Einze-lens-'tractor-gun

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Conrad Farnsworth
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Einze-lens-'tractor-gun

Post by Conrad Farnsworth »

These past few months, off and on, I've been designing and fabricating an ion gun with a built in einzel lens/extractor electrode (hence the name). I would like to present the nearly final, sans JB Weld, first fabrication to the community for criticism. I designed this gun knowing virtually nothing about the mathematics behind the physics of an einzel lens. This is partly due to my lack of knowledge and experience with calculus. The dimensions are nearly random and I hope to run experiments to find out what voltages will create a focal point where. Although I know it's far from perfect I have never constructed something so beautiful.
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Steven Sesselmann
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Re: Einze-lens-'tractor-gun

Post by Steven Sesselmann »

Conrad,

Nice job, I hope it does what you expect it to.

A computer simulation before building might have been a good idea, but hey, that advise is from someone who usually builds first and then calculates what went wrong after ; )

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
Conrad Farnsworth
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Re: Einze-lens-'tractor-gun

Post by Conrad Farnsworth »

Steven,

Thanks for the kind words! I considered using simion for the simulation but didn't want to spend $2000. I have considered possion superfish but never made the time to sort through the logistics of it.

Sincerely

-Conrad
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Doug Coulter
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Re: Einze-lens-'tractor-gun

Post by Doug Coulter »

In theory, practice matches theory. In practice, it often doesn't because the use of the theory was too oversimplified. It's generally obvious what you missed in hindsight, thus the emphasis here (and on my site as well) is - get thee to the lab and try it!

I did a little work with einzel lenses and electrons to learn what little I know about them. Haven't done it with ion sources yet - just simple pullme-pushyou stuff.
Why guess when you can know? Measure!
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Dennis P Brown
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Re: Einze-lens-'tractor-gun

Post by Dennis P Brown »

I will have to check later on some papers I have on the subject but your design would appear to have very weak focusing. If memory serves, the hole diameter to tube length for the unit is important - the longer the tube (for a fixed diameter), the stronger the focusing. The electric field matters, of course, but for equal fields, the hole diameter and tube length are the key factors, Flat plates will not focus very well due to their very long focus distance.

By the way, nice overall design.

I'll edit once I find the papers later today.

Update: Computer with the papers is down ... hope to fix by Thursday.
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Doug Coulter
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Re: Einze-lens-'tractor-gun

Post by Doug Coulter »

Termans Radio Engineer's handbook has about the best lens design stuff there is (for electrons, but easy scaling factors apply for ions - charged is charged).

In practice, you'll be working with this attempting to accelerate and focus electrons the other way as well, something to keep in mind.

Moore, Davis, Kaplan's Building Scientific Apparatus also has some Einzel-specific stuff, but it's not a patch on Terman's 1943 works, which are outstanding in all of raw information, plots, and explanation of the why - including some not too difficult math to really get into it. Too many plots and nomograms to scan and put up here, but in most cases the gap length vs diameter vs change in eV is more important than tube length. Which is why the flat plate with hole version is universally used in all CRTs from almost the very beginning. But usually the holes are a lot smaller vs gap.

This should be a good setup to learn the difference between way oversimplified theory and actual practice. Remember, ions repel one another but attract electrons (which also repel one another - the real behavior is a heck of a lot more complex in this environment.

I recommend getting both books as starters. Charged particle lens design has gone all "trade secret" in the last few decades due to electron microscope profit motives (largely magnetic lenses in that case but still...) - it's real hard to find good data these days in current form without paying WAY too much for a book that may or may not have it (the academic vanity press, in other words).
Why guess when you can know? Measure!
Conrad Farnsworth
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Re: Einze-lens-'tractor-gun

Post by Conrad Farnsworth »

Dennis and Doug,

Thanks for the reply! If there was an area where I shot myself in the foot then it was probably with the washers. Either way it will be a learning experience. The distance I have to work with is about 6" to a foot if I recall correctly. I'll do some more reading on the lenses and see how it turns out.

-Conrad
Edward Miller
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Re: Einze-lens-'tractor-gun

Post by Edward Miller »

We could in theory setup a shared workstation virtual machine on Amazon Web Services that the contributing users could use to do their simulations.... Let me know if this is interesting enough for enough folks. It would be easy enough to access, you just reserve your time and remote desktop in via Logmein or similar.
Conrad Farnsworth
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Einze-lens-'tractor-gun UPDATE!

