Vacuum Quality

14 03 2009

Reading this page on what it takes to make fusion with a fusor, I noticed this bit:

A vacuum pump capable of reaching pressures of 10-3 Torr (1 micron Hg) or deeper. A 2-stage mechanical pump is usually good enough. Lower pressures require oil diffusion or turbomolecular pumps in addition to a mechanical pump.

This seems to indicate that fusion is possible even with a 10-3 Torr vacuum. 

With this is mind I’m going to move forward with using the chamber with the current rubber gaskets. I may be able to get as low as 10-6 torr, which should be good enough to begin research, and maybe create fusion.

UPDATE:

Although 10-3 torr IS good enough for the fusor, the polywell requires the full 10-8 torr. See comments.

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31 responses

14 03 2009
A. McEvoy

A fusor will indeed operate at 10e-3 Torr or thereabouts. The problem is that you are not trying to make a fusor at all, but rather a Polywell, which will not operate at this pressure. Even 10e-6 is about 2 orders of magnitude too high in all reality. The mean free path of the electrons (which you do not inject in a fusor) is much too small at high pressure.

Fusors are much easier to build and create fusion with, but they will likely never reach Q=1 (this has been “proven” and I agree), which is basically the whole motivation for the Polywell approach.

Small rubber gaskets can maintain a higher-than-usual (>10e-6) vacuum seal over large gaskets, which is why I asked about the other flanges on your chamber. If you can convert the large seals to ConFlat you might be able to get away with leaving some of the smaller flanges rubber-sealed. If any of the flanges are ConFlat then you are in a much better overall situation.

15 03 2009
FAMULUS

Consider me disabused! Back to the drawing board.

15 03 2009
A. McEvoy

Don’t get me wrong, you should definitely try pulling vacuum on those gaskets and see how well your current system performs. However, while you are finding a few things (adapter flange for your turbo/chamber connection, backing pump, turbo controller, etc.) you should also try to find someone in your area who has some experience with making ConFlats from scratch. It will probably take a few hours of calling and asking machinists about this, although maybe you can find a vacuum specialist right away by asking around the right crowds.

If you find someone who is confident about doing this, before you get the other things you need to start pumping, I would just go ahead and do it since that is where you will need to go eventually. Also, make sure you can find a company that already makes a copper gasket of your dimension so you don’t have to order custom. Good luck!

15 03 2009
FAMULUS

I’m guessing the most challenging conflat to make would be the large top lid, 27″ diameter and it requires machining the main chamber body.

It might be possible to weld on conflat portals over the existing portals.

I’m not opposed to Re-Baying this chamber, however I don’t see a suitable chamber on the used market at the moment.

However it may be perfectly reasonable to weld closed SOME of the portals. I’d be perfectly happy wending this thing up so some degree if it resulted in a 10-8 torr chamber. You start by welding on the stuff you know you will need, like viewport, high voltage feedthroughs, gas feedthroughs, etc.

15 03 2009
A. McEvoy

It’s certainly up to you what direction you want to go. Just keep in mind that with welds you are stuck with that configuration forever. You will need to have a whole ton of stuff stuck onto that chamber and if you weld a portal shut in one critical spot you can be up a creek without a paddle. Also, welding a viewport may be unadvised due to high heat + glass.

I would shop around for machinists, new chambers and the other essentials before you commit to welding. As you said, if it results in a 10e-8 chamber you can get away with some inconveniences, but sometimes a minor inconvenience can turn into a show stopper.

15 03 2009
FAMULUS

These look like decent ultra high vac chambers:

http://www.tfstechnologies.com/Vacuum_Equipment/vacuum_chambers.htm

15 03 2009
FAMULUS

Some detailed background on conflat flanges:

http://www.ancorp.com/familyPDFs/file725200881048AM63.pdf

15 03 2009
FAMULUS

The industry standard conflats top out at 16.5″ OD. So doing a conflat for the lid of the chamber is strictly custom. Custom gaskets too.

15 03 2009
ike

i remember reading about the emc2 vacuum chamber using rubber gaskets too somewhere , so i figure the necessary vacuum cant be that high. of course, doing it “just right” by going to ultra high vacuum eliminates a possible source of error, but thats not trivial so maybe you should just investigate about the vacuum requirements a bit more.

15 03 2009
FAMULUS

Found this:

http://talk-polywell.org/bb/viewtopic.php?t=294&highlight=emc2+vacuum+chamber

Tom Ligon says:

MSimon, forgive me if I’m dropping into the conversation late and am missing something.

I know you have calculated the amount of fusion won’t produce enough helium to sneeze at, at least for the present. That’s not why the UHV level is not the real problem.

