“I believe that there is a subtle magnetism in Nature, which, if we unconsciously yield to it, will direct us aright.”
– Henry David Thoreau
No disrespect to Henry David Thoreau (or, as the kids today call him, Lil’ Thoree), but for physicists observing atoms in a closed compartment, magnetism often directs us “awrong.”
Magnetic waves can pose significant problems when it comes to creating an ultra-high vacuum environment. How many times have you been manipulating atoms in your pressure chamber only to find that your viewport’s magnetic permeability is interfering with your observations?
Kovar Kovar-ringed viewports may be pretty; in our opinion, they have the brightest smiles, but the magnetic charge they emit (a whopping 1.26 10−4 on a good day) can produce havoc in your vacuum vessel.
What to do if you need a viewport with a magnetic permeability of less than µ<1.01, or even lower, to maintain a particle-free UHV environment…hmmm
In recent years, it has become the scientific standard for physicists to use fused silica viewports mounted to 316LN conflat flanges. Employed by nuclear research laboratories such as CERN (the European Organization for Nuclear Research), 316LN is an alloy that contains 18%-20% chromium, and 8-10.5% nickel, in addition to other metals, with a magnetic charge of µ<1.01.
At MPF, we carry viewports in a variety of materials with 316LN conflat flanges. Less magnetic than their stainless steel peers (I’m looking at you 303, 304, and 316), the advantages of 316LN viewports reside in their versatility: UHV viewports can be welded to a number of stainless steel and metal components.
At MPF, however, we know that some of our partners in the scientific community require even lower magnetic interference in their vacuum vessels still. Like Max Eisenhardt, they seek complete control over magnetic fields.
Enter: titanium. The magnetic permeability of commercially pure titanium possesses a range of between 1.00005-1.0001. Titanium offers the lowest magnetic permeability available, but can be a bit of diva. You have to pick all the brown M&Ms out of the bowl of this viewport, and they simply don’t play well with others.
Unlike 316LN, if you decide on a titanium viewport, you need all of your components to be titanium. Don’t weld 303 stainless steel components (or any other non-titanium components) to this ultra-high vacuum viewport and think you’re just going to walk away without trouble. But, again, titanium conflat flanges are the way to go if you want the least magnetic interference in your vacuum cavity.
MPF offers a number of viewports with titanium conflat flanges, which we test with one of our favorite toys, Ametek’s XRF Elemental Analyzer, to ensure that you are receiving a viewport with a magnetic permeability of µ< 1.0001 (the lowest you can go). So, our only question for you now is: how non-magnetic do you need your UHV environment to be?