Ion Gun test rig

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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John Futter
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Ion Gun test rig

Post by John Futter »

Just finished building a test rig to test various ion guns @ work.
The system uses leftovers scrounged from around the lab to give a viewable measurable ion gun evaluation system. View port end is insulated from the main system by nylon bolts and iso 150 red plastic gasket to measure ion current. In photos the left end is the measurement end and the right end is the ion source dejour. The ion source is seperated from the main earthed system by a 30kV vacuum break and the HV terminal in the centre of the pipe work is connected to an Einzel lens for focusing. Below is a 1000 litre /sec turbo, backing pump and 40kV isolation transfprmer for the terminal (ion source) end.

The ion gun being tested in the pics is a variation of Andrew Seltzmans one descibed elsewhere in this forum. The test run here is using carbon monoxide gas in the ion source at a pressure of 8 by ten to minus 5 millibar. Pics show 2mA ion current on the viewport glass @ 10kV acceleration. Ion source running @ 1.1kV 3.8 mA.
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test rig view port end2.jpg
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Chris Bradley
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Re: Ion Gun test rig

Post by Chris Bradley »

Very nice!

What is the significance of using carbon monoxide, John?
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Re: Ion Gun test rig

Post by John Futter »

Chris
Two reasons amoung others come to mind
1./ It was the first gas bottle of low cost gas I could put my hands on.

2./ We use CO to get Carbon beams for implanting in our implanters twice as efficient as CO2

3./ I would have been killed if I'd used HE3, O18, N15 as these make my salary insignificant if wasted!!!!.

I could have used air but it doesn't give such a good colour

The system is pumping down over the weekend with argon in the bottle for the ion source.
I did run it before leaving work but the vacuum wasn't that good yet after being back up at air to make some adjustments to the ion source (remove short from not cleaning ferris bits off it ). Air to to 4 by ten to minus 5 after 20 mins but too much air molecules to make meaningful measurements
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Chris Bradley
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Re: Ion Gun test rig

Post by Chris Bradley »

Good reasons!
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Steven Sesselmann
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Re: Ion Gun test rig

Post by Steven Sesselmann »

John,

This looks cool...

Love to see it, but Auckland is a bit far away...

Maybe I should send you one of my ion guns for testing on this rig

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
John Futter
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Re: Ion Gun test rig

Post by John Futter »

Steven

I'll have to check whether we have an ios6" to NW25 but if it exists i'll do it.

Extraction could be a problem but I'll find a way to overcome it.
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Doug Coulter
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Re: Ion Gun test rig

Post by Doug Coulter »

John,
Real nice rig, this has needed to be done for a long time, and I'm surprised you're able to run that source at such low pressures, now I have to go build one of those too! I've been playing with a microwave/ECR source, but nothing comprehensive to report, other than it works. I've not yet had it stable enough at real low pressures that I want to make good measurements on it, so all I can say is it lights up and makes ions pretty well at lower pressures than most of my other attempts.

What is really needed for us fusor guys, (IMO -- everything I say is!) is now to add a mass spec in the ion beam and try it with its ionizer both on and off. Again, IMO there's going to be a large difference in results depending on whether you just ionize the gas contaminants, make D2+, or make D+, and this would tell you that. And oh yes, how much unionized gas gets through there and builds up in the tank.

To make this test the mass spec has to be exposed directly to the beam -- so we don't see things like wall recombinations from ions bouncing off walls before getting there.

Sadly, my mass spec isn't in a good location to do this test, and last time I moved it from tank to tank, I dinged it, which in turn dinged my wallet rather severely, so I'm not in a rush to do that again.

Meaning, I'll do it someday, but not this week, for one thing I need to finish building a collection of different ion sources to do the tests with, as after this test, it needs to go back to its safer place before running things like fusor in the same tank. You don't want very hot H or D ions bashing the ceramic insulators and causing them to be reduced chemically, for example -- unless you like burning multiple thousand dollar bills to sniff the smoke.

Perhaps there's a simpler way to get this done though -- a very crude magnetic system might get the answers that I at least am interested in, after all, it's pretty easy to tell apart things that have a 2::1 ratio (like charge/mass of D2+ vs D+) and to see what fraction of gas is charged at all, I'd think.

