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 »

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
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Re: Magnetron Ion Gun

Post by Dustinit »

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 »

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 »

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 »

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 »

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
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EthanH719
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Re: Magnetron Ion Gun

Post by EthanH719 »

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 »

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 »

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.
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Carl Willis
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Re: Magnetron Ion Gun

Post by Carl Willis »

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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EthanH719
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Re: Magnetron Ion Gun

Post by EthanH719 »

Alright, although I would like to build a true RF ion source some day (thank you for correcting me about the difference bewteen magnetron and RF), I think the hot cathode idea will be the simplest for my first ion gun. Is this just a tungsten filament running across a feedthrough at a couple amps? If it is, will this filament work, because I would rather not expose myself to beryllium dust trying to cut open a microwave magnetron.

model OB-3 from

http://www.tungsten.com/orphan.html

Also, does the deuterium have to be running directly through the filament, or can the gas inlet be mounted on another port?
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Doug Coulter
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Re: Magnetron Ion Gun

Post by Doug Coulter »


One of the ion sources I used to make a gage was simply a filament from a tiny quartz halogen bulb (6v 12W from a lawn ornament) that I cut the top off of (diamond wheel on the lathe tool post grinder). As tungsten is a lousy emitter of electrons without some help, I tried both the Ba/Sr/Ca carbonate mix they use for electron tubes (which decomposes at high temps to Ba0 etc which is what really does the emission), and a Y2O3 coating, which is what I stuck with so I could run it yellow hot instead of hotter -- the Ba mix doesn't like being exposed to air once activated, and only lives about 3 times of that before it won't reactivate, while the Y2O3 doesn't have that problem. With about 70-100v on the "anode" electrode, this gave some good electron current and reasonable filament life -- tungsten really doesn't like being run with even trace amounts of water present, and quickly dies -- and all vacuum systems do have some water as the main contaminant unless very heroic measures are taken. This is why exotic materials are used in mass spectrometer filaments -- the assumption is that you're going to be measuring something other than perfect vacuum, and in less than perfect vacuum (or inert gas only) tungsten just doesn't live long.

If you can afford it, the high alloy wire from type C thermocouples makes a much more robust filament (26% Re) that doesn't have the water disease nearly as badly. The fat 2 amp filament in the bulb does go bad at some point, even though it's running at much less than ratings. You can get that thermocouple wire pretty thin so it doesn't need as much current to light up, say 5 mils or so. The high Re content also makes it lots less brittle so you won't get as frustrated with it breaking when you try to form it.

The reason I let gas in right through my sources is that in molecular flow vacuum levels, that's there the pressure is highest and the gas easiest to ionize well. No particular other reason, its a minor "tweak" on something.

FWIW, for a non RF ion source I suspect the one championed by Andrew Seltzman may be best.
But I haven't tried it myself. Look here for details, for some reason I couldn't find the thread here on this board.
http://www.rtftechnologies.org/index.htm

Other tricks I've tried include ionizing the gas right at an inlet orifice while it is still at high pressure, and easy to ionize with low voltages. The problem with that one was the insanely tiny clearances (less than .01") needed to get to the gas before it expanded to low pressure, and the sputtering onto insulators nearby. Another was the typical magnet crossed field source, called a magnetron in an overload of that word to mean too many different things, IMO. Sputtering killed that one too. In either case, a few more revs of mechanical design so that all insulators were shadowed from sputtered metal vapor may have made them acceptably reliable, but I got disgusted and came up with my RF one that is really only a slight improvement on an older design in Rev Sci Instruments.

For those who don't know, here's the tungsten disease:

Hot tungsten disassociates water vapor on the surface. The resulting oxygen oxidizes the tungsten. Tungsten oxide has a low boiling point and flies off. When it hits the tank or bulb walls, it is reduced by the hydrogen, and water results again. Repeat till filament burns out. This preferentially happens at the place it's hottest, and thinnest already. This is one reason the early incandescent bulbs didn't use tungsten...they couldn't get the lamps free enough of water to make them reliable. The other was I suppose that tungsten wasn't either cheap or readily available then -- but even iridium was tried, and that has neither property itself, nor does osmium.

