Magnetron Ion Gun

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
EthanH719
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Magnetron Ion Gun

Post by EthanH719 » Sun Feb 21, 2010 10:30 pm

I am trying to build a fusor with a magnetron to increase the neutron output, but having some trouble actually getting it to work. I ripped a magnetron and power supply from and old microwave oven and inserted the magnetron into my vacuum chamber. There is a high voltage feedthough on the outside to power it, and I have a variac hooked up to the magnetron transformer input. The magnetron produced plasma at 40v. 7a. variac input, and pressures greater than 20 microns, but this is too high to operate the fusor. At about 3 microns, which is the pressure where I can create a stable plasma at 25kv, the magnetron uses 120v. 3.21a, but does not produce plasma becuase of the reduced pressure. Is it still acting as an ion gun although I cannot see a visible plasma, and will it still increase neutron output at this pressure?

Thanks, Ethan

Dustinit
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Re: Magnetron Ion Gun

Post by Dustinit » Sun Feb 21, 2010 10:53 pm

I have quite a few concerns with this posting.
Firstly, putting the magnetron inside the chamber without grounded mesh on the viewport will cause it to irradiate the viewer. Any feedthru will pick up the RF and re-radiate the microwaves.
The magnetron requires cooling which it will not get in a vacuum and the magnets will depolarise from overheating. The feedthroughs into the magnetron are not insulated inside the casing (in the filterbox) and will breakdown.
Without some setup photo's I'm very concerned about the safety of the setup. Microwaves will overheat the corneas and will blind you as some of the first symptoms. Make sure you know what you are doing and leak testing as you go.
Even experienced people get someone else to check their work to be sure, to be sure.
Please post some pix.
Dustin

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Carl Willis
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Re: Magnetron Ion Gun

Post by Carl Willis » Sun Feb 21, 2010 11:05 pm

Do you have any photos of what you have done?

When people talk about a "magnetron ion source" they are referring, usually, to a situation in which a magnetic field is applied in a direction orthogonal to the electric field in order to force particles to take longer trajectories through the gas, promoting ionization and increasing the density and temperature of the ion-source plasma. In this forum that has historically been the usage, as it is elsewhere; for example:

http://www.iop.org/EJ/abstract/0029-5515/46/1/016

A microwave oven magnetron tube is just another particular use of the magnetron principle, using the concept to help generate RF oscillations.

Anyway, it sounds like you are attempting to excite the interior of a vacuum chamber with RF from a microwave oven magnetron. I can understand how this might cause some additional glow-discharge breakdown if the pressures are high enough, but in the absence of enough pressure to sustain such discharges I don't think there is any basic principle by which it might be helping to achieve more fusion. Some photos would be interesting...

-Carl
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EthanH719
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Re: Magnetron Ion Gun

Post by EthanH719 » Sun Feb 21, 2010 11:36 pm

I was hoping you would answer this one Carl, as it seems like you have considerable experience with ion sources judging by your other posts. You are correct in assuming how I am trying to use the microwave magnetron, although I don't think my setup is really of any use now that I understand the pressure has to be high enough to create a glow discharge. Would you reccommend I use a setup like your ion gun for Carl's Jr.? I looked at your diagram, and is it just a positively charged electrical feedthrough surrounded on the outside by magnets? Would I have to charge it to over 20 kv to get plasma, since this is the main grid voltage at the pressure I am using? It seems like the max you charge it to is 1200v -- how does this work? As for safety, yes I am using a grid over the viewport and one on the vacuum inlet to prevent leakage. No, it is not cooled, but I only run it at about 10 sec. test runs (heat will probably destroy it eventually). No, I can't really get pics cause its in the chamber and the viewport is too small, unless you want one of the outside.

