Quick and dirty PIGatron

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Maciek Szymanski
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Quick and dirty PIGatron

Post by Maciek Szymanski » Fri Nov 06, 2020 4:11 pm

This started completely fusor-unrelated, but as far as it goes it seems more interesting. I just needed to test some old micro channel plates at work, so I needed some reference ion source. The plates are used to detect laser generated ions but doing laser shots just to test if the plate is ok or faulty is rather cumbersome. And the ion burst is high energy (some MeVs) but very short, so synchronized photographic registration is a must. For other hand low energy - continuous source would make visual testing possible and easy.
So I started to think about a very simple penning (pig ion gun or pigatron) type ion gun:

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The idea is to ionize the stream of gas flowing through the gun from left to right by the mess of electrons accelerated by the electric field and following spiral paths due to the axial magnetic field. This is cold cathode gun and operates on single low current HV supply. The gas inlet and extractor electrodes are grounded, making it very easy connect do the chamber. I decided to build a simple proof of concept prototype:


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The parts of the PIGatron. The insulator is a plexiglas tube with a hole for exit of the anode wire. The extractor is made form a random KF25 fitting.


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The assembled gas distributor. The small screw is for grounding connection.


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Assembled core of the PIGatron. The end pieces and the anode wire are glued with low vapor pressure Loctite epoxy.


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The complete assembly with the ring magnets.


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PIGatron mounted on the vacuum chamber.


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It’s alive! The bare clip is for grounding the gas distributor, the one with the insulating tube is for anode HV.


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The view into the eye of the PIGatron. The bright core with diameter smaller than the aperture of the extractor is clearly visible.

To my surprise the thing worked from the first shot. I’ve not made any measurements yet, and for sure there are needed some improvements:
  1. The clearance between anode and grounded electrodes is too small. Very small variation of gas flow or voltage causes the discharge to stop or to arc between electrodes. But maybe after some time under vacuum it will improve, as the epoxy will outgas.
  2. The gas distributor holes are way to big (0.5mm), and probably to widely spaced. The flow adjustment would be much easier with smaller holes.
  3. The extractor hole should be also significantly smaller. The ions hitting the extractor will generate secondary electrons and improve ionization. And the extracted beam should be also less diffused.
  4. It will be nicer to have this device dismantlable (with o-rings and longitude bolts for example) instead epoxied.
But generally it seems to be easy to build and operate working ion gun. And due to grounded extractor electrode quite easy to integrate into fusor, needing only a simple positive HV supply.
Last edited by Maciek Szymanski on Fri Nov 06, 2020 5:00 pm, edited 2 times in total.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Maciek Szymanski
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Re: Quick and dirty PIGatron

Post by Maciek Szymanski » Fri Nov 06, 2020 4:28 pm

And by the way - despite the fair from optimum gas flow system (to big distributor holes and imprecise valve) it was possible to keep the discharge while the main chamber pressure was on 3E-4 mbar level.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Dennis P Brown
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Re: Quick and dirty PIGatron

Post by Dennis P Brown » Fri Nov 06, 2020 5:53 pm

Impressive build - the pigatron is very nice.

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Richard Hull
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Re: Quick and dirty PIGatron

Post by Richard Hull » Fri Nov 06, 2020 10:43 pm

The pigatron is a great ion gun for simple systems and rather easy to put together.. Good work! This subject is directly related to more advanced fusor ideas. Focused, weak ion streams are better than no ion streams or haphazard, unfocused streams. The simple fusor, as we tend to build, are volumetrically distributed spherical ion generators. This is not great, but good enough to do fusion in spite of the volumetrically mixed energy ion/deuteron production.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Quick and dirty PIGatron

Post by John Futter » Sat Nov 07, 2020 4:11 am

Maciek
You do not need to put multiple holes for gas distribution
You are in the molecular flow region at these vacuums so the gas fills (expands to fill) the void immediately. So one small hole would be ample and help with gas regulation.
Also as you have surmised the exit canal could be smaller I use 2.7mm for our penning ion sources and you also need some length of this to get the pressure differential to the rest of your system.

But nice write up well done!!

