Single-digit micron pressure in plastic belljar?

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.
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Richard Hull
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Re: Singe-digit micron pressure in plastic belljar?

Post by Richard Hull » Thu Feb 11, 2010 2:40 pm

When you have only an 8 micron air/oil vacuum and try to do fusion, you have to waste a lot of D2 to arrive at an 8 micron vacuum that replaced all that air/oil or at least, say, 90% of it with fusion fuel. You have to do this every time you start up. Starting at 10e-5 or 10e-6 torr and pushing in D2 to 8 microns you have about a 99.99% D2 environment instantly.

I often, when using a mechanical only, had to push D2 in for 15 minutes to get to my max fusion point. The micro maze would take it to 0.5 microns , ~5X10e-4 torr, but you still had to push D2 in for a few minutes to get the best results.

With a Diff or Turbo, you get instant gratification.

Remember, back in the early days 96-99 I used only a single mechanical pump. I got real good at it too. I was a VCIW (Don Lancaster's term - voice crying in the wilderness.) zero-point-zero amateurs were doing fusor work then.

The micromaze was a mile high advantage and I recognized that, so, when I built fuosr IV I used one of the small diff pumps I had obtained in the interim. Many of the earliest fusioneers who did fusion used only a mechanical pump and maybe a micromaze.

Richard Hull
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Rich Feldman
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Re: Singe-digit micron pressure in plastic belljar?

Post by Rich Feldman » Fri Feb 12, 2010 10:45 pm

Thanks for all the advice. Here's an update, a bit more info, and 1 more question.

1. There's no hard requirement for single-digit micron pressure or particular gas composition - we'll just see if what we get is enough for 8th grade science fair goals. One project is to unspool Scotch tape & detect x-rays -- Camara et al did their published work at 1e-3 torr, and we know it does NOT happen at 1 atm. http://www.wired.com/science/discoverie ... _xray_tape Another is to generate an e-beam visible in the gassy space. This guy made cold cathode CRTs that work at 150e-3 torr! http://www.sparkbangbuzz.com/crt/crt6.htm . I have a lecture-hall "fine beam tube" that makes a nice purple beam with a hot cathode & less than 100 volts anode potential. Not sure what its fill gas pressure or composition are.

We'll see (this is the kid's project, I just provide vacuum) if an ex-TV electron gun can be fired in belljar without burning up its filament. Perhaps the critical weakness of the plan. We will test the waters with some light-bulb filaments in the chamber (the more W mass, the better. I wonder what else might serve as a getter?)

2. When the KF fittings come in we'll try mating our manifold directly to a belljar baseplate
with 1" round hole (that receives one side of KF-25 centering ring) and a bulkhead clamp -
which could be just a big flat washer loose on manifold pipe, with a few bolt holes in a circle. That would reduce the number of threaded connections & expensive flanges.

3. For gasket material: If I can't find smooth silicone sheets at a kitchen or restaurant supply store, or suitable diaphragm material at hardware store, will face a bewildering array of choices at McMaster. Must choose thickness, chemistry, durometer, etc. and can't afford to buy one of each. How 'bout 1/8 inch, silicone or neoprene or buna-n, about as hard as rubber bands or inner tubes?
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Re: Singe-digit micron pressure in plastic belljar?

Post by DaveC » Fri Feb 12, 2010 11:37 pm

Rich -

They sound like some interesting projects.

1. Silicone Rubber from Mc Master is the way to go. Get it soft as a pencil eraser. Hardness is not too critical, anyway. Durometer of 50-70 Shore A, I think.

2. With the CRT electron guns, these are all tri-oxide cathodes, which are quite possibly irrepairably ruined by their first air exposure. So be prepared for that. A good pump down to usually in the -5s, -6s (Torr) may allow you to dewater them safely with some filament heat... bright red to dull orange temps. Remember, these are LOW TEMP filaments - typically 900 -1100 K. Heater voltages are usually 6.3 or 12.6 VAC.