If your boss is mainly into making implanters though, he probably won't care to invest in these kinds of tests? Maybe things like this can matter there too?
Why guess when you can know? Measure!
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Re: Ion Gun test rig

Post by John Futter »

Doug
This setup is only for testing ion guns and focusing arrangements.
Parameters that we want to measure / see are.
1./ stability of the source over time
2./ Ion current
3./ visually check ion beam dynamics /stray ion paths
4./ suitable for both positive and negative ion sources

The reason for the ISO6" ion mount spec is this is what our implanters use. So after successful testing on this rig the ion source can be put on the implanter and the mass selection magnet swung over the entire AMU scale measuring ion current for all responses this includes single and multiple charge states.

We are buying a small residual gas analyser and it may be put on this system for specific tests. I understand that it can detect gasses down to 1 by ten to minus 14 millibar giving ppb if one is running an ion source at around the ten to the minus 5 millibar.
Edit
Doug I will do a H2 run on the implanter with this gun in the near future to get the ratio of H2 to H+ but I suspect they will all be H+
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Doug Coulter
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Re: Ion Gun test rig

Post by Doug Coulter »

Thanks, John.

The highest numbers I've seen for monatomic ions from a diatomic gas are in the 90% ranges, and aren't from this kind of source. Most sources also let quite a lot of just plain neutrals through, including the ones that do make the 90% monatomics of the ones they actually do ionize.

You see complaints about this, and ion source lifetimes all over the literature - I assume you're looking for long life, and if you don't roast the magnet or kill an insulator due to sputtering this design should last a good while. One paper I'm looking at right now for ion traps complains that they *only* have two vacuum systems in use to pump out all those neutrals as they disturb the ion trap performance, for example. They have one right after the ion source, then another a long way away down a beam pipe for their ion trap, and it's not enough, even though there are some orifices that ions can be focused through, but neutrals tend to bounce back from.

An RGA will indeed do the job here -- and please try it with it's internal ion source both on and off if you can (my Pfeiffer lets you turn the filament on and off, for example -- which makes it possible to take two measurements, one total content, one just what's already ionized and then subtract).

Any time ions hit a wall, or a neutral, strange things happen, from just sticking to the wall, to bouncing off as neutrals, to sticking to some other gas and making things that show up on my mass spec as stuff that "cannot exist" and indeed won't out there in the world of chemistry, but a single weird molecule or really, just some gases stuck together (not covalent bonds) can and do exist until they hit something else and re-arrange for lowest energy -- we're usually creating ions with enough energy to force chemical reactions, which are normally in the single digit eV range and down.

This is something that makes interpreting a mass spectrum real tough. You just see all sorts of things, from CO2 with an H stuck on (very temporary, but it's there) and stuff like that, even H3 (not tritium, just three H's) and so forth. You kind of have to look at the big peaks and then do all the possible combinations in your head somewhat to figure out what all those other ones are.
Why guess when you can know? Measure!
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Re: Ion Gun test rig

Post by John Futter »

Doug your points noted
another reason for the test rig is to optimise the extraction electrode so that only charged ions are transported with the neutrals being swept into the pumping system. This is also the reason why we want our ion sources to operate at such low pressures. This to maximise charged ion production and minimise neutrals. The Einzel lens also helps by allowing a larger diameter beam through most of the system reducing space charge effects until eventual focus on target. Also in the implanters we normally operate with target currents in the few to tens of uA to stop target heating, the test rig allows operation such that there are so many ions you can see them. The view port got very hot yesterday with only 10kV acceleration, a beam of this intensity in the implanter @ 70kV would probably melt the same holder stage
From what we saw yesterday from the anode layer source we could get a focused diameter of around 3mm @ 2mA ion current. With no focus potential the anode layer source showed a slowly diverging annular beam. Unfortunately I just threw a surplus Einzel assembly into the setup and the focus point is around 300mm in front of the view port. I now need to put the setup into Simion so that I can shift the Einzel to the correct position.
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Re: Ion Gun test rig

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Drool!
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Doug Coulter
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Re: Ion Gun test rig

Post by Doug Coulter »

John,
those are real impressive results indeed. I've been playing around with all the same stuff, including Einzel lenses, and my main problem with them was beam blowups at high space charges, even with electrons moving fast, and therefore kind of spread out in space for a given current. The one I made can have all 3 electrodes at different voltages if I want, and I was able to get focal length to vary over a wide range in the normal Einzel hookup-- I did this pre-acceleration to get that happening, as after I put some serious kv on the electrons, the lens was kind of a null effect device at reasonable lens voltages.