The big chamber in the old EMC2 lab had turbos, and a couple of the turbos had a chiller device on them. This was not a full cyrotrap, it was simply a vaned device that could freeze out water vapor. The chamber was electropolished, and brand new, without a bakeout, could pull down to the mid e-8 range on one of its 6 turbos. The manifold was pulled down by two roughing pumps with Roots blowers to improve the throughput. All the turbos were on big gate valves, and the whole system was rigged so that it could be used with half the turbos down for service and any other component out of action.

And I should have had more pumps!

I don’t judge the crossover to UHV by the pressure. For me, you are in UHV when water vapor is a trace gas, and you are dominated by hydrogen. Cryotraps are good for reducing water vapor, but if you need the cyrotrap, that means the system is still producing water vapor. You are much better off if the system is inherently clean enough that has stopped producing H2O, and also hydrocarbons if those are present. A bakeout at up to 400C is the traditional answer. UV lamps are supposed to also be very good at getting water out of the machine. If a system can get under 1e-7 and the dominant background gas is hydrogen, you are getting into the UHV range. Better, you have kept the machine clean and run plasma it until the dominant background gas is deuterium, you’re where you want to be.

But operating these machines, it is not your ultimate pressure that is the problem, it is dealing with huge gas loads evolved from metal parts when the plasma lights off. You need to get rid of that to keep the desired conditions. Aside from opening high-conductance pipes to the hard vacuum of space, I think turbos will probably be the answer. Lots and lots of them, with attention paid to conductance.

15 03 2009
ike

well, i guess that proves my memory wrong :)

hey, at least the polywell will be able to do the bake out without any additional equipment required.

btw: your magnet casings, you wrote they feel solid but are they really? i read that pinhole leaks are something to really wory about

15 03 2009
FAMULUS

Also found this:

http://talk-polywell.org/bb/viewtopic.php?t=502&highlight=conflat

Tombo,

I’ve used both a helium leak checker and a residual gas analyzer. I conclude helium leak checkers are a waste of time. The RGA will do that job perfectly well, but it will also distinguish between a real leak and various types of outgassing.

Once the actual leaks are sealed up (the RGA will show prominent nitrogen and oxygen peaks, and will easily show helium or argon test gas, usually with an audio indication of peak height just like a leak checker), the real utility shows up. The dominant peaks will initially be a water triplet, telling you it is time to stop wrenching and start bakeout.

A lot of newer systems use UV lights instead of heat, which blasts water off faster and far more efficiently. One key to all of this is to have a really smooth interior finish. Some grades of stainless already have a good start on this finish, and I had the big chamber at EMC2 electropolished. You want minimal surface area, hopefully no porosity, and nothing hidden from the UV light.

Wire brushing is one of the worst things you can do. It smears metal over depressions, making pockets that are very difficult to clean out.

Running a plasma will clean things up fast.

Once the water is gone, the primary background gasses will be hydrocarbons and hydrogen. Meticulous cleaning before assembly helps minimize the hydrocarbons.

It should not take all that long to leak check the system, but it can take a very long time to understand why the pressure won’t go down if you don’t have an RGA, and can’t tell what the source of the gas is. Once you identify the source, you know what you have to do.

When we got the big chamber at EMC2, in which WB4 and WB6 were run, I had it under vacuum in a few hours. Pumping it down, I found one 2 3/4″ conflat fitting was not tightened, thanks to the RGA. That took an hour or two to find. The chamber then pulled down to the “high eights” overnight on one of its six 1000 l/s turbopumps, on a Roots blower and a large forepump. I was delighted by how fast it pulled down and how quickly it dried.

15 03 2009
FAMULUS

ike: The casings are porous stainless steel infiltrated with brass. They hold water. I would not be surprised if they were porous to nitrogen.

Are you concerned about liquid nitrogen leaking out of the core, or just the general outgassing of the metal?

15 03 2009
ike

both. i mean, since it s porous i guess it was some kind of an indirect laser sintering process, which means additives have been used. im no expert in that area, but to me that just screams god knows what kind of contaminants embedded everywhere. then you have two different materials with different coefficients of expansion which opens up the possibility of miniscule cracks forming. exposing the contaminants and possibly making the whole thing leaky. and then across the single metal grains…. but dont hold to what im saying. im just speculating a bit.

15 03 2009
FAMULUS

Ike: maybe you were thinking of this re:rubber gaskets:

http://talk-polywell.org/bb/viewtopic.php?t=460&postdays=0&postorder=asc&highlight=conflat&start=45

I’m seeing a large door with what look like O-Ring compressing clamps.
That O-Ring looks too big to buy molded in 1 piece but must be glued from cord stock.

AND I’m seeing conflat flanges.

Can we really get away with oring seals at these vacuum levels?
And if so then why bother with conflats?
Conflats that big are a bear to get tight.

