Great American chamber bake-out

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
AnGuy
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Great American chamber bake-out

Post by AnGuy » Wed Oct 05, 2005 12:24 am

I think I already know the answer to this but:

Is it worth while to bake out a vacuum chamber with open 6 inch flange (open to atmosphere). The 6 inch flange is intended to be attached to a turbo pump, which I don't want to be exposed to high temperatures. I am not sure If I will be able to attach the pump while the chamber is very hot without burning myself. I also have a vacuum gauge with a 1.33CF flange that has plastic for the electrical connector. That will have to also be connected after the bakeout.

I could close the 6 inch port during the bake out with a flange blank, and perhaps pump down using a roughing pump and back fill with dry argon or nitrogen, and then attach the turbo. Ideally I would use a 6 inch gate valve, but they are extremely pricely and rare on ebay.

Second, can I rough pump down the chamber through the turbo's exhaust port, or do I need to use a port on the vacuum chamber?

Any advice is welcome.

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Frank Sanns
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Re: Great American chamber bake-out

Post by Frank Sanns » Wed Oct 05, 2005 2:37 am

Heating a vacuum chamber open to the air will not do a thing for you. It would be a waste of time. The time would be better spent being sure everything on the inside of the chamber was clean and degreased. Connect everything then pump down. You can draw slowly out of the turbo pump if you need to but I don't think it is the wisest because any debris that is left over in the chamber can be sucked into your turbo pump and this is not a good place for it. Once the chamber is pumped down, huge air currents will not sweep large debris into the pump so at high vacuums drawing from the bottom of the turbo is normal.

Don't be too worried about a bake out. They are necessary when you go down to 10-6 and beyond but they do not gain you much at 10-3 where a fusor runs. Pump on your chamber for a few hours or overnight and you will be in good shape. Once is is pumped out, keep it under vacuum. Keeping nitrogen or argon in the chamber will keep the water out but will not prevent gas molecules from diffusing back into the pores of the metal chamber. Dry gas is better than nothing but not as good as a tight chamber and a good vacuum.

Sounds like you are getting close to first light. If you have your power supply and grids ready, don't miss the opportunity to see what the plasma looks like as the vacuum comes down.

Frank S.

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Re: Great American chamber bake-out

Post by AnGuy » Tue Oct 11, 2005 12:40 am

>Heating a vacuum chamber open to the air will not do a thing for you. It would be a waste of time.

This is pretty much what I assumed, but it doesn't hurt to get a second opinion!

>Don't be too worried about a bake out. They are necessary when you go down to 10-6 and beyond but they do not gain you much at 10-3 where a fusor runs. Pump on your chamber for a few hours or overnight and you will be in good shape.

Well, the reason I considered a bake-out was the high outgassing issue I've experinced. I recently tried to pump down using just a roughing pump (Welch 1402) for about 10 or 12 hours and with in 48 hours the pressure rose from 15 milltorr to 520 milltorr. I left it for another week and the pressure rose to about 540 milltorr. I tried spraying methanol all over the outside and I saw no pressure increases. so I am pretty sure it outgassing issue.


>Sounds like you are getting close to first light. If you have your power supply and grids ready, don't miss the opportunity to see what the plasma looks like as the vacuum comes down.

Not quite. I still have to purchase the Turbo pump, electrical feedthrus, finish construction of the grids and power supply. After stumbling on how much effort it takes to pump down the chamber, I am going to spend extra time constructing the grids and other internal workings. I am going with a multiple grid configuration so I can test several ideas with a single pump down.

Now I understand why every loaths having to open up the chamber to make changes. It takes damn forever pump down the chamber to a stable pressure!

Thanks,

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Re: Great American chamber bake-out

Post by DaveC » Tue Oct 11, 2005 3:07 am

This brings up a prime reason for having a HiVac valve on the fusor. So you can leave the Turbo running, while you change out things in the fusor. Then, all you have to do if rough out the fusor, and then open the highvac valve and your vacuum will be in usuable range in a minute or two.

If you were to put a small diameter (like 1/4" SS line with Swagelok fittings and small bellows sealed valve) around the High Vac valve, you could actually use the Turbo to pump directly, via the small diameter line. Then once the pressure is down, open the main valve... and you're ready for work.

If you intend to do fusion, you will need some sort of throttle valve and a needle valve to inlet the D2. So the bypass line is a good investment.

Dave Cooper

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Richard Hull
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Re: Great American chamber bake-out

Post by Richard Hull » Tue Oct 11, 2005 2:20 pm

The key to getting right back into action, bypass line or not, is the rapidity of action in changing things in your chamber. Time and air moisture are very evil enemies to pumped system components once re-exposed to air.

Before opening a system as, John suggested, be sure to have every component and tool you will need at hand so that the work can proceed very rapidly and avoid at all costs touching the interior of the device with your bare hands, once opened. From seal break to reseal should be a matter of a couple of minutes at most.

If you take much longer to complete the task or if you run into a snag that drags on for tens of minutes, you will have a proportionally longer pump time. After an hour, consider the thing pretty fouled with water is you live in a moist climate. (East Coast).