If the output.is abnormally low, a Tri-oxide Cathode, can sometimes be regenerated, once or twice. It's done by heating up to about 2700K with some extraction voltage on... a few volts on the control grid, and perhaps a few hundred on the "anode" which is part of the lens assembly inside....it's the strucutre connected to a wire feeler that makes contact to the "dag" inner coating on the CRT. The so-called 2nd anode lead connects both to the (+) HV. But pressures need to be below -4'sTorr.

One surprise here is sputtering of the control grid ( brightness) G1 onto the cathode which rapidly poisons it, and that's it. Usually G1 sees only negative potentials for this reason. I think you can go with 0V on G1 safely, however.

Bottom line on CRT e-guns, they do not like hydrocarbons, or high temperature, so have few on hand while everyone learns about them.

3. Bare tungsten needs above 2200 K to begin to emit in a serious way. Tungsten with Thorum, or Zirconium oxide, or Cerium oxide alloys emit at lower temps by up to 500K. (I like the W-2%Th best, but it is slightly radioactive and that frightens some.)

Building a test cathode out of simple flashlight lamp is a great learning experience.... but again, tunsgten doesn't last real long in micron pressures of air. If you can do a N2, Ar or He flush...first... that will lengthen your play time.

But all the improvising is a major part of the fun and excitement.

Check out Richard Hull's notes on Micro-Maze... it may be just the thing you need to get good results all around, with the mechanical pump, only, as long as you don't plan to use H2 or D2.

Dave Cooper

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Re: Singe-digit micron pressure in plastic belljar?

Post by Carl Willis » Fri Feb 12, 2010 11:48 pm

Rich,

Regarding your question about sealing materials, look for an industrial gasket supply company in your phone book. We have two in Albuquerque. Custom flat gaskets in our linear accelerator and related vacuum work are usually cut from Viton sheet from these sources. The sheet itself is surprisingly affordable. We just drive over and pick it up as needed. I have made gaskets at home for diff pump mounting (used to be custom Edwards gaskets) from this as well.

Pre-made standard gaskets, in all kinds of shapes, are available from a local Parker or similar dealer ("Southwest Seal" in Albuquerque). This is an expeditious and less expensive alternative to ordering from Kurt Lesker (or probably McMaster-Carr), although the gasket dealers tend not to know the vacuum hardware standards and you have to order the o-rings by dimension. I recommend Viton. Buna-N also gets used in vacuum gear quite a bit, but lacks the temperature and chemical stability and has worse outgassing and permeation properties. Viton, when dropped, does not bounce. Buna rubber bounces. If you want to get a matching elastomer for a NW, ISO or ConFlat flange, bring the flange to the store, or better yet, an example o-ring. It is easy for them to pick out a suitable alternative from stock, even if they are clueless about the standards for vacuum work specifically. I have no idea if there is a standard, stocked size of flat gasket suitable for a bell jar, but there's a good chance.

Good luck on your educational projects.

-Carl
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Re: Single-digit micron pressure in plastic belljar?

Post by Rich Feldman » Mon Feb 15, 2010 7:32 am

This is to report my biggest "Doh!" moment of the year.

Readers of the 4hv.org forum followed my story a couple months ago, about repairing a refrigeration-service pump that my dumpster-diving friend had scrounged. The cartridge was practically seized, so the motor could not start. No damage was evident on disassembly, but a part was missing, so I wasn't the first to open this case. The shaft turned freely, until the bolts holding cartridge to aluminum case were tightened. Had my predecessor disassembled the cartridge too far & got the stack mis-registered? Tapping with a steel hammer while progressively tightening the bolts & briefly running the motor seemed to fix it. Then came February ...

Things were going well with the DV-85 pump. Yesterday was the first time I ran it for more than a few seconds. Had a donated Convectron gauge and was testing some vacuum connection parts & methods for the science workshop program (while waiting for KF-25 fittings to hook up the fancy pump). With a KF-16 ring set between the gauge and the 1/2" ball valve, I reached an indicated vacuum of 20 millitorr after a few minutes for the new & old plumbing (and gauge) to settle down.

