Accelerator column

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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George Schmermund
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Accelerator column

Post by George Schmermund »

The quest for an accelerating column for my Van de Graaff generator has made some progress in the last few days. In order not to give unfair advantage to the VDG over the micro-cyclotron project, I decided to level the playing field by making the column on the back patio using only simple hand tools. The only electrified tool was a hot air gun. Of course I could have done just as well with a torch or alcohol lamp, so the rules were bent only slightly. All the parts so far have been right off the shelf from Home Depot.

The parts list includes five 3/4" to 1/2" copper reducing couplings and one 1/2" x 12" piece of polyethylene tube. Total cost: ~ $10. This being a 'proof of concept' effort, I made no attempt at dimensional accuracy or surface preparation except for removing the scanner labels. I mused about using Scotch Brite on the edges of the copper parts, but they looked pretty good already and I was more interested in the assembly process than vacuum cleanliness.

The assembly proceeded by using the hot air gun to soften the end of the PE tubing to the point at which I could force it over the wooden handle of a screwdriver that was secured in a vice. This gave me the desired shape for fitting over the 1/2" end of the copper coupling. Using a hand cutter designed for cutting plastic irrigation tubing, I allowed a slight overhang of the PE before parting it. The PE started with an I.D. that was smaller than 1/2" before being shaped by the screwdriver handle and now had an O.D. slightly larger than the 3/4" I.D. side of the fitting. By lining up two couplings in the vice, with the section of PE nesting between them, I was able to extrude the couplings together. Some amount of force was required to produce this extrusion and the parallel faces of the vice kept the couplings in remarkable alignment throughout the process. After repeating this tactic on all five couplings I was rewarded with an assembly that was very stiff and rolled on a flat surface like a piece of drill rod. This outcome surprised me because I was also dreaming up an assortment of methods to keep the assembly straight when all the parts were together. Now it's like sighting through a gun barrel and with no remedial help from me.

An advantage to this type design is that the ion beam should see a very smooth voltage gradient because the electrodes overlap. This overlap should shield the beam from being influenced by charged insulators caused by rogue charged particles.

The next step was to test the electrical resistance between each coupling. My insulation breakdown tester is limited to 5 kV DC and each section passed with flying colors. This gives me a rating of 25 kV minimum for the column as it sits. As a non sequitur, I'd like to add that I think it's a good thing to make an x-ray image occasionally, so this project was chosen to scratch that itch.

In the next installment I'll try to describe making some simple corona barriers and then festoon the column with 200 M ohm resistors to act as equipotential bleeders. I'm aiming for at least 10 kV per section when the inside volume is under vacuum. As there is no real heat generated at the currents that I'm planning to use, the PE should provide good service under reasonable vacuum. We'll see.

P.S. - Chris B, you've trashed all of my ideas about what will or won't work in constructing a vacuum chamber.
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Doug Coulter
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Re: Accelerator column

Post by Doug Coulter »

I really like this one a lot, cool idea George!
The main reasons to have more than two electrodes are just to split up the HV and solve some corona to streamer transition problems, and to give you a special section (the first) to get focus right by adjusting the drop there. After that, the other "lens elements" don't do much as the focal length is more or less shortened by the *ratio* of the increase in voltage per lens. So if the voltages are all equal, the first is zero to 1/n the power supply, big jump, and this is the main focus. The next is n to 2n, so also a fairly big jump. Next is 2n to 3n so not so much focus effect, and so forth.

If you don't get this hot, it should be fine for vacuum. Back in the day, apezion wax was used for example. The only change I would make to this (and only after I saw trouble) would be some rounding off where there might be a fairly sharp edge that would cause too much corona loss to the air.

I bet you can get nearly 10kv/stage here, maybe more, depending on the details of the plastic pieces.

FWIW, for our electron gun/accelerator, we did discs with slightly too small holes punched in copper flashing and just forced them over the elements -- that may cheat your rules, as we used a real punch and a press we have here, but it's still pretty simple. Of course, you could just make the holes right sized and solder these on (before doing the plastic I suppose). We were running about 10kv a stage, and didn't need to do much to treat the disc edges -- just smooth them out on the sander. Disks were about 2" diameter.

Doug
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DaveC
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Re: Accelerator column

Post by DaveC »

George -

Very slick and pretty! A clever variation on the glass egun designs.

Dave Cooper
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Steven Sesselmann
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Re: Accelerator column

Post by Steven Sesselmann »

I like this too, I have been working on the same design myself..

Would it work to have these copper sleeves as passive electrodes, or do they need to be actively connected via a series of resistors?

Steven
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lutzhoffman
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Re: Accelerator column

Post by lutzhoffman »

So Cool !!!!

Simply "art" in terms of simplicity, and functionality. You could even drop this in oil if you wanted, if you used DP HC oil as an insulator then it could even seal any micro defects at the same time.

The closest thing that I have seen in this area (Which really stands a chance of actualy working) was an UHMW plastic disk design with viton o-rings, and sheet metal electrodes between them. They used long pre-tensioned nylon threaded rods to keep it all under compression. This worked up to about 200KV with a 30 inch length if I remember right.

