Micro-cyclotron

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George Schmermund
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Micro-cyclotron

Post by George Schmermund » Wed Mar 17, 2010 3:08 am

A couple of storms blew in last week and the weather turned brutal. Don't believe everything you hear about San Diego being in perpetual summer. Temperatures plummeted down into the 50's and I huddled around the fireplace for over a week. Got some good reading in, though. Eventually cabin fever crept upon my brow, so the decision to brave the elements was set into motion.

I'll start with some pre-storm info to catch things up. The VDG accelerator column (as previously reported) was a good start in its own direction. However, work on the micro-cyclotron (uC) needed to be accelerated (pun) so that the projects would be back on equal footing. I had ordered a pair of NIB N50 magnets from 'magnets4less.com'. While waiting for them to arrive, the larger transformer was chiseled and filed into shape. (See previous cyclotron post for details).

When the magnet package arrived, I opened it up and saw a pair of shiny 1" thick by 2" diameter N50's separated by plastic spacers. They were nestled in foam peanuts and looked for all the world like the baby Jesus sleeping peacefully in the Nativity scene. I lifted the joined pair gently out of the box and admired them in seraphic wonderment. It didn't take long to realize that I was actually fondling a hand grenade from which someone had already pulled the pin.

Getting the magnets into the yoke was an experience that goes beyond imagination. I thought I knew about NIB's from working with the collection of N42 types that are stuck, somewhat permanently, to various things around the lab. Well, those smaller N42 magnets are barely even the shadows of the large N50's. The closest I can get to a simile is to say that trying to safely intercede between the ferocity of a pair of large N50's is like trying to clean a loaded large caliber hand gun that has a hair trigger and the hammer cocked. Take from this what you will. Somehow I can't see these magnets being available to the general public for long.

Anyway, sparing you guys the clinical details of the magnet nightmare, I'll move onto the effort of the remaining challenge to make the major (and most intimidating) parts of both the VDG and uC accelerator assemblies on the back patio and with only hand tools. Being that the VDG effort became svelte and simple, I had to come up with something comparable to those results for the uC. The answer came to me during an ethanol fired, cannabinoid enhanced, vision. We're not talking Johnnie Walker wisdom here, this was the real thing!

Einstein said that things should be made as simple as possible, but not simpler. Reflecting upon this profound sapience, I was guided to the understanding that cyclotrons were overly complex. It was like when I was a kid rebuilding a carburetor. There were always parts left over. Obviously the manufacturer had designed a device that was also overly complex.

To this end it became clear that, with the right materials for construction, a uC could be fashioned quite simply. The magic trick is to replace the major parts that fit between the pole faces with a material that is both, and at the same time, an excellent conductor and also an excellent insulator. Hey, I'll call it a Hot Fudge Sundae! The magic material is double sided printed circuit board. This material and the supplies needed to make the PCB do it's stuff are readily available at Radio Shack, Fry's, eBay, etc.

With the PCB materials I can etch the dees and any ion source traces on alternate sides of the board. Ancillary circuits for tuning and such can also be etched in place. The core material of the PCB is a good dielectric and easy to work with using only hand tools. As can be seen in the photos, a piece of copper pipe can be cut to form a pair of half circles that will support and separate the two sides of the PCB between the pole pieces. These pieces can be easily soldered into place to form the dees and a self contained vacuum chamber. All electrical connections can be attached from the outside without the need for vacuum penetrations (except for the ion source, of course).

A ceramic post can be placed at the center of the of the dees intersection for support if necessary. I expect the deflection of the PCB to be minimal because of the small area under vacuum. This stuff is really stiff and has the inherent characteristics of a constrained layer damping material. Sealing the space between the dees for the purpose of the electrical and vacuum requirements can be accomplished by using solder paste and a hot plate. The gap between the pipe sections is equally simple. We now have a vacuum chamber, dees, and electrode system that is basically a one-piece throwaway unit. It will be easier to replace than fix.

This handcrafted trick Sundae wouldn't be complete without some 'whipped cream and walnuts', so I'm going to add a specialty pump to the package. The pump will consist of a cryo-sorption pump module that will allow the operation of the uC to run as a quasi-closed system. The singular nature of this type of pump for this application is that it has a huge capacity for pumping everything vaporous and gaseous except for H, He, Ne, and their isotopes. From a normal pump-down of atmospheric gases and vapors, the restriction of best vacuum is limited by the partial pressures of He and Ne. This can be as much as several mTorr. If the system is first back-flushed with argon, the pressure can easily drop to below 1 mTorr. Any of the villainous H2O vapor, in a leak free system, will be permanently trapped in the sieve until the pump is regenerated. This will also make the electrolysis of D2O the method of choice to source the D2.

Assuming that the system is leak tight, a previously regenerated pump module, if properly attached to the uC, will never return to atmospheric pressure when it is thawed out. This is a great advantage over conventional pumping requirements. I'll just mix the Ar with H2 or D2 in the appropriate amounts to get the pressure I need to make everything work. It will also take care of any issues regarding the out-gassing of an unbaked system. The preliminary targets can also be sealed in to keep the system hermetic.

The images include a bonus picture that may be helpful in scaling this project.

Standby for further breakthroughs!
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lutzhoffman
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Re: Micro-cyclotron

Post by lutzhoffman » Wed Mar 17, 2010 7:55 am

Simply Totally Cool l!!

