ion source doesn't work

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DanielSandner
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ion source doesn't work

Post by DanielSandner » Sun Jan 10, 2016 5:00 pm

Hello,
At the moment i want to build this ion source: http://www.rapp-instruments.de/Beschleu ... neugen.htm
Until now i only use my 2-stage-Rotary Pump, but the tube is flanged to the (yet without oil) Diffusion Pump.
A few weeks ago i had around 5 Pa, i made this picture (the left glas-tube is connected to +25kV) so i thought everything is great. Image But now with 1 Pa, (the Glow discharge is gone) i use a selfmade Needle valve to get a discharge in the left small tube, i only get a very small glowing but nothing like rapp wrote (even with -25kV at the main tube).
Is this getting better when i turn on the Diffusion Pump?
Sorry for my bad english,
kind regards Daniel Sandner

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Dennis P Brown
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Re: ion source doesn't work

Post by Dennis P Brown » Sun Jan 10, 2016 6:40 pm

That low a pressure - 5 pascal with a two stage pump? That is pretty good but maybe a bit low for a good glow discharge (as you have seen) so maybe raise the pressure up to 20 pascal and the glow should get brighter. Not sure what you are expecting? As the pressure is reduced, the glow will get smaller and not too surprise that it appears to disappear at 1 pascal - what I would expect. The pump is working as it should but that is an extremely low pressure for a simple two stage oil pump - maybe the gauge is not calibrated.

DanielSandner
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Re: ion source doesn't work

Post by DanielSandner » Mon Jan 11, 2016 7:57 am

Hello,
I have an Alcatel2012A, http://www.idealvac.com/files/manualsII ... Manual.pdf As you can see in the datasheet the pump should reach an even lower pressure, but the pump is pretty old so I'm glad about 1 Pa. The gauge is very reliable.
My problem is the missing pressure difference between the capillary and the main tube. Rapp instruments had 0,8 mbar in the small tube, and 10^-4mbar (with oil diffusion pump) in the main tube, so you have a glow discharge in the capillary and ions but you can also accelerate them in the main tube. Is my capillary too thick/schort?
Btw i'm very impressed by your own accelerator Dennis.
kind regards Daniel

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Dennis P Brown
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Re: ion source doesn't work

Post by Dennis P Brown » Mon Jan 11, 2016 11:33 am

Rated values for pumps are generally lower than what the pump usually does. So your 1 Pascal is very good for a typical old pump and will work very well with your diffusion pump. Maybe your gas inlet for the gun is too small. Are you measuring current? If the current is very low either the power supply isn't doing its job or the gas flow is too large/small for the current. By the way, your 1 pascal is almost exactly what a fusor operates at (typical fusor's operate at 0.6 to 0.7 pascal.) The ion gun's glow depends on current, voltage and gas flow into the ion gun; and some on rate you remove gas.

Thank you for the complements and I hope it actually works soon!

What are your plans for the ion gun?

DanielSandner
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Re: ion source doesn't work

Post by DanielSandner » Tue Jan 12, 2016 1:31 pm

The power supply is a flyback transformer, but i had to put a 100MOhm resistor between the HV and the small tube because otherwise the small tube gets too hot.
Maybe i should use a filament or a coil so i can work at the actual lower pressure, because if the gas inlet of the small tube is big enough for a high beam current the pressure in the main tube should be high too (so the mean free path is very short).
I want to portray a metal surface onto a fluorescent screen, its a bit similar to the https://en.wikipedia.org/wiki/Field_ion_microscope.
kind regards, Daniel Sandner

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Dennis P Brown
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Re: ion source doesn't work

Post by Dennis P Brown » Wed Jan 13, 2016 11:35 am

I am no expert on ion guns, to say the least. But a 100 Mega-ohm resister!? That will guarantee that the current even from a flyback transformer is micro-amps. Again, no real knowledge but that is far too high a resistance, I would think. Just by ohms law and say the flyback transformer can really put out current at very high voltage - say an 1.0 amp at 10,000 volts - which then means the current is simply: I = V/R or (1*10^4)/(1*10^8) or 10^-4 amps. Considering the flyback may not provide that much power, your current is likely even lower.

That is rather low current - 100 micro-amps. No wonder you aren't seeing any significant glow. I guess that is why high current ion guns are so often water cooled. Maybe use forced air cooling and lower the resistor to just a mega-ohm?

Maybe consider using a current limited NST that puts out 30 milli-amps (and under zero load might give 7.5 kV)? That will both limit the current to remain low and provide enough voltage.

DanielSandner
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Re: ion source doesn't work

Post by DanielSandner » Wed Jan 13, 2016 1:24 pm

Thanks for your answer but i think that is not the main reason.
The flyback gives around 25kV and 3 mA, and for such a small tube (4mm diameter) this leads to 80 degrees (Celsius) in a few seconds. Furthermore rapp also used a 100MOhm resistor and his project was very successful. The pictures in my first post, where the glow can be easily seen was also made with this resistor. I will write more later after I've tested it with the oil diffusion pump (I tested it yesterday).
Cheers, Daniel

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Werner Engel
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Re: ion source doesn't work

Post by Werner Engel » Wed Jan 13, 2016 6:49 pm

Did you try to talk to Mr. Rapp? Maybe he has an idea?
By the way: I ordered a very interesting book regarding Ion Sources. It should arrive in 3-4 weeks in my local bookshop. If I find something after browsing through, I will tell you : http://www.amazon.com/The-Physics-Techn ... 3527404104

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Richard Hull
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Re: ion source doesn't work

Post by Richard Hull » Wed Jan 13, 2016 6:51 pm

It all hinges on how many ions you want or must have. Normally, in ion gun and ion production schemes, after you get the needed voltage, it is all about current and lots of it.

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.

DanielSandner
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Re: ion source doesn't work

Post by DanielSandner » Thu Jan 14, 2016 7:17 pm

Hello,
great news, yesterday I tested the hole tube with my diffusion pump and i saw the beam in the hole tube. The next days I want to test a new power supply and check the focus of the beam (with a screen). After that i want to reflect the ions on a metal ball and test if I see anything at the screen. If you like I will write more about the following idea and experiments (after I've done them). If this project is successful I'll come back to my fusor, promise ;)
kind regards, Daniel Sandner

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