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Re: Magnetron Ion Gun

Post by John Futter » Mon Feb 22, 2010 4:05 am

Ethan

unless you have a matching network it is doubtful that you will get enough field concentration to achieve what you want -- unless you have access to a simulator that has very good circuit models for what you are trying to do

do a Web search on magnetron ion sources-- there are plenty most do not use a microwave magnetron as this limits natural automatic tuning that the normal magnetron ion sources rely on. ie any ion source that is excited by DC and the plasma chamber is inside a magnetic field, then you will get with the right DC and magnetic field an oscillation at RF ie the magnetron effect

there are a few ion sources that are excited at 2450MHz but all of these use a triple stub tuner and a tapered transition into the plasma chamber. The trick is to concentrate the E field to a point where there is sufficient energy present to ionise the gas de jour
Check out my earlier post "Quizz what am I building"
and in this I was lucky i guess, that after many pages of calculation derived from( Mathie microwave filters and coupling structures) or something like that, came out working more than well enough

will post the exact ISBN # and book/ author name tomorrow

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Steven Sesselmann
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Re: Magnetron Ion Gun

Post by Steven Sesselmann » Mon Feb 22, 2010 8:47 am

Ethan,

Alternatively you could cut the magnetron open and use it to make a powerful ion source that is safer, like John Hendron and I did.

viewtopic.php?f=12&t=5013&hilit=starfire#p32321



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

EthanH719
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Re: Magnetron Ion Gun

Post by EthanH719 » Mon Feb 22, 2010 3:40 pm

Steven, are you suggesting I cut open a magnetron to remove the tungsten filament, then just mount that inside an electrical feedthrough? How many amps (DC, right?) would I need to run through it to get significant ion production? I have a 10v 100A dc supply, but running that at full power would probably do more harm than good.

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Chris Bradley
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Re: Magnetron Ion Gun

Post by Chris Bradley » Mon Feb 22, 2010 5:42 pm

Please be advised that some magentrons have beryllium [oxide] in them as insulators, so if you just go cutting into something that you don't know what's in it, then bear this in mind.

I recommend you go make your own ion sources, like those of Andrew Seltzman, or the RF induction plasma Carl used. I don't see what you are hoping to gain from this configuration, but it is a bit of guess work on our behalf until you show some pics/schematics of your intent. In any case, I advise against cutting magnetrons up. You're just ruining a magentron and risking inhaling beryllium if you're not sure why you are doing it, and if you are sure of what you're doing then you need not ask.

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Doug Coulter
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Re: Magnetron Ion Gun

Post by Doug Coulter » Mon Feb 22, 2010 6:26 pm

Or just use this, which was pretty easy to make and which works well, day in and day out on my currently working fusor, which runs very often these days (10 runs last week).

http://www.coultersmithing.com/AuxCP/uWaveIon.html
Also discussed here:
viewtopic.php?f=12&t=5022#p32330

This is about the 5th or 99th source I've made and tried out, and this one beats them all for monatomic ions, pressure ranges over which it works, total ion output, and reliability. None of the other techniques ever came close to how well this works, and it avoids all the objections above quite nicely.

If you made enough RF field in the tank proper to ionize your gas, you'd have more RF than DC input in there! Those E and H fields would utterly disrupt normal fusor operation, and cause sputtering off the metal tank walls and antenna probe besides. Which is very hard on all your insulators and windows to say the least.

If you have a choice, an electrode-less discharge is much better on all counts.
Why guess when you can know? Measure!

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Carl Willis
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Re: Magnetron Ion Gun

Post by Carl Willis » Mon Feb 22, 2010 7:42 pm

Hi Ethan,

>is it just a positively charged electrical feedthrough surrounded on the outside by magnets?

Yes. This follows directly from the examples in the Takamatsu paper I provided a link for. It is extremely simple. It is called a "magnetron ion source" because it uses the magnetron crossed-field principle, and involves no RF at all. I've noted that it is common to confuse the word "magnetron" in this context with the use of RF (from magnetron tubes). But magnetron ion sources are fundamentally different from RF ion sources.

I was not terribly pleased with the results from that ion source. It did have an effect on operation and as my various photos show, it produced a beam that can be seen traversing the fusor. But it is not a prolific source of ions. If you can afford something else, you might want to pass the basic magnetron source concept.

I recommend checking out Andrew Seltzman's posts for examples of RF ion sources and a simple magnetically-enhanced DC ion source that is quite similar in principle to a magnetron ion source. Doug Coulter has an RF ion source using a microwave oven magnetron. I have a magnetically-enhanced RF ion source described in this forum also. Phil Fostini's posts from a number of years ago also show some interesting ion source ideas. Steven Sesselman has done some work with hot-cathode source ideas. I think Jon Rosenstiel did some further work with the Takamatsu-type magnetron sources, beyond where I went with the concept.

-Carl
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