Fusor Forums

Kurt J Lesker actually sells big tungsten filaments pretty cheaply (about $5 ea) as evaporation sources. They tend to be a little too beefy for most small power supplies, but you may not need to heat these huge things up to white hot, either.

I have another source telling me the reason tungsten isn't good in hydrogen (or water vapor) is that it reacts, not sputters, with hydrogen, which makes a volatile compound that then evaporates. With water vapor, what happens is the water H and O get back together on the glass, leaving the tungsten there. It is supposedly one reason Edison didn't use the stuff, there weren't vacuum pumps good enough then for it to have a better lifetime in use.

I know I've burned out a few small ones pretty fast in the vacuum produced by a new Sargent-Welch 2 stage pump, and was not even bringing them to full brilliance. At orange heat, 1 mil wire is lucky to last one hour.

Hydrogen (or helium) is what you have in the tank if you want a glow but *no* sputtering, except with tungsten.

Tungsten filaments are used in Hydrogen Thyratrons with no ill effects and in some are also used to heat the Hydrogen reservoir to release the Hydrogen to allow 100kw conduction in large tubes.

It is the well known and hated tungsten-water cycle that is so deleterious to such items in vacuuo, as Doug mentioned.

Unfortunately water is a tough cookie to get rid of in systems going up and down from vacuum.

Hydrogen embrittlement in all metals is also well understood, but doesn't impact us in vacuum tubes or fusor use as the stuff is not flexed, nor does it usually bear a load.

Richard Hull

Water vapour can be a real problem - how our test guys get rid of it is baking the chambers in a huge oven for a weekend connected to an RGA so they can tell when it has all gone. Not sure the wife would appreciate me popping a small chamber in the oven for the weekend!

They also do a couple of things to keep it out - one is flush the chamber with N2 when venting, rather than air. Then, keep the ammount of time the chamber is open to a minimum. Not a cheap option, though CO2 may be an option for the hobbiest - easier to get rid of than H2O.

A good pump to remove H2O is the cryo pump (used by all the silicon chip manufacturers), and it has occurred to me that, with no moving parts, it may be possible to construct your own - they struggle with Hydrogen and Helium, Helium should not be a problem, but Hydrogen can outgas from stainless steels - and they use zeolite to capture that - which I think that you could add easily.

Anyone got an idea on how to construct their own?