But it worked fine before one final stage of real high voltage acceleration, kept the beam in the tube till it got there which was my goal then. Once it got really tuned, I could even let in some gas to see the beam without messing it up totally, which was really cool looking (since I have a tank of neon, I used that, of course). The fun thing I did with that by accident was to reduce (?) some of the glass target I used to see the beam spot -- pyrex glows fine when you hit it hard. Looked fine in the tank, but when I took it out there was this big white spot where I'd affected the glass, some kind of powder which was deliquescent and after wiping off the result after a couple of days (it was just a wet place by then), no visible damage at all. Very weird, but fun. I don't think I implanted any neon in it, but who knows? And it wouldn't explain the powder. Something funny happened for sure.

I do sometimes wonder why any design data for magnetic lenses is so scarce out there, I've been looking and looking and not finding very much that's specific enough to be useful. Seems like the microscope guys like their trade secrets, or I'm looking in all the wrong places. There's one good overview in the Scientific American amateur science projects disk, that's about it, and that's not really enough, but it is helpful for just getting a feel of what's needed at what energy. All the e microscope papers I've found kinda leave out some of those details...either it's so easy it's not interesting, or something they want not to reveal it seems.

I've not tried this type source myself yet, AndrewS described it quite well,and IIRC he provided (or heck. maybe I just found, I just don't recall) some interesting links where you can affect the beam pattern out of the raw source by moving one of the pole pieces back and forth a little, from converging to diverging, maybe he'll pop in on this and let us know that link again -- it was to some small company that makes them, in an animation they had on their site, it was pretty cool. It might help you get into your lens aperture better.

Be sure and let us know results! I think this kind of thing is too often not paid enough attention to here.
And ion sources I've invented have been my nemesis so far, they work great but don't last long -- everytime I fix one failure mode, another appears. I did a neat one I still like that used a tiny capillary tube and a very short gap to a hole in a charged plate, that allows very low tank pressures -- we ionize before the gas has time to expand to the much lower tank pressure, and it makes cold ions due to the post expansion, with shock diamonds in the beam -- looks real cool, but has a number of failure modes, mainly due to metal loss off the electrodes -- even if you're careful to keep it from getting on the insulators, it makes the gap too big at some point. So my current design on this one only lasts some tens of hours at full power.

For completely other reasons I've been wanting very low pressure ions too, and very few neutrals, so I am awaiting any results with high interest! Of course, I'd love to be able to ion implant some things myself, I already do sputtering and evap here for fun and games. I want to find a source that doesn't need another pump to keep my main tank pressure low as I want, and I want that very low.

So far, my best results have been with another type of source, one that uses an oven magnetron I finally managed to get running continuous instead of pulsed at about 5% or less rated power, which is plenty, and it gave me about a 10x factor lower pressure than I could get with any DC/magnetiic field, and I tried some real large fields of both where other problems appear. I've since added ECR magnets to that, and plan to get a good measurement soon, but upcoming HEAS means I've stopped work for the moment and am getting ready for that. And I'm on to the next rev in design anyway, now I want to do this right in the tank, instead of in a quartz tube in the microwave cavity, and I think I can if I can seal the tuning rod well enough.

Note, if you're getting your window hot you'll have trouble at some point. I do two-three things, depending. One is just put a cheap piece of thick pyrex inside the tank to shield the window, It's cheap and can be dunked into acid to eat off anything you may deposit on it, not something I'd do with my fancy hinged window/door. It you ruin it, not a big deal. I've also used even cheaper mica I get from McMaster-Carr, as "india glass" but the last couple batches haven't been real clear -- they are also where I get the thick cheap round pyrex glass. You can also of course put some metal screen there with a charge such that you just repel off any particles, but that may mess up your experiments by making a field in the tank. But all these things are far better than a shattered main view-port and an air inrush accident.
Why guess when you can know? Measure!
John Futter
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Re: Ion Gun test rig

Post by John Futter »

Doug
Thanks for the comments.
I'm painfully aware of most things you mention, and because of that we will not be operating the ion guns above 2mA on target until we get a mesh screen to take most of the charge. The sreen will be fine SS mesh which may glow hot to yellow hot later on.
Andrews info on the anode layer source gave the links to the Russian web site that showed the focusing effect of moving the outer pole piece.
Early days yet
1 we have to shift the einzel lens - Simion CAD run required to save time
2. Mesh screen on the target window to keep window temp below 100 C
3. Install pneumatic gate valve above turbo connected to alarm out on high vac gauge
4. redo anode layer source with screw thread adjustment of outer cathode for tuning beam
5. improve gas feed in ion source so that at higher gas flows it doesn't setup an arc in the gas feed tube (quartz)

edit
Over the weekend I has a few ideas to make the assembly of the anode layer source alot easier
1 I couldnt source a NB magnet with a hole in it and drilling a 4.1mm hole through it without breaking it took ages so the magnet will in future rest in recesses top and bottom to locate it.
2. outer cathode to be on a screw thread to optimise
3 gas feed to be metal tube into bottom of ion source not into middle of gas space
6. model extraction electrode (Simion) for annular ion exit to improve extraction from 25% to at least 90% of active ions. as well as finding the right values of voltage / current in the ion source for various gas loads.