Richard Hull
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Re: Great American chamber bake-out

Post by AnGuy » Wed Oct 12, 2005 4:55 am

>Dave Cooper: If you intend to do fusion, you will need some sort of throttle valve and a needle valve to inlet the D2. So the bypass line is a good investment.

Thanks, I already have a UHV needle valve for D2 and I have a throttle baffle for the turbo on the way. My chamber has four 1.33 conflat ports three 2.75 ports, one 6 inch conflat and one 8 inch conflat..The 6 inch port will be used for the Turbo, the 8 inch will be reduced to a 6 inch and used for the HV feedthru. One 2.75 will be used for a view port, another for a 4 pole eletrical feedthru. One 1.33 will be used for the vacuum pressure sensor, another for the D2 input (attached to a needle valve). The unused ports will be blanked off.

However I am still trying to keep the number of connections to a mininum. I have two UHV 2.75 valves which I could used to attach to both the Turbo and the Rough pump (via 2.75 Tee). However I don't see any significant reason why I can't just rough pump down thru the Turbo output port. Is there a solid reason why I should not do this?

>Rich Hull: From seal break to reseal should be a matter of a couple of minutes at most.

I don't see that being possible. Speed and accuracy are pretty much mutually exclusive when it comes to my skills.

>Rich Hull: avoid at all costs touching the interior of the device with your bare hands, once opened.

Do you have a prefered brand of gloves you can recommend? I've been using latex and nitrile gloves when machining SS to avoid nasty SS swarf. But these gloves have a small amount of powder (even the "non-powdered" version). Is there a vendor that sells gloves that are "vacuum" certiified?

Thanks

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Re: Great American chamber bake-out

Post by Richard Hull » Wed Oct 12, 2005 1:57 pm

When machining, I use no gloves and handle all items freely. I sort of expect a certain amount of cuts and scraps in this biz. I am far more concerned about something flying out of a chuck or out of a milling situation than a cut.

Once all machining is done all items are degreased according to vacuum standard practice.

Once assembled, evacuated and disassebly is mandated, all components are handled with clean, white cotton gloves.

Speed is easy during re-opening sessions with good planning in design and preparation prior to opening. Without speed or in the case of a major operation, a long re-pump session is just going to forever be in the cards.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Great American chamber bake-out

Post by DaveC » Wed Oct 12, 2005 7:20 pm

A.G. -

You CAN rough through your Turbo, foreline connection, but this is really not desirable for at least two reasons: One is the large pieces dirt and trash, hazard. Many turbos have a screen to keep nuts, bolts and lost washers out of the impellers. Good investment for all to make.

But the real reason, for roughing through a separate line, is that the high pressure gas, is wet, possibly oily, but in general a lot more contaminated than the foreline gas will be after things get down to a few tens of microns. Some of that contamination will coat the interior of your Turbo and despite the flogging the impellers give to everything, their is still some boundary effects that partially screen the surfaces of the rotors and stators.

Some Varian manuals describe use of a clean gas at relatively high pressure...high for a turbo.. that is... as a way to scrub off the blades and interior parts. If the thing pumps down to the pressures you need and does it quickly, then it is clean enough, and the rest is just procdures for a dull Saturday, or something.

For High Vacuum, high voltage assembly, we have always used the latex powder free gloves, to minimize contamination.

Richard's caution about not touching with bare hands anything inside is the most important caution of all. The grease (everbody knows this don't they?) from your fingers is fairly volatile, and will be an outgassing source for literally days.

There used to be a poster about the vapor pressure of a fingerprint. (Varian or Welch? ) Once had one in the lab.

I am always leery of the cotton gloves, from the standpoint of lint, but there is a nearly lint free type. And their softness is a plus on polished surfaces. Matter of choice, here, I think.

One thing to remember about the rubber gloves, is that they can easily become contaminated on the outside, with grease oil, donuts, coffee, and other common lab "materials".

A lot of opitcal folks, like the finger cots. I don't, but they do let the rest of your hands breathe while work their damage on the circulation to your fingers. They probably are not the best for vacuum assembly as your palms are not covered.

Sounds like you have your gas inlet metering system well in hand.

Dave Cooper

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Re: Great American chamber bake-out

Post by Todd Massure » Thu Oct 13, 2005 8:24 pm


Could I expect to get less outgassing from the ceramic parts in a fusor than the metal per given area? Does the feedthrough require any different preparation and handling than the metal?

-Todd

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Re: Great American chamber bake-out

Post by AnGuy » Thu Oct 13, 2005 11:52 pm

>When machining, I use no gloves and handle all items freely. I sort of expect a certain amount of cuts and scraps in this biz. I am far more concerned about something flying out of a chuck or out of a milling situation than a cut.

I use gloves because I usually ended up with a half a dozen tiny, razor sharp stainless splinters when machining with stainless. The gloves definately help. The splinters are often extremely difficult to remove or even see if your hands are dirty.

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