Maintaining a vacuum in the absence of a pump.

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fabian bunbury
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Maintaining a vacuum in the absence of a pump.

Post by fabian bunbury » Tue Aug 07, 2018 7:20 pm

Hi Everybody,

I hope this is the right place to put this thread. I'm new to the forum so I've put it in the new user's chat area.
The other day a friend of mine saw me working on my fusor and seemed impressed, so I had the idea that I would try and make one for him. At first, this felt like a nice idea but on further thought, I realized that it had some major caveats. The one that I think will cause the most difficulty is that I don't want to give him my vacuum pump, so I was wondering how easy it is to maintain a vacuum without a pump for a long period of time(perhaps using some kind of system of valves). Ideally, I would pump the vacuum and then give him the fusor which would be sealed so it stays evacuated.

Does this idea seem at all plausible, or does it just seem like the fantasy of a newbie?

Thanks.

MatthewL
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Re: Maintaining a vacuum in the absence of a pump.

Post by MatthewL » Wed Aug 08, 2018 12:51 am

A fusor can't run without a vacuum pump in operation. Although, you could construct a vacuum chamber and have it retain a vacuum for quite awhile without a pump, but with no plasma or deuterium. If he doesn't know much about fusion, then giving him one doesn't seem to have much value, and could possibly be dangerous.

-Matthew

fabian bunbury
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Re: Maintaining a vacuum in the absence of a pump.

Post by fabian bunbury » Wed Aug 08, 2018 6:39 pm

Hello Matthew,

Thanks a lot for that fast reply. Although your answer has mostly dissuaded from trying to build a non fusing but attractive star in a jar for my friend, I'm interested to know why it isn't possible to produce plasmas without an active vacuum pump?

I'm really sorry if this question stupid and if you think it is please don't feel the need to reply! :)

Thanks.

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Dennis P Brown
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Re: Maintaining a vacuum in the absence of a pump.

Post by Dennis P Brown » Wed Aug 08, 2018 8:00 pm

Yes, it can be done but requires extremely good vacuum - not just no leaks but no "virtual" leaks. That said, to build a fusor without flowing deuterium is also very difficult (and not worth the effort for most.) If you can build a extremely leak tight system, clean it so there are no virtual leaks, and take to to fairly high vacuum, as well as install both an electrode and window, then yes, one can maintain a plasma - to see an example, just look for those globes that are sold which have a 'dancing' light (a plasma.) They aren't even all that expensive. But again, the effort to do this task could take a very long time and a simple vacuum pump will not give you a clean enough vacuum to last. Just let them buy one of those plasma lighting globes.

ian_krase
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Re: Maintaining a vacuum in the absence of a pump.

Post by ian_krase » Thu Aug 09, 2018 6:26 am

Maintaining a hard vacuum in the absence of a pump is *comparatively* simple, by which I mean it's finicky and I don't know how many people here have ever done it.

First, the chamber must be hermetically sealed -- no rubber gaskets allowed. Everything must be brazed, welded or glass-sealed together and made of ceramic, metal, mica, or glass.

Second, the inside of the chamber must be fully baked and outgassed.

Third, there must be a "getter" -- a small amount of a highly reactive solid (often barium-based) that collects any air molecules that do leak in. (an alternative is to build a tiny ion pump into your chamber, which isn't strictly "pumpless", but achieves more or less the same goal. It has no moving parts.)



The problem is doing this not with a hard vacuum, but with a small amount of gas. A certain amount of talk has been given to this. Basically, it's very difficult to make something that won't, over time, either run out of gas or gain gas.

Neon tubes have a lot more gas pressure than a typical fusor. Mercury vapor systems, as used in lamps, fluorescent tubes, etc use toxic mercury (though not very much) in liquid form; the vapor pressure of mercury is then the gas pressure in the tube.

(A normal vacuum tube with hard vacuum is continuously running out of gas faster than it is gaining gas).

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Richard Hull
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Re: Maintaining a vacuum in the absence of a pump.

Post by Richard Hull » Thu Aug 09, 2018 2:38 pm

Ian is right. If you truly have a perfect seal on a vacuum device and if it is properly out-gassed, (baked or bombarded), including all the metals within it and also gettered when finally sealed, you can have an effective vacuum held virtually forever within it.

Vacuum tubes of old managed this well as the only metal within were the tube elements. The vessel was all glass which is relatively easily freed of gas atoms. As all vacuum tubes, be they electronic or of the lighting type, accelerate electrons in space through out their useful lives, tend to bury remnant gas atoms and this makes the tube much harder, (higher vacuum). For a normal electronic vacuum tube this is very good. However, for some critically gassed tubes with special characteristics, like hydrogen thyratrons, this can be bad. Needless to say, the fusor, in its best operational regime, must always be critically gassed. The largest hydrogen thyratrons have an internal gas storage system that demands a special voltage to re-gas the tube at each turn-on cycle due to gas absorption in normal operation. Ultimately, enough gas gets buried to the point that the special characteristics of the tube are impacted and the tube will need to be replaced. As critical timing pulse switches designed for use in radars of WWII, early hydrogen thyratrons had to be replaced after only 10-20 hours of use.

In amateur hands, the above ideal, as relates to a sealed, gassed fusor, is just not tenable.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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.

fabian bunbury
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Re: Maintaining a vacuum in the absence of a pump.

Post by fabian bunbury » Thu Aug 09, 2018 3:19 pm

Wow! Thanks for these descriptive replys, they've given me a lot to think about, and I'm grateful for the efforts.

Fabian.

ian_krase
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Re: Maintaining a vacuum in the absence of a pump.

Post by ian_krase » Fri Aug 10, 2018 6:09 am

Of course there are also mercury thyratrons. The biggest ones of these, called ignitrons, have a pool of mercury covering the entire bottom of the tube, which serves as an electrode that can't melt under high current because it is already molten.


It seems like you could make an (obviously non-fusing) "star in sealed jar demonstrator" using mercury. A small pool reservoir of mercury would be in the bottom of the tube, and it would have a heater and temperature controller. One can get mercury's vapor pressure through the full range of fusor internal pressures by varying the temperature between 25 and 100 Celsius. A special getter would collect any non-mercury gases.



It seems to me like beam-on-target systems can probably be made fully sealed. You would have a hydrogen source of some form integrated with the ion gun, and getter and/or ion pumps around the target / in the accelerator column.

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