spectroscopy),
is is worth considering mu-metal as a possibility.
Mu-metal is a nickel-iron alloy (77% Ni, 15% Fe, plus Cu and Mo) which
has extremely high magnetic permeability at low field strengths.
Mu-metal can be used as a very effective magnetic screen as it exhibits
high attenuation at low levels of the interference field (i.e earths magnetic
field).
There are certain criteria which relate both to the physical properties
of mu-metal and the way that the chamber must be constructed. These must
be considered before embarking on a mu-metal chamber design. For instance
dished ends or hemispheres should always be used as opposd to flat base/
top plates. Thus has the added advantage of eliminating the need for internal
shielding.
Design for Effective Screening
Mu-metal vessels provide an internal volume with low residual magnetic field (typically <5 milligaus) and are desirable on experimental systems involving low energy electrons (<100ev). These experiments generally demand a number of probes and detectors, requiring many ports on the vacuum vessel. A hole made in the mu-metal envelope destroys the shielding effectiveness. The penetration from the external field extends internally to a distance approximately equal to the hole diameter. With a length at least equal to its diameter. The solution is to provide a mu-metal port on the vacuum vessel with a length at least equal to its diameter. Bell-jar type chambers must be fitted with a perforated mu-metal base plate to allow for pumping and provide access for drives, feedthroughs, etc. There must be substantial mu-metal to overlap between the base plate and the chamber walls and the mu-metal / mu-metal contact should be as tight as possible to form a complete magnetic circuit.
Chamber Fabrication
Mu-metal/ mu-metal welds are relatively straightforward if the material has been processed correctly. After welding, the chamber is annealed with a vacuum bake to 1100oC to restore the magnetic permeability. This precludes the fitting of the conflat flanges before baking as the knife edges would soften to an unacceptable extent. Therefore the flanges are welded on at the final stage of fabrication, including the following measures:
- A short steel skirt can be welded to the end of each port prior to a high temperature annealing: the flanges can be readily welded to theses skirts afterwards.
- A combination of welding and brazing is used to ensure the chamber is leak tight.