15 03 2009
ike

most probably those contaminants would have a vaporization pressure that is negligible, so thats kind of irrelevant. still, i think this is a possible problem to keep in the back of the mind.

15 03 2009
ike

from http://en.wikipedia.org/wiki/O-ring

In vacuum applications the permeability of the material makes point contacts quite useless. Instead, higher mounting forces are used and the ring fills the whole groove. Also round back-up rings are used to save the ring from excessive deformation [3] [4] [5]. As the ring feels the ambient pressure only at the seals and the ring feels the partial pressure of gases only at the seal, their gradients will be steep near the seal and shallow in the bulk (opposite to the gradients of the point contact [6]). See: Vacuum_flange#KF.2FQF. For high vacuum systems below 10-9 Torr, copper or nickel O-rings have to be utilized. As rubber becomes hard and brittle at low temperatures, in vacuum systems that have to be immersed in liquid nitrogen, indium O-rings are used.

15 03 2009
ike

in reverse, that means as long as you stay below 10-9, rubber is fine i guess.
but where’s the tradeoff? maybe you should add a book about vacuum engineering onto your shopping list after all.

15 03 2009
FAMULUS

Perhaps the main problem with rubber gaskets is they cannot withstand the 400C bakeout. But maybe this is irrelevant if we can use the plasma to bakeout the chamber.

15 03 2009
ike

or its just that the amount of outgassing scales with the exposed surface area of the o rings. so as long as you stay under a critical number… but that means you’d need hard numbers you can calculate with. lots of hassle.

or how about this? : “Metal wire gaskets made of copper, gold or indium can be used”

replacing the rubber with copper wire?

15 03 2009
FAMULUS

BTW, there is an electropolishing shop a few blocks away from my lab. Their tank would accommodate my chamber. I’ll get a quote just for knowing’s sake.

15 03 2009
FAMULUS

I get the impression Metal wire gaskets are pretty specialized. I doubt they can be swapped in for rubber gaskets. But I don’t know for sure.

15 03 2009
15 03 2009
ike

oh, another little tidbit i found on a quick googlesearch: “The use of aluminium wire as a vacuum seal between steel flanges has been studied. Gaskets made by twisting together the ends of a 22 s.w.g. wire could be baked to 200° C before they leaked on cooling. High temperature working was possible with butt-welded loops of aluminium wire providing the gasket was highly compressed. Such seals did not cold weld but, at baking temperatures above 400° C, the aluminium plastically flowed and adhered to the mating flanges. The “cemented” joint was vacuum tight at low and high temperatures (20-550° C) although the securing bolts became loose after baking. Such a seal leaked only when the aluminium reached its melting point. Information is given on the minimum compression force which must be applied to an aluminium wire in order to form a cemented joint at elevated temperature.

Several uses of the aluminium-wire seal are described.”

15 03 2009
ike

that kind of leaves the question how copper behaves, and why conflat flanges are preferred over those.

15 03 2009
FAMULUS

Page 22, WB6 report:

2) Ensure gas solenoid switch is closed. (tank pressure < 1E-6 torr)

Page 21:

while the vacuum tank pressure would start at the low 1E-7 torr range.

15 03 2009
ike

“standard Cu conflat gaskets, heat treated Cu conflat gaskets, Al conflat gaskets and Al wires. For the conflat gaskets the flanges were machined with the standard conflat flange geometry having a knife edge. For the aluminum wires a groove was cut into the flanges. After a leak check of the connection at room temperature several cycles were made between room temperature and 4 K. Afterwards the gasket was exchanged and the whole test repeated. In addition some flanges have been tested down to temperatures of 1.8 K using superfluid He. At room temperature the measured leak rates are below 10-10 mbar l/s for all type of gaskets. With the flange immersed in liquid or superfluid He the leak rate for all flange connections sealed by aluminum wires stay below 10-9 mbar l/s. In contradiction for all types of conflat gaskets significant leak rates of up to 10-6 mbar l/s have been observed accompanied by damaging of the knife edge. This behavior can be explained by the different thermal Isolation expansion coefficients for NbTi, stainless steel and the conflat gaskets. Based on the results of the cold tests the aluminum wire gasket has been chosen for the new flange design”

from: http://tesla.desy.de/mvp/publications/papers/padua_97.pdf
win! seems like wire gaskets are a real alternative and deserve a closer look.

15 03 2009
FAMULUS
15 03 2009
ike

im getting out of the loop from here, there’s still plenty of tensor algebra homework to be done.
and its kind of late in germany :)

16 03 2009
A. McEvoy

That RGA on ebay looks like the display unit only. There aren’t any pictures of the RGA head that goes on the chamber.

16 03 2009
FAMULUS

yes. the RGA units that come with the head are around $2K.

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