Ion Gun test rig

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

Post by John Futter » Fri Sep 25, 2009 6:03 am

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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Ion source test rig.jpg
Ion source test rig.jpg (671.45 KiB) Viewed 3224 times
test rig view port end2.jpg
test rig view port end2.jpg (621.21 KiB) Viewed 3224 times
ion source running.jpg
ion source running.jpg (482.53 KiB) Viewed 3224 times

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Chris Bradley
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Re: Ion Gun test rig

Post by Chris Bradley » Fri Sep 25, 2009 7:08 am

Very nice!

What is the significance of using carbon monoxide, John?

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Re: Ion Gun test rig

Post by John Futter » Fri Sep 25, 2009 7:25 am

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 » Fri Sep 25, 2009 9:33 am

Good reasons!

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Steven Sesselmann
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Re: Ion Gun test rig

Post by Steven Sesselmann » Fri Sep 25, 2009 9:39 am

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 » Fri Sep 25, 2009 10:05 am

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 » Fri Sep 25, 2009 1:31 pm

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 » Fri Sep 25, 2009 6:58 pm

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 » Fri Sep 25, 2009 7:56 pm

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 » Fri Sep 25, 2009 8:27 pm

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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