Post by Conrad Farnsworth »

I'd like to start off saying it's good to be back

It seems as though I've opened up Pandora's box with this apparatus I've created. There are several mysteries and problems that came with it, but overall I'd say with some more testing the kinks can be ironed out.

The problems/mysteries are as follows:
1) Exposed high voltage next to control surfaces (DOH!)
2) Increased X-Ray output (see 3)
3) Evidence of thermionic emissions without any evidence of heating
4) Longer run times
5) Einzel lens reducing X-Ray output temporarily
6) Plasma Rods in inner grid
7) Einzel lens failure



1) This is by far my largest problem and it's all user/designer error. I should probably be dead right now. At approximately 2 AM Thursday morning I was (ironically) in the process of turning off the power supplies between runs for my experiment. When I had reached for the control to my inner grid supply I was suddenly slammed with 1kv at AT LEAST 50mA (a friend of mine says 100) from my ion gun supply. It was the worst shock I have ever experienced in my life and I truly believe that if it had been my left hand been contacting ground and not my right leg I would be dead right now.
Lessons learned:
Always keep DEADLY high voltage away from control surfaces, in this case it was rather unavoidable but next time precautions will be taken.
NEVER work alone.
NEVER work late at night.
ALWAYS exercise extreme caution. The things that try to kill you from day 1 will still try to kill you no matter how long you've been around them.
Proper lab attire is a must. I was wearing my Uncle Eddie bathrobe and crocs during this incident. I don't think anyone wants to be found dead in crocs and a bathrobe.

2) I'm not sure if I'm noticing this more because I'm measuring it more or if it has actually increased. Either way I've created a sort of X-Ray flashlight that maxes out my geiger counter at distances as far as five feet. A couple different tests have been run to determine that this is NOT EMI.

3) The only time I have ever measured X-Ray radiation in my fusor is when I was observing inner grid heating. Now all of the sudden it appears as though the entire inside of my fusor is staying cool (nothing glowing red) while at the same time generating copious amounts of X-Ray radiation.

4) At an 83% duty cycle (5 min on 1 off), I have been able to run my fusor for over an hour collecting data for my experiment. After I collect all the data I need, I will do a marathon run to see exactly how long it takes to begin heating something.

5) Out of curiosity, I measured the very edge of my X-Ray beam while at the same time switching on my einzel lens. Surprisingly, the amount of radiation decreased by a noticeable amount for a few seconds, then proceeded to climb again. Perhaps the X-Rays originate from the ion gun and the lens is acting like a sort of vacuum tube grid.

6) This is a really neat effect that I have never seen before! Instead of a ball of plasma in the grid, I saw what was too little balls of plasma side by side. When I changed my viewing angle, I noticed that they weren't balls, but rather two parallel cylinders pointing straight at me and straight back at the ion gun. I'll try to snap a picture of them.

7) With the lenses machined to such tight dimensions in respect to the nipple I was inserting them into, I knew to expect arcing, but what I am unable to explain is their failure (after arcing several times) to respond to input voltage now. It would seem that a mechanical failure would put at least one lens in contact with the stainless steel which then would cause a short or make arcing more prone to happen, but this is not the case. Now no matter which delivery method (PSU or variac) I am unable to observe any sort of current draw at any voltage.

With this configuration I have seen my maximum neutron rates rise from 300 CPM to 900 CPM at 6". This could either be attributed to the ion gun (inconclusive data so far). Or the defunct diffusion pump which is causing major pressure fluctuations that are skewing my runs.

-Conrad
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Conrad Farnsworth
Posts: 135
Joined: Wed Jul 07, 2010 9:35 pm
Real name: Conrad Farnsworth

Re: Einze-lens-'tractor-gun UPDATE!

Post by Conrad Farnsworth »

Image Descriptions:

1) Grounded Inner grid extremely high pressure, ion gun on einzel lens off.
2-6) High pressures and the effects the einzel lens had on the plasma. (4-6) are at a different pressure.
(7-12) taken at a different viewport position and the inner grid had been re-positioned.
7) Just the ion gun fired up behind the grid.
8) Just the grid.
9) Ion gun at higher pressures.
10) Idiotic ion gun supply position.
11) Ion gun and inner grid at higher pressures.
12) Ion gun and inner grid at fusion pressures. The optimized image prevents one from seeing the two "rods" mentioned before.
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