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750 KV CW Accelerator Example / Finding Corona Rings?

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LutzH

Tue Dec 15 2009, 10:24PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

I have finally found a good picture to share, which nicely shows what I am eventually building: It is a DC linear particle accelerator, using a FWCW HV multiplier to generate the required HV. Most of my other HV activities like designing, and building a HFHV transformer, the HF IGBT driver, and others, tie into this project. I received a big boost recently when I landed an accelerator tube on ebay. So no more having to build the accelerator tube from Pyrex rings, sheet metal electrodes, and PVA glue. If someone is interested in home building an accelerator tube, I will still be happy to share how to do it right, so that all my research on the subject can be used by someone. What I am building will look almost the same as this unit except mine will have less "beefy" HV diodes, mine will be 2CL2FM 20KV 100ma diodes, about 350 of them ïŠ

Pictured is a 750 KV FWCW electron accelerator, this one was built by RRCAT in India recently, their web site is great to visit also because they did a lot of work with HFHV design, and they are happy to share their knowledge. Their site is searchable, and the HV stuff is fairly complete, with schematics etc. The main difference between the one which I am building, and the one in the picture below, is that mine will have an ion source on the top, and a rotating insulating rod running a 400Hz generator inside of the HV terminal, driven by a motor in the base of the column. The HV terminal is not shown in the picture, but it is a hemisphere with a radius on the bottom just like on a VDG generator. What I like about this picture is that it shows how the components of the VM are arranged. In the upper edge of the picture you can see the bottom of the segmented accelerator tube. The VM sections pictured here have been lowered from their normal position around the tube.

The idea of being able to re-arrange the smallest bits of matter, is simply fascinating to me. Radiation protection with this project is an issue of staying alive, so it will NOT be neglected. The problem is not radioactivity, the big problem is the acceleration of secondary electrons from the target back up the tube, can turn it into a giant x-ray machine if they are not suppressed.

The anti-corona rings which I am trying to find, are really frustrating me. You would think that something so simple would be easy to find? I need two sizes of rings; both are essentially just a round hoop of SS tubing with joined ends. The two sizes that I need are 14" OD, and 16" OD rings of SS 1" tubing. I need six of the 16" rings, and eight of the 14" rings for the project. Any 14 and 16" SS tube rings made from 1-1.5" size SS tubing will work fine. Yes I know I can have them made, but its just to expensive, starting at over 1000 USD. I have even seen some perfect examples on the bottom of bar stools as a foot rest, and on wheel chairs, but try to get just the ring minus the bar stool? I did and had no luckin this area. I thought about copper pipe filled with sand for bending, but I really want SS, Aluminum is out due to its inferior HV properties when compared to Copper and SS, I even put this to the test with a table top VDG generator, Cu and SS terminals gave about 10-20% higher voltages in both high, and low humidity. The problem with Al seems to be related to the insulating oxide layer.

I hope that someone out there can give me some ideas on where to search for such a SS ring product. I have tried searching under: SS and metal Hoops, rings, and even toroids, but no luck. I am hoping that if I do the net search using the correct wording, that then it will bear fruit. I for example recently found some nice HV SS toroids finding them listed under SS "Welding Donuts", they cut these up to make extreme angle exhaust pipe welded bends, but as is, they are great for HV terminals up to around 120KV. SS balls can be found in the garden section as "Gazing Balls", but no luck on the hoops which will form the anticorona rings on my project? Any ideas anyone? "Thanks in Advance"

rp181

Tue Dec 15 2009, 10:43PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Nice picture.
Forming rings would be quite easy your self. I would go about it like this:
1)find a tube of a sort with the ID you need.
2) drill the end of the ss tube all the way through.
3) bolt the end to the former, so the pipe sticks out on a tangent
4)grab the end, or roll the former to bend it around the former, around 1.25 turns (wind like a tesla coil)
5) cut where the pipe needs to join
6)drill holes on the other end of the pipe
7) clamp together the ends using a piece of tube cut lengthwise

All the commercial ones are used with sheets spun on a lathe, and two halves welded together and polished. If you can find access to a lathe, you can try that.

LutzH

Wed Dec 16 2009, 12:19AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

Thanks for the suggestion, I still have hope that I will not have to make them, but if I do then I may try your suggested method.

