Tungsten Emitter

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.

Re: Tungsten Emitter

Postby David Kunkle » Tue Sep 15, 2015 5:49 pm

Their website doesn't say much about them, but it does say 120V primary in there.

http://www.bmisurplus.com/products/2174 ... ransformer

Don't know if it's a different model than what Biehler got his hands on, or if the company info is wrong on the primary V.

Worst case if it is 240V, I should be fine if it'll put out 6V and 70A running on 110V. I intend to try the filaments in the above pic/first post. I'll see how well they behave with the new power setup.
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Re: Tungsten Emitter

Postby Jerry Biehler » Tue Sep 15, 2015 8:25 pm

I looked at the schematics and they are 120v, I thought they were higher. Good to know now before I hooked it up!
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Re: Tungsten Emitter

Postby Jeroen Vriesman » Wed Sep 16, 2015 11:18 am

Your emission will be much better if you can use thoriated tungsten. (So you need less heating for the same amount of electrons, and your wire lives longer).

You do need to activate it, which simply means running the wire very hot for a short time, a very thin layer of thorium will form on the surface. Use tungsten with 1 to 2% thorium.

Some info: http://electrontubestore.com/thoriated-tungsten.pdf

The thin layer of thorium reduces the work function from 4.5 to 2.6, since this is in the exponent of the emission current function the difference in emission is big (up to a factor 1000 at the same temperature).
During activation you can use the resistance of the wire as a temperature indication.

It is true that Ba-Sr oxide coatings have an even lower work function, but thoriated tungsten can be re-used after being under atmospheric conditions, Ba-Sr oxides are usually applied in the carbonate form and turned into oxides under vacuum by heating, the oxides would react with water under atmospheric conditions.
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Re: Tungsten Emitter

Postby David Kunkle » Wed Sep 16, 2015 8:05 pm

I'd be up for the thoriated W. Found a few industrial type sites with filaments- look like big $$ for a pre-made filament. Only thing I can find on ebay and google is the same- electrontubestore.com. About $100 for 10' of wire- overkill for what I need. Then coil this to make a filament with more area? They do give the electrical resistance for each wire type. Sounds useful, but not sure how to figure out how much power to run it at.
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Re: Tungsten Emitter

Postby Jeroen Vriesman » Thu Sep 17, 2015 9:02 am

contact for 2% thoriated tungsten wire (it's not on their shop site, but they do sell it for about $350/kg, I don't know the minimum order, I think it's 1kg)

http://www.aliexpress.com/store/511041

By measuring the resistance you can estimate the temperature, so you can find the power you need.

If the minimum order is too much, maybe share one order with some others on this forum? 100g per person for $35 + shipping seems a reasonable for me.
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Re: Tungsten Emitter

Postby David Kunkle » Thu Sep 17, 2015 6:48 pm

Nothing thoriated came up at that site. If it is $350, I'll wait and see how it goes with the filaments I have. Doubt I could find enough people to split it with to make it worthwhile.
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Re: Tungsten Emitter

Postby Roberto Ferrari » Thu Sep 17, 2015 7:14 pm

David

Pay attention to Tom Rapp' solution: he took out a hot cathode from a discharge lamp (quartz envelope):

viewtopic.php?f=12&t=9668&p=65042&hilit=Rapp#p65042

Sealing it with an o-ring looks working fine!

Regards

Roberto
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Re: Tungsten Emitter

Postby David Kunkle » Wed Sep 23, 2015 2:05 pm

Have the Variac and filament transformer hooked up along with the filaments from the first pic of this thread. The transformer came jumpered between 2 and 3, filament is wired to 1 and 4- just like Rich Feldman's pic of it on the previous page. At full power on the variac going to one filament only, the filament glows about as strongly as the coils in a toaster. When I was using the 60W bulb filaments, I didn't seem to be getting electrons emitted until 1/3 to 1/2 full power of house current; and the filament gave off a pretty intense light. I'm guessing I'm not getting many electrons right now with this setup. Don't even need the Variac in there at this rate- I get the same result if I leave it out of the loop.

Is there a way to get more power out of the transformer?
Are these filament just sucking too much power for what I need? (I was able to get them burning brightly, but uncontrollably, with house current- then they'd always melt in short order!)

[
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Re: Tungsten Emitter

Postby Dan Tibbets » Wed Sep 23, 2015 3:10 pm

I wonder if a Microwave Oven transformer would work. Reversing the leads would give a ~120 volt primary and ~ 6 volt secondary. Small wires on secondary would not handle much amperage and a ballast resister would be needed to limit amperage to a fraction of an amp(?). Or, tear out the original secondary- and replace with a few windings of heavy wire. I used a small automotive booster cable. With about -6-7 windings I was getting about 5 volts and plenty of amps. I don't know how efficient it was, but it made for a modest spot welder. For that matter, how about a regular spot welder from Harbor Freight or other retailer?
Alternately, how about a 12 volt battery? Current should be ~ 1/10th that of 110 household outlet through the same filament resistance , modified by any ballast, or effects of gas present as mentioned.

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Re: Tungsten Emitter

Postby Jerry Biehler » Wed Sep 23, 2015 5:15 pm

The transformer is rated for 12v at 70 amps, a microwave transformer or battery is not going to do any better.

The filament you have is probably just designed for higher voltage than the transformer can put out. You could try shortening the filament somehow. That would lower the resistance and operating voltage.

Also you should be running the filament in a vacuum, it should run brighter since there is no conduction from atmosphere and to get electrons out of it you should be negative biasing the transformer secondary, dont expect many electrons from just a glowing filament, especially at a low operating voltage.

You can also just pick up some e-beam gun filaments, they are not that expensive since they are a consumable in the systems.
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