That was fantastic, but when I tried to repeat it this morning the motor could hardly turn the pump. The machine had reverted almost to its as-scrounged state. Well I needed it for a Sunday afternoon session at the science workshop. So drained the oil, opened the case, and messed with the all-too-familiar cartridge bolts as reported above. That seemed to do the trick, but after a minute or two of run time (at science workshop) the pump started dogging again.

Back home after a generally productive session, I was eager to tackle the pump, but reluctant to repeat the oily disassembly job. How about running in different positions? With pump standing vertically on its nose (motor end up), it seemed to run a bit smoother. Now stand it on its tail, which had been handy when the case was open to access the cartridge.

But this time the case was closed & full of oil. The bent-pipe handle on these pumps is screwed directly into the "crankcase" and serves as the air exhaust port. Or, in motor-down orientation, the oil drain port. At least the oil drains slowly because air has to glug in through the same pipe -- if the pump is not turning! When I pulsed the motor for a second, it pressurized the case & drove the whole pint of oil out through the handle, in a stream like that from a garden hose. I think this is where Homer Simpson would say, Doh!.

Until I have time to get some more clean oil, I will re-consider what defect of material or assembly could make a good pump (as attested by Lutz on the other forum) turn bad.

-Rich
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Re: Single-digit micron pressure in plastic belljar?

Post by Rich Feldman » Mon Mar 08, 2010 8:40 am

Here's a semi-final report -- hope it doesn't break a double-post rule.
Today we got the bell jar down to 20 mtorr, presumably mostly air, using my Plan B configuration. Measured X-rays from unspooling Scotch tape. Observed E-beam from a television CRT electron gun impinging on a shard of broken fluorescent light.
The vacuum system:
8" square of 3/8" HDPE baseplate (deflects about 2 mm). Path to pump is 24" of 1" ID hose with KF-25 flanges on each end. Vacuum gauge connection is a hose barb to 1/8" MPT adapter screwed into baseplate. Electrical feedthrough is 4 pieces of insulated AWG20 solid wire passed through another 1/8" MPT fitting & sealed with 5-minute epoxy.
The x-ray system (in picture): Tape-winding machine made by 8th grader using a DC gearhead timing motor.
The e-beam system: must wait for another post (I have questions for you all).
Thanks for the support.
-Rich
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Re: Single-digit micron pressure in plastic belljar?

Post by Doug Coulter » Mon Mar 08, 2010 10:49 pm

Looking good! I'd have used a cheap canning jar for this, and probably gotten a bit better vacuum, I've been using them for little, dirty tests for years, no problems -- they are cheap enough to toss in the trash once all gunked up with things. You can even solder a pipe into one of the lids for the vacuum pump, and do similar things for feedthroughs. They are fairly thermal-shock resistant -- not perfect, but they don't break by accident without a heck of a good reason.
Why guess when you can know? Measure!

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Re: Single-digit micron pressure in plastic belljar?

Post by lutzhoffman » Tue Mar 09, 2010 2:01 am

Really Cool!!

Its not just the equipment its the experiments done, I think this was a good choice, anything to keep their minds engaged. Your basplate looks like it could accept a future glass bell jar upgrade also. Good show, you could maybe even try different gasses, like Ar, and He, with a small HF high voltage power supply, for some really cool plasma effects, many students really enjoy this.

Two things that you may consider for a deeper vacuum, if you desire one, are:

1. Add a simple dry ice cold trap, with some activated charcoal. You fill the trap, heat it some, and evacuate to clean it out. Then after cooling you put it in dry ice / alchohol slurry, and your vacuum will drop way down, sort of a poor mans DP. If you can get LN2, then even better!!!

2. Run premium oil with a low VP in the Alcatel pump, Inland 20 will get you way down. Some folks recommend warming the pump a little when doing this, I never needed to on mine though.

Keep going it all looks good.....

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