If you were making a lot of these would you consider winding one big tight coil, and then cutting this up into the individual hoops, and then joining the ends in the manner that you suggested?

Thanks....Lutz

rp181

Wed Dec 16 2009, 12:38AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

It would be better to wind one long coil, less waste of metal too.
The fact that there will be tension when the ends are pushed together will help keep everything tight too.
Keep in mind after you wind it, it will spring back some. to get it together, I would clamp one end of the tube, and have someone position the other end while tightening the joint.

jpsmith123

Wed Dec 16 2009, 05:41AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Hello,

Is that the accelerator tube in the center of that structure shown in the picture? I'm a little confused as it looks like it's made of solid metal (although it's hard to see it very clearly).

Is the structure shown in the picture designed to operate in the open air, or does it go in a tank with pressurized air or SF6 or something?

In any case, I don't think you should rule out aluminum for use in any kind of grading structure. A table-top VDG is probably not a very useful test bed as it has such a high impedance that a small increase in corona loss makes a noticeable decrease in terminal voltage. Unless your design is really faulty, I would think that your corona loss will be small compared to the beam load.

Ross Engineering uses aluminum for their corona toroids:

Also, I'm wondering, how did you determine what size rings you need, did you simulate it with a field solver or copy an established design you saw somewhere or something?

Lastly, I'm wondering, what kind of ions do you want to accelerate?

teravolt

Thu Dec 17 2009, 05:07AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

They have aluminuim refigerator tubing at OSH. you could use that to do what rp181 is sugjesting

LutzH

Thu Dec 17 2009, 10:21PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Thanks for the ideas, Aluminum I want to stay away from. Before making the rings I want to first try to find them in the form of some mass produced product. I did find some listed as "hand rail" for wheel chairs for about $40 each, that is close to the price of the material plus shipping.

I will try some other search terminology to try to find the bar stool foot rest rings which would be perfect in size :) The key is searching with the correct terminology, which usualy works. Thanks for the ideas, so far they have gotten me to the point of being able to build them, if I end up having to.

govtcontact

Sun Dec 20 2009, 05:35PM

Registered Member #343 Joined: Fri Mar 24 2006, 02:45PM
Location: Los Alamos NM
Posts: 10

jpsmith123: I believe that the unit is in a pressurized vessel (the yellowish metal ring with holes near the bottom of the pic). In fact, as close as the stages look to each other, it would have to be pressurized.

The use of aluminum in an accelerator is not a major issue, as long as it is super clean. I use aluminum in my 0.5 - 3.5MeV RF S-band LINAC at work for holding targets and what not. Only SS is used for the beam-line, mainly because that is what our UHV vacuum tubing is.

I'll have to research some more on what your trying to do exactly so I can give better help.

LutzH

Tue Dec 22 2009, 02:57AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

Ref: jpsmith123

How ironic, my accellerator tube came from Los Alamos according to NEC, they think it was used as a HV feedthrough, instead of as an accelerator due to the total lack of radiation discoloration.

My plan is to use an SF-6 gas mixture at STP, even without pressure this will allow for 2-3 times the potential of air. I would like to investigate Tetrafluoroethylene as an insulating gas, it is everywhere in dust off cans etc. but I cannot find anything about it, so maybe it sucks, but maybe not.

Another reason for choosing SS is that it is much more easy to work, I can silver braze it, even weld it with fuel gas and Solarflux, and it has better structural properties, like resistance to dings, corrosion etc. So it is my prefered material, maybe under ideal conditions Al is as good, but its a pain in other respects with the tools that I have.

Let me know if you ever know of a spare ion source, I will probably have to build one. Doug at the fusor.net site has a real nifty design using a common microwave magnetron, just the ticket. A duoplasmatron would be nice though. Thank You.....Lutz :)

Steve Ward

Fri Dec 25 2009, 08:04PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Id consider using standard plastic hose filled with water to make corona rings. It should be fairly easy to come by but i guess the biggest challenge would be to join the ends together cleanly and leak-proof.

Im surprised about the difference in aluminum vs copper or ss. Are you sure it was just because of the surface finish differences? A brushed aluminum finish probably does have a significantly lower breakdown than a polished one. Im not sure why the oxide layer would matter at all.

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