This is however even better, an accelerator for the masses! If it were placed in a 6" PVC pipe with a multiplier, and some simple KF fittings, then maybe 100-150KV could be done with just a little pressure, or with some oil, or teflon powder dry fill. Now add Carl's or Doug's ion source and bingo!!! Good Show, thanks for sharing

Aloha.....Lutz

PS: I have a bunch of 35KV 3 nF new HV caps for sale, which I no longer need, if you want to build a multiplier to go with it : )
George Schmermund
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Re: Accelerator column

Post by George Schmermund »

I think something fundamental is being overlooked here. I'm a cheapskate and didn't want to spend more than about 10 bucks and limited the construction time to less than 1 hour. This was a 'proof of concept' effort. If I was a spendthrift, I would have blown about 20 bucks and made the column 10 electrodes long, etc.
Anything obvious in high vacuum is probably wrong.
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Doug Coulter
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Re: Accelerator column

Post by Doug Coulter »

I don't think I missed it George. The wonderful simplicity of this (science teachers projects come to mind right off) is what's so cool about it. The fact that a bunch of HV and or accelerator freaks jumped on your basic idea and immediately wanted to do this and that with it for some custom use is sign of pure genius in the underlying idea -- it's not a tweak, it's something others tweak, something fundamental. Or at least it is to this old crufty engineer. The "wires and parts that aren't there" are the finest ones, IMO.

Doug
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lutzhoffman
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Re: Accelerator column

Post by lutzhoffman »

Just like Doug said, "ditto"
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Steven Sesselmann
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Re: Accelerator column

Post by Steven Sesselmann »

George,

At the hardware store today, I couldn't help myself, so I went to see if they had these copper fittings, and sure, excactly the same ones.., so I got a handful to play with

Might try machining some plastic fittings, I don't like the idea that the plastic is exposed to the ions, so I am thinking of machining it so that the plastic sleeve is shorter than the fitting.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
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lutzhoffman
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Re: Accelerator column

Post by lutzhoffman »

Hello:

I do not want to take anything away from the elegance of the simplicity of this idea. So what to do? Do we really want to get out the sharpie, and try to improve the Picasso?

What the heck, we are only talking about doing it, and not actually ruining the art. So I have done my best to exclude anything which you cannot get at the local hardware store, and I have come up with some variations, which may be in keeping with the spirit of the original thread.

My ideas are based on the PE is basically doing 2 things: One providing vacuum tight sealing, and two acting as an insulator. Yes I suppose there is a third: Providing rigid support for the whole thing. So instead of proposing an exact variation, I would rather suggest a line of thought, towards some possibilities:

Since the PE is just an insulating, sealing bushing, then any combination of cheap hardware store parts which accomplish this function is fair game. I could think of quite a few parts which could work, like the already o-ring fitted PVC bushings found in the irrigation section. The local refrigeration / AC supply shop could also provide more options. In my neck of the woods they have many things not carried by the hardware store. like vacuum fittings, silver brazing alloys, and large diameter copper fittings.

Then there is the possibility of just using a couple of fat o-rings, with some epoxy between them to cast a sealing, insulating bushing. In other words cast the bushing into place out of epoxy, by using the o-rings as a "sealing dam" to keep it in place until it sets. Afterwords the o-rings could be left in place.

The third possibility is the exact reverse, by using insulating reduction fittings with conductive copper bushings. In this case you would have to solder a washer into them.

None of the ideas that I could think of: Even come close overall to the original idea, they are all inferior in some way. My point of posting them is only to maybe stimulate some thought on the subject, in keeping with the spirit and intention of the original posting.

Take Care......Lutz
George Schmermund
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Re: Accelerator column

Post by George Schmermund »

Steve - Sounds good to me. I'd have done the same thing if I wasn't so lazy. Once again, I was only testing the parts and method of assembly.
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Dustinit
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Re: Accelerator column

Post by Dustinit »

I like this.
Looks like it could be directly coupled to the stages of a cw multiplier too.
RF acceleration might also be possible by extending each coupling a bit towards the source although the freq would probably be too high with such short stages.
Dustin
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Re: Accelerator column

Post by ggombert »

Hi,
I was wondering if anyone had done anything more with the concept that George came up with for a low cost accelerator column since this thread was originally posted?

Questions that come to mind are:

-- The first stage would probably need a variable resistor to focus the beam; the rest of the elements could use fixed resistors (perhaps the 200 M ohm) that is mentioned in the original description.
-- Would the proposed assembly method allow 10-5 vacuum without any additional sealing (or would something like this be required)?
-- Would simple corona barrier be a smaller hole in the ½ inch coupling made with a piece of copper/brass and attached before the sections are assembled?
-- Of the many good ion sources described in the forum what would be the simplest to use with this sort of accelerator column?

This seems like too neat a simple concept not to at least try and carry the design a bit further to a working prototype.

Cheers,
Glenn
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Re: Accelerator column

Post by John Futter »

Glen
Good questions
The simplest ion sources would be either Carl Willis;s HFRF driven one or Doug Coulters Microwave oven derived source. other possiblities include cold cathode penning type.

1x 10 e -5 Millibar should be very easy to get with what George has presented. Bear in mind that you would want to operate the tube an order of magnitude below this for best results (beam spreading due to neutrals).

A 50 l/s turbo would be heaps for this

and yes the top tube section is generally referred to as the Focus electrode
ggombert
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Re: Accelerator column

Post by ggombert »

Hi,
The attached file describes a simple ion source that is similar to what was used on the original Amateur Scientist article in 1971. It was built by a group of students that modeled what they built loosely based on that article. Would this type of design be suitable for use with the accelerator column discussed here or is something more sophisticated required?
Thanks,
Glenn
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John Futter
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Re: Accelerator column

Post by John Futter »

Glenn
Yes will work albeit badly
if a permanent magnet was put around the glass tube then you have a cold cathode penning type which will produce around 100x as many ions as what you presented
Reading all of this especially the chapters on ion sources should put things in perspective
viewtopic.php?f=6&t=3115#p12625
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