Up until now most of our ion sources are cold cathode penning type. These where introduced to us by our principal scientist Andreas Markwitz who did his Phd under Bauman in germany who invented / optimised this sort of ion source. We get around a hundred hours of operation before having to service these (new front and back cathodes) and typically these run at around 2mA ion+ current output around 85 to 95% of the arc current in the source. The higher the current in the ion source the sooner you have to service it, integrated ion current is directly proportional to cathode wear.

Again The extraction electrode is the secret to getting only excited ions to move to target the neutrals do not see it and the hole in the centre of the extraction electrode is small (3-4mm) allowing differential pumping to the rest of the system.

System was sitting at low ten to minus seven millibar today ready for the Argon test but I/ we had other things to do maybe tomorrow-- will try to take better pics of ion beam trajectory.

Edit 2
Bear in mind the first ion source tested on this rig was a negative gas ion source and it is now being modified and will go on the system between iterations of the ion layer source. It involves changing out all the Hv psus for negative versions
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Doug Coulter
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Re: Ion Gun test rig

Post by Doug Coulter »

On the magnets with holes, try any of amazingmagnets.com, magnetsrc.com or even scitoys.com.
I've gotten the ones in the pics below from amazing, and had fine dealings with all three of the above.

Here's the source I'm currently testing. It's a CW microwave driven, ECR source in a cavity, which has a quartz tube going through the cavity which has gas inlet at one end via capillary tube (2" by .005" hole), and is open to the tank on the other end. This lights up down to about 1e-5 mbar and below, at very low magnetron input/output power. Best I've done so far, and it all runs cool and looks like being real reliable.

However, on the first test we found that the ECR field we needed also bashed all the nice ions into the quartz tube wall before they reached the tank -- no good. So we added a little piece of Cu tubing inside the quartz right at the exit point and put about -3kv on it (from the maggie power supply via ballast, a few mA were drawn at about 3kv on this), grounded the capillary tube, and bingo -- ions coming into the tank very nicely, and making a big difference in fusor operation over a foot away from this coupling. I doubt however, that my "e lens suction design" which was a guess, is optimal at all, and we'll try a couple more things before rev two, which will have the cavity in the tank, skip the quartz tube and maybe not have this problem anyway.

In that case, we'll just make the cavity bottom out of screen wire, so the ions can get out over a wide angle anyway, and may not need the electrostatic "suction" element. I have to add though, that this added a good factor of being able to run lower pressures yet with some ripple on the magnetron output which would let it "go lights off" at low pressures easier. Much better with a little bit of keep alive DC in there too.

Getting an oven maggie to go CW and not fry kind of took some convincing, but we have that going on now -- and it only takes around 20ma HV current for this to run well, and with somewhat reduced filament voltage. It doesn't even oscillate well unless the cavity is tuned right, but when it's right, it's right and everything stays cool for long operations, and it stays right from time to time. We have noted that some maggie tubes mode hop a lot, and we had a selection to pick from here to get one that works really good despite some slight tuning errors. We get them from dumpstered maggie ovens, more or less free.

I used a hall effect magnetometer to get the field right on that magnet yoke -- you can slide one of the pole pieces and magnets back and forth until you get the numbers you want -- when you do, stick a shim in there and it's done for good. The magnets make about a 2.5 order of magnitude difference in the lower pressure limit, and we don't use differential pumping here yet (trying to avoid the need, as that would be past what most of this group will accept, even me, and I have the extra pumps).

At any rate, in a "lousy geometry" experiment with this about 18" away from a fusor grid and not pointed quite at it, we got real good results right off, and are now able to run the fusor at about half the pressure we could before -- I have high hopes that with good geometry (this setup could almost not be worse than it is for what we're doing with fusors), we'll be able to keep a fusor lit off at the lower limit the ion source will run at which is where we are heading here as part of our overall research.

As I said, this runs real cool and there's no evidence of any sputtering onto insulators after hours of continuous running, so it may be of interest to you. Looks like good high ion currents are possible, but I've not done quantitative measurements yet. It does work better than other sources we've tried, subjectively, but we've not tried the type you're working on yet either.

We will, of course. Real soon, as your results are in the same ballpark, and with less gear to clutter up the setup required.
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