Halogen bulbs reverse this process. Tungsten evaporates some, and is combined with the iodine or other halogen. At heat, this decomposes back into tungsten and iodine again, which happens at the hottest and thinnest part of the filament. Unlike all other filament bulbs, this actually means the life is shorter if they are run below design temperature! At design temperature, they are somewhat self repairing! They have to run hot to work, and with that halogen, you have to have a quartz bulb, so "quartz halogen" is nearly one word in the lamp business.
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Steven Sesselmann
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Re: Magnetron Ion Gun

Post by Steven Sesselmann »

Ethan,

You can build a a power supply from microwave junk.

The heater coil on a MOT is perfect for heating the filament, and you can rectify the HV coil with one diode, to use for the negative bias.

The filament runs at about 4-6 volts to glow orange, and the bias needs to be around -1400 volts. You then feed the gas in through the filament, the gas is then stripped of electrons as it passes through the planar stream of electrons. A very efficient ion source.

Note1, It needs good cooling!

Note2, Never heat the filament in air, it will burn out immediately.

Note3, Be careful, Microwave Oven Transformers are dangerous, dont play with them unless you know what you are doing.


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Re: Magnetron Ion Gun

Post by Starfire »

Ethan

It is better to run the gas through the filament - the idea is to strip as many Electrons as possible and no better way than to make them pass through the hot filament and stream to the anode - this is what makes the Starfire so efficient.
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Re: Magnetron Ion Gun

Post by EthanH719 »

Ok, so to make a power supply for this out of a MOT, do I just keep the original circuit intact, but remove the capacitor and place a wire between these connections, the magnetron in the picture being replaced with a filament across the two leads? Also, where can I get yttrium oxide coating, and do I really need it?
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Re: Magnetron Ion Gun

Post by EthanH719 »

Found the yttrium oxide from rare earth products for $27 25g. How exactly do you coat tungsten with this, just run it through the powder?
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Steven Sesselmann
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Re: Magnetron Ion Gun

Post by Steven Sesselmann »

Ethan,

You need to rearrange the MOT parts slightly.., and forget the Yttrium Oxide, when the filament glows orange, it emits more electrons than you can handle.

The attached Diagram shows a simple rearrangement of components, you may put a high ohm ballast resistor between the filament and the MOT HV, to prevent over current.


Steven

PS: Attached diagram originally from John Hendron
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Re: Magnetron Ion Gun

Post by Starfire »

Ethan

Your circuit shown will short on alternative half cycles if you short the cap as shown - the current will simply pass through the diode on half cycles - better to use the circuit as shown by Steven - it will rectify half wave and with the cap will produce DC albeit noisey
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Doug Coulter
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Re: Magnetron Ion Gun

Post by Doug Coulter »

Here's a pic of a working ion gage I made, same idea more or less. Those are two 6v, 2a bulbs I busted to get the filaments -- nice trick, and they're already mounted up well and very rugged. I used two bulbs so I could compare two coatings, or coated vs uncoated on a single vacuum run.

Pure W does NOT emit well at all at orange, Steven must either have had thoriated tungsten or a coated filament without knowing it. Plus, I bet that what's really happening is a bit of plain thermal ionization which starts a garden variety gas discharge going, then when ions hit the filaments there is secondary emission that knocks out several more electrons per strike. You don't get even a microamp emission at orange heat from pure W, I know, I've measured, and besides, about a century of practice in the design and making of electron tubes backs this up -- see almost any old EE book that mentions things like this, I like the 1943 Terman book or one of RCA's tube design handbooks (they printed up some internal documents to bring their own designers up to speed, I am not talking about the tube *users* book here). Sorry Steven, you're just dead wrong on this one. It may work for you, but not for the reasons you think.
Here's a little background, but the tube design handbooks you can access at the link I gave Steven have a lot more.

http://en.wikipedia.org/wiki/Thermionic_emission


The circuit they are showing doesn't have a current limit, and you're going to have problems with that. Normally the series cap in a maggie oven volt doubler is what limits current. With the circuit shown you're going to have to have some other current limit or there's going to be smoke. The maggie transformer is designed to core saturate at about 89-92 volts input and will get very hot at full input even with no load, by the way, so here I either use a variac or a 100-200 watt 120v bulb in series with the primary. Even in an oven (and under full load the saturation is less) they don't run all day long, just minutes, and that with a big fan on them....so watch for that. Also, with too much current input, the thing will go negative resistance on you, finally limiting out at arc welding kinds of speeds and feeds -- about 20 volts at horrendous amps jumping an inch gap and evaporating metal.
So you do need some current limit in there....gas discharges are pretty complex and have taken up several entire careers of smart people, and we still don't know it all.