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Maciek Szymanski
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Re: Quick and dirty PIGatron

Post by Maciek Szymanski » Sat Nov 07, 2020 10:51 am

I’ve used multiple holes in the gas distributor because on all drawings and photos of the PIGatrons I’ve always seen multiple holes. The original Farnswort’s cold cathode guns even used only peripheral holes. But I think you have made good point with the single hole.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Maciek Szymanski
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Re: Quick and dirty PIGatron

Post by Maciek Szymanski » Sat Nov 07, 2020 7:28 pm

I’m going to these it with the smaller output aperture (I can put a reduction ring without dismantling the glued joints) and maybe with a more precise valve. And then try a bit better engineered version:

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“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Maciek Szymanski
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Re: Quick and dirty PIGatron

Post by Maciek Szymanski » Fri Dec 04, 2020 6:34 pm

I’ve built an improved version and made some simple experiments. Firs of all - single gas aperture of 0.3mm diameter and anode-cathode distance of 12mm gives much more stable operation. The vacuum sealing of the insulator tube with NBR o-rings and sealing of the anode HV pin with rosin-beeswax is good enough to pump down the chamber to 1e-5 hPa with gas regulator closed. In operation the pressure rises to 8-9e-5 hPa. The operating voltage of the anode is in range +600 to +2000V. Haven’t measured the current but expect to be in 10mA range.
I’ve put the ion collector in front of the PIGatron. The collector consisted of copper plate with a fine metal mesh in front. With effective area of 200mm2 I’m assuming it should collect almost whole beam. The ion current stabilized at about 500nA when the grid was biased -400V. Assuming only single charged ions (surely not true, but it’s just crude estimation) this gives the beam of 3.12e12 ions per second. Not very impressive.
As the second experiment I’ve put the fusor grid into the chamber and a phosphor screen in place of the collector. With -5kV applied to the grid there was no measurable current flow in the grid circuit (I’ve very rudimentary current measurement circuit) but on the screen there was a clearly visible image of the grid created by secondary electrons knocked out from the grid by ions. There was no visible discharge in the chamber of course.
When the grid was replaced by a simple copper plate with a 2mm hole it was possible to observe with well adapted eyes a faint glow on the ion gun side of the plate and and the ion beam itself. I attribute it to the excitation of the neutrals (which are quite abundant) in the ion beam by the secondary electrons knocked out form the plate and accelerated towards the ion gun. There was no visible excitation on the opposite side of the plate, while the electrons were still emitted and observed on the screen.

The observations from those crude experiments makes me think, that even a very simple pig-gunned fusor may be an interesting animal. It should be possible to reduce the grid current requirements as compared to standard brute force ionization method. But for sure it needs more investigation. If anybody is interested I can provide the production drawings of the PIGatron (but note that it’s metric design and will need adaptation for the imperial hardware).


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The assembly drawing of the improved PIGatron.


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Working PIGatron connected to the vacuum chamber.


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The experimental setup. On the table of the pump unit - the ion current nanoammeter. On the rack two HV supplies - the lower one for the PIGatron, the upper one of biasing the collector grid.


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The vacuum chamber setup. On the left the PIGatron with gas regulator (air) on the right extension tube with ion collector.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Richard Hull
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Re: Quick and dirty PIGatron

Post by Richard Hull » Sat Dec 05, 2020 4:35 am

Great work! I have always said a crudely gunned system would be a system easily capable of moving head of the simple fusor that most operate here. You just can't add ion sources without expecting better operation. The issue is operation, itself. Lots of new things to take care of and keep in balance. Maybe not a job for superman, but at least one for maybe a well programed micro-controller. Before any programming can be done, a human being needs to become good at controlling the system. This is the only way a person could program the device as he would be sensitive to nuances needed during controlled operation.

All the best in what seems a bold step forward.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Maciek Szymanski
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Re: Quick and dirty PIGatron

Post by Maciek Szymanski » Wed Dec 09, 2020 4:30 pm

Although completely not fusor related, today the PIGatron proven itself in it’s intended purpose: testing the micro channel plate detectors:


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The PIGatron is mounted on the far left at the end of the extension tube. The MCP is mounted inside the round CF100 chamber with HV feed troughs on sides (MCP in and screen bias). The viewing hood is the proof that any junk will find it use. Two months ago I’ve found few of those ugly, smelly rubber viewing hoods for old Russian oscilloscopes. I’ve thrown away most of them, but left two just in case. And here it is.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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