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ion source doesn't work

Posted: Sun Jan 10, 2016 12:00 pm
by DanielSandner
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

Re: ion source doesn't work

Posted: Sun Jan 10, 2016 1:40 pm
by Dennis P Brown
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.

Re: ion source doesn't work

Posted: Mon Jan 11, 2016 2:57 am
by DanielSandner
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

Re: ion source doesn't work

Posted: Mon Jan 11, 2016 6:33 am
by Dennis P Brown
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?

Re: ion source doesn't work

Posted: Tue Jan 12, 2016 8:31 am
by DanielSandner
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

Re: ion source doesn't work

Posted: Wed Jan 13, 2016 6:35 am
by Dennis P Brown
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.

Re: ion source doesn't work

Posted: Wed Jan 13, 2016 8:24 am
by DanielSandner
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

Re: ion source doesn't work

Posted: Wed Jan 13, 2016 1:49 pm
by Werner Engel
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

Re: ion source doesn't work

Posted: Wed Jan 13, 2016 1:51 pm
by Richard Hull
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

Re: ion source doesn't work

Posted: Thu Jan 14, 2016 2:17 pm
by DanielSandner
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

Re: ion source doesn't work

Posted: Thu Jan 14, 2016 3:23 pm
by Werner Engel
yes, please Keep us informed and also make pictures!

Re: ion source doesn't work

Posted: Fri Jan 15, 2016 6:54 am
by Dennis P Brown
Always want to hear more about a system. Thanks for sharing what you have done. Remember to have at least a baffling system to reduce back streaming of DP oil into your system.

Re: ion source doesn't work

Posted: Mon Jan 25, 2016 1:50 pm
by DanielSandner
Hello everybody,
sorry for the late reply but I had many exams at school, but in a few weeks there are holidays.
Now I use the oil diffusion pump, luckily I bought it with a suitable baffle, for a hour every day, and the ion source works pretty well. To allow longer runs i have to improve my cooling system.
Image
As you can see in the picture I have a plasma discharge at around 1 mbar and beyond the capillary a pressure lower than 10^-3 mbar. The lightning disc is a piece of an fluorescent screen to test the intensity of the beam depending on the distance from the capillary. I use a flyback transformer, now without resistor but with limited power on the primary, and the tube keeps pretty cool even after 30 seconds, and you can see the ion beam easily.
cheers, Daniel Sandner

Re: ion source doesn't work

Posted: Wed Jan 27, 2016 8:18 am
by Dennis P Brown
Looks very good. Glad this has progressed so well.