Talk-Polywell.org

Hello All,

In CSI's videos they talk about nitrogen or helium filled background gas.

Is it a common practice, for people to fill their vacuum chambers with gas like this before they do pump down?

No? Yes? If, so, why?


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Did anyone see anything in the CSI talks indicating, how long this electron cloud was stable for?

They talk about long-term... but its hard to tell what they mean by this.

they probably do this to remove atoms/molecules that might chemically interact. notice helium and nitrogen are both noble gases - they are at the furthest right of the periodic table, meaning all their electron orbitals are filled. meaning they don't form molecular bonds with anything. they are chemically inert.

this way the few atoms that are in there in the near vacuum )(well, something like 99.9% of them) aren't going to e.g. (in the case of oxygen) bond with the hydrogen to make water, or oxidize the chamber to make rust, or combine to make ozone when there's an electrical spark, etc.

i don't know for sure, but i'd imagine this is common practice. it's certainly a good idea.

Nitrogen a noble gas. That is a good one.

Anyway, IIRC the reason to back fill is to dry out the chamber.

EDIT

http://www.vacuumlab.com/Articles/VacLab27.pdf

Stubby wrote:

Nitrogen a noble gas. That is a good one.

oh geez, haha! i must have been thinking neon or something...

EDIT: in case anyone missed the joke, I just called a CHNOPS element ( http://en.m.wikipedia.org/wiki/CHON ) "inert".

Stubby wrote:...
Anyway, IIRC the reason to back fill is to dry out the chamber.
...

This is the correct answer. At a molecular level, water, nitrogen, oxygen,and other things stick to most surfaces. You don't notice this every day because this is a very small amount, a few atoms thick. This is called adsorption. Note this is different from the more familiar absorption in which the materials dissolve *into* the surface. Absorption of water into the rubber of sealing o-rings can also be an issue.

At high vacuum, you are generally under the vapor pressure of all the gunk adsorbed onto the surfaces inside the chamber. Water is especially problematic. This all starts to boil off and can add up to a significant gas load; you won't reach your ultimate pressure until it is all gone. You either have to wait on the order of days for the system to naturally clear itself out, or you can perform a 'bakeout' where the chamber is heated to a few hundred degrees to drive the adsorbants off surfaces in a few hours. This requires semi-specialized hardware that can handle the temperatures.

Long story short, a plasma also blasts this gunk off surfaces into the reactor volume and you generally have really poor performance until everything is cleared out.

The point of nitrogen is to try to mitigate all this. When you bring the system up to atmospheric pressure for servicing, you re-pressurize by bleeding in dry nitrogen from a bottle instead of plain old wet humid atmosphere. Dry nitrogen more or less plugs up all the little nooks and crannies and keeps water out of them. This means the system will generally cycle back down to low pressures much faster next time if you are relatively quick about getting it closed back up.

It is a preventative measure, and would not do any good if you pumped it in before pumping down for the first time. This is all very common for high vacuum systems. The best option is to just keep the system pumped down 24/7.

Why would someone use helium instead of nitrogen? Isn't nitrogen far more abundant and thus cheaper?

I have never had personal experience with anyone using dry helium to backfill.

However, helium is a standard tool to check for leaks. It is a small atom that easily squeezes through cracks. It is also generally not found naturally in atmosphere making it a good 'tracer'. So, you bleed bits of He around the outside of the chamber and observe a residual gas analyzer, basically a real-time mass spectrometer. If you see a spike in He inside the chamber, you know the location you just passed over is where the leak is coming from.

This is very common, to the point of there being off-the-shelf self-contained little systems to do this

All of the above. Also, Helium (He4) is often used as the fill gas for various plasma studies. You cannot fuse it nearly as easily as deuterium or Helium3 or even hydrogen (via the CNO cycle), but it is a good monoatomic ionizable gas.

Dan Tibbets