How do you get mercury into a toroid?

[peetss]

Also, what type of toroid material could withstand temperatures of near 0 Kelvin?

I have a very interesting idea but it requires an answer to both of these questions.

[TinselKoala]

Well, if you are working that  cold, the obvious answer is to freeze it into that form. Make a mold like a donut or bagel sliced in half. Pour the mercury in and freeze it. Make 2 and stick them together, weld with molten mercury and re-freeze solid.  Now you have a toroid of solid frozen mercury. Freezing is no problem; mercury freezes at about -40 degrees C. Getting it down to 0K is harder, though. Mercury's T sub c, the superconducting transition temperature, is just below 5 K. You will need liquid helium and a good cryostat, as well as some practical practice handling cryogenic liquids. Liquid Helium is commonly available in big cities but is pretty expensive and doesn't stick around very long, no matter how good your cryostat is.