Here's a link to some of this info:

http://www.coultersmithing.com/OldStuff/pdfs/dox.html

The ideal voltage for making ions is in the 70-150 volt range, ask any ion gage or mass spectrometer.
(Dustin, do you have a comment here?) At 4kv the cross section is much lower -- the electrons whiz by the gas atoms too fast to do much.

I'm pretty sure that something else is going on there, not straight electron ionization of gas, but hey, it may work fine -- anything that gets to drawing some current will....The main thing to look at is how wide a range of gas pressures it will work over.

Yes, you can just make a slurry in water of the coating and drag the filament through it, or use a little bitty paint brush. I use cataphoresis which is what the pros do, mainly to learn how, and it does work better.

To do that (should you need to) you put the thing in a beaker with the stuff, agitated as it doesn't dissolve, with real pure (store distilled is good) water and put perhaps 70-100 volts negative on that, with positive on an anode -- I use a little piece of the same lead sheet I use for rad shielding, but a lot of things would work. You'll see a couple of milliamps, and this drives the stuff right into the pores in the tungsten -- it really stays on. You only need just about enough to see, too thick and in insulates and then falls off. Pure water is actually a pretty good insulator, if you have a clean setup you won't draw a lot of current or see bubbles. A fleck of dust will make it not pure, though.

If you're somewhere where there's not a scary customs barrier, I can mail you a little of the Y2O3, I have a pound, and a gram does about 50 of these. I don't want to mail some white powder to someplace they'll think it's anthrax or coke.

BTW, the .005" diameter wire from type C thermocouples, the high Re one, also works well and lasts longer than pure tungsten does, as the Rhenium prevents the water/tungsten cycle. Expensive but you may be able to scrounge a little somewhere, and you don't need much.

Here's a gif from the Terman book. This is correct beyond any question. FWIW, 1000C or 1273K is very high up in the yellow heat range. Orange is around 800C....Most electron tubes operate around 700C or so with coated filaments, and thoriated tungsten is mostly used around 1000c.
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Chris Bradley
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Re: Magnetron Ion Gun

Post by Chris Bradley »

What does "magnetron" in this diagram mean? I'm a bit confused - seems to be a magnetron without its magnets, nor cathode and anode either?
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Re: Magnetron Ion Gun

Post by Steven Sesselmann »

Chris,

Not sure which diagram you are referring to, but to clarify the Starfire ion source merely uses the filament and power supply "from" a commercial magnetron, as it happens to be the perfect shape and very robust. So not a magnetron per se.

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Re: Magnetron Ion Gun

Post by Chris Bradley »

It was a post under (and in respect of) Ethan's, with "magnetron" printed on his image.
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Re: Magnetron Ion Gun

Post by Carl Willis »

Chris:

"Figure 2A" represents the standard circuit of a household microwave oven.

The magnetron tube is depicted as a high-vacuum diode, of which it is a variant.

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Re: Magnetron Ion Gun

Post by EthanH719 »

Doug, you say that the ideal voltage is 70-150v? Doesn't a magnetron filament transformer only produce 3-6v (with the 2kv bias)? Yes, I do have a variac to limit current, might also be a good idea to stick a light bulb in there. I found some barium carbonate laying around, so I'll probably just use that for the coating. Also, will the fact that my vacuum chamber is charged to 15kv during operation effect the ion gun? I had to do this so I could get 30kv total across the chamber and inner grid without using a ton of current.
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Re: Magnetron Ion Gun

Post by Doug Coulter »

That 70-100v number is for the electron velocity, not the filament heating voltage. You can try more or less, Steven seems to be saying he's using the full couple kV for this.

You'd have to ask Steven about that other question, or fill me in with a bunch more detail for me to answer. If I understand right, Steven's design uses the tank itself as the anode for the electrons, so this would make some problems perhaps....

I'm avoiding chambers that are off ground for now, but I do plan to do some interesting things in quartz tubing (pumped by the same system) with split supplies so I can use less voltage from ground on either end (which what I think you meant by "current" -- very much not the same thing, think of voltage as pressure and current as flow in a plumbing analogy and you're close).
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