Tungsten Emitter

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David Kunkle
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Tungsten Emitter

Post by David Kunkle »

Looking for a decent passive electron emitter. Didn't find much on a search. Been using 60W incandescent bulb filaments and house current. They actually work OK, but I'm getting tired of them frying. The supporting wires seem to get hot, bend, and then short out on the chamber walls.

New plan:
I already have some of the W filaments in the photo below.
W filament.jpg
From what I can gather for these filaments, all I need is a Variac that can handle the amperage, and then run it at just 2 or 3 V?
What kind of amperage would you expect these to require?

Thanks for any input.
If your experiment needs statistics, you ought to have done a better experiment.

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Rich Feldman
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Re: Tungsten Emitter

Post by Rich Feldman »

On 120 volts, your household lamp guts will run hotter in vacuum chamber than in original argon-filled bulb. To be quantitative, you could use one as a Pirani gauge. At what reduced voltage and current does the filament's resistance in vacuum match its resistance on standard voltage in original bulb?


You should use a step-down transformer together with the variac, to have good control resolution at that kind of voltage.
How about using, uh, a filament transformer? A low voltage lighting transformer could serve.
Variac could go on either side of the fixed transformer, but its current would be much lower on the primary side.

You can estimate of the amperage requirement without actually heating one of the new filaments.
Hot:cold resistance ratio will be about the same as that of a light bulb filament at similar temperature.
Power will be similar to a light bulb filament of similar size and temperature or emission current.

When you go for first light, be careful. Once upon a time, I burned out a beautiful didactic Crookes tube using a variac and 2.5-volt filament transformer. Had forgotten that that tube's short, robust ribbon filament wanted much less voltage. :-(
All models are wrong; some models are useful. -- George Box
Jerry Biehler
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Re: Tungsten Emitter

Post by Jerry Biehler »

Like rich says, you need a filament transformer. A variac will have almost no control at that low end. And if you intend to put a bias on it to actually get it to emit more electrons you will need an isolated filament transformer that can handle the voltage you plan on running the filament at. BMI Surplus has a bunch of these Airco Temescal filament transformers. They are intended for use on electron beam guns for deposition. I got one for $75 from them and I think the darned thing is brand new. They are good for at least 10kv isolation from the primary and are rated at something like 6 volts at 40 amps.

http://www.bmisurplus.com/products/2174 ... ransformer
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Steven Sesselmann
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Re: Tungsten Emitter

Post by Steven Sesselmann »

Search for electroplating transformers, that's what I was using, a great big monster, 6V 50 Amps. For the filament you can pull one from a commercial microwave magnetron. Somewhat tricky to extract, but after a few you become an expert.
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
David Kunkle
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Re: Tungsten Emitter

Post by David Kunkle »

No electroplating transformers came up on ebay. I decided on a used Airco filament transformer from BMISurplus for about $100. 6-12 volts in the secondary.

Also, a new 110V Variac, 20 A, 0 -130V output, on ebay for $120. I was using a cheapo Menards dimmer on the 110V bulb filaments. Worked well enough, but half the time a filament would short, it would take out the dimmer/rheostat too. Hopefully, between the 2 things, I'll have a much easier time of it with the emitters.

Thanks for the help guys.

Dave
If your experiment needs statistics, you ought to have done a better experiment.

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Jerry Biehler
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Re: Tungsten Emitter

Post by Jerry Biehler »

You can also use a phase angle SCR controller like a Eurotherm 425A to control a resistive load on the transformer. Thats how I am controlling the filament on my e-beam gun.

Note that the Airco transformer is 240v in.
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Steven Sesselmann
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Re: Tungsten Emitter

Post by Steven Sesselmann »

David,

That's probably because its old school, they are called "plating rectifier" in the plating trade :)

http://www.ebay.com/sch/i.html?_from=R4 ... r&_sacat=0

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
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Rich Feldman
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Re: Tungsten Emitter

Post by Rich Feldman »

Note that the Airco transformer is 240v in.
Jerry, is David's new transformer the same model as what you got? Would be handy if it comes with a dual primary (120 P / 240 S), but today's BMI catalog pictures look unpromising. If 240 only, David can still easily get up to half or 7/12 of nominal voltages, which ought to be plenty.

The pictures clearly show dual secondary windings, presently jumpered in series. Red potted shape suggests that there might be HV isolation between the two secondaries. Don't know about e-beam applications, but that could be handy for powering rectifier filaments. David: if you ever connect the secondaries in parallel, it would be bad to get the phasing wrong.
21745_1_.jpg
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Jerry Biehler
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Re: Tungsten Emitter

Post by Jerry Biehler »

No, they are intended for running on 208/240 only. They were never expected to run on 120v since the power supplies that these worked with required three phase 208 or 240 to run. You could run these at 120 and get 6v out probably up to about 70 amps. They are rated for 840VA at 240 in. The windings are certainly isolated from each other, I just dont know how well. they also have shielding between the primaries and secondaries too.

These are intended for electron beam deposition systems like the old Airco Temescal CV-8 and CV-14. I am putting together an e-beam system in my large chamber at home (26" dia, 30" tall, 2000l/s turbo) to do dielectric coatings and stuff. I ended up getting a Thermionics electron beam power supply that will output up to 1.5 amps at 10kv. To power it I bought a 12.5kw 3 phase generator, the e-beam gun I am using can only handle up to 12kw so I should be fine. More info here: https://hackaday.io/project/190-vacuum- ... deposition
David Kunkle
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Re: Tungsten Emitter

Post by David Kunkle »

S_Sesselmann wrote:David,

That's probably because its old school, they are called "plating rectifier" in the plating trade :)

http://www.ebay.com/sch/i.html?_from=R4 ... r&_sacat=0

Steven
Found 'em. Only thing is, even for a used one, it's still quite a bit cheaper to go with a filament transformer.
If your experiment needs statistics, you ought to have done a better experiment.

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David Kunkle
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Re: Tungsten Emitter

Post by David Kunkle »

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.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
Jerry Biehler
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Re: Tungsten Emitter

Post by Jerry Biehler »

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

Post by Jeroen Vriesman »

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.
David Kunkle
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Re: Tungsten Emitter

Post by David Kunkle »

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.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
Jeroen Vriesman
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Re: Tungsten Emitter

Post by Jeroen Vriesman »

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.
David Kunkle
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Re: Tungsten Emitter

Post by David Kunkle »

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.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
Roberto Ferrari
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Re: Tungsten Emitter

Post by Roberto Ferrari »

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
David Kunkle
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Re: Tungsten Emitter

Post by David Kunkle »

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!)

[
If your experiment needs statistics, you ought to have done a better experiment.

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Dan Tibbets
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Re: Tungsten Emitter

Post by Dan Tibbets »

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.

Dan Tibbets
Jerry Biehler
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Re: Tungsten Emitter

Post by Jerry Biehler »

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.
David Kunkle
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Re: Tungsten Emitter

Post by David Kunkle »

Been occupied with other things lately, but finally got to test my new setup with the variac and filament transformer. Turns out there's not nearly enough juice to run either the light bulb filaments or the filament in the first pic of this post. All the way up on the variac, and they just glow red like a heating element.

Decided to cut out the filament transformer and only use the variac. Perfect! Excellent control and I get a very intense output from the large coiled filaments.

Being a cheapskate, I originally used a Menards dimmer switch before which gave good control for the light bulb filaments. But those filaments would burn out easily, short, and kill the dimmer switch. And I had lousy control over the large coiled filaments. Very touchy, and they would melt every time.

With the combo of the variac and the big filaments, I'm getting very fine control and intense output at 35V for extended periods without melting them.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
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