Are you planning a future Synchrotron-cyclotron upgrade ?

Just kidding, or maybe not........This goes to prove that small can be just as cool as big, in nuclear physics, thanks for posting.

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Steven Sesselmann
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Re: Micro-cyclotron

Post by Steven Sesselmann » Wed Mar 17, 2010 10:18 am

George,

Looks like the beginning of a cool experiment, keep us posted..

Steven
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Doug Coulter
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Re: Micro-cyclotron

Post by Doug Coulter » Wed Mar 17, 2010 3:23 pm

Truly a thing of beauty George.

Here is a picture of my own humble effort along that line.
In the picture, I'm trying to measure the actual field via NMR, which I did before I got turned onto a very nice hall effect magnetometer by Chris Bradely. At any rate, I found that a field that should have been about 12k gauss was only about 8k, so I stopped work for some head scratching. Seems the magnet vendors (or the math they provide) is a bit optimistic. I did come up with a way to more or less safely handle these magnets, shown here. First you stick them on a thin piece of sheet metal, which saturates so easily the "pull" is manageable. You then slide that onto the yoke, which in my case has a small milled cutout for that to keep things aligned. I built a little wooden guide to help do the sliding in a more controlled fasion -- George is right, these things are dangerous and a whole new level of magnet. Which can't be turned off, so you have to deal with it, or do like the big boys and magnetize *after* you put it together.

The big insight I think for either of these is that there is no need for a vacuum vessel in the magnetic gap, so it can be thinner and you need fewer Oersteds -- either of these will fit through a door into one of our vacuum tanks. This is a gimme that Nature hands us. Because we need less Oersteds, we then can make it smaller yet and so on.

In my case, I made Dees thusly -- I took a 2" square plate of thick Ti, drilled it out for the ID, then spot welded thin Ti sheet top and bottom, then cut the whole thing in half on the metal saw. I planned to insulate that with alumina plate -- the high temperature version of PCB material. As you can see in my version of this, the magnets have a piece of iron between them and the heat too, but I suspect that very short runs (with a thermocouple on things) will be needed to avoid hitting the Curie temperature on them anyway -- a "real" one might need some plumbing for cooling. Since you cannot machine these magnets in any meaningful way, I also thought I might want something I could machine to get the field profile needed for one of these to control the vertical monkey motion cyclotrons exhibit. But with these particular dimensions, I found the field profile is already about right with plain old flat pieces. All the iron shown here is very soft stuff -- 1008, and I've copper plated most of it to reduce the outgassing and rust issues.

Another nice thing about these -- you can put the final RF stepup coil inside the tank, and only need to feed that with a low impedance/low voltage source -- so your feedthrough doesn't have to be good for a bunch of Kv at RF.

While most cyclotrons have an explicit (and often troublesome if the literature is read) ion source, at the pressures they run I'd think that at most a few random ions would get the process going and the beam itself would make enough to be self sustaining.

This design I wanted to use the Dee walls themselves as the target -- Ti when cool will hold quite a bit of D, so you have a beam on target device with no need for the extra complexity of a beam extraction setup.

I have since bought some larger magnets and plan to really test this at some point, after I do some more fusor work here. If you can't get the gauss you want, you can make the thing a bit bigger and get to the same energy anyway.

Oh, and once together this is much safer, but don't bring it near any big chunks of magnetic material, they have a way of flying up to the thing and your fingers might be in the way of a fast moving hammer!

The screws you can see on the top of the yoke are not to hold this together -- no need for that, the magnets do fine...they are there to jack the top off so you can take it apart!
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Why guess when you can know? Measure!

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Carl Willis
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Re: Micro-cyclotron

Post by Carl Willis » Wed Mar 17, 2010 5:56 pm

Hi George,

Cute project.

It's definitely pushing the envelope for what can be expected to be reliable in the way of materials, particularly the epoxy PC board as insulation for the dee potential. But that will just add to the beauty of the thing if you can get it to work in some fashion.

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Chris Bradley
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Re: Micro-cyclotron

Post by Chris Bradley » Wed Mar 17, 2010 8:03 pm

Your enemy will be contamination from those PCBs - from solder mask (? or whatever the copper surface treatment is) and unbonded epoxy.

I only barely 'get away with it' and my volume:surface area is enormous compared with what yours will be. I also appear to get direct atmospheric diffusion through the material, which tends to kick up the penolic on the way through, and mine is 25mm thick rather than 1.6mm! Leave as much copper as you possibly can on those boards... (I presume you are planning it with one-sided UV types?)

Are you going to measure the field you have there? You may find it is surprisingly small, but it depends on how much flux gets cross-conducted to the yoke.

Lots to do... may be better to find a suitable containment vessel to accommodate the whole assembly. Either way... good luck! (I think you may need it!)

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Doug Coulter
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Re: Micro-cyclotron

Post by Doug Coulter » Wed Mar 17, 2010 8:16 pm

Ah, I missed that George was going to use the PCB stuff to be the vacuum chamber too -- that's a step above what I had planned in some ways.

It might be pretty good if it stays very cool. Fiberglass is a heck of a lot better than phenolic in this situation. If one side is left all Cu, you'd have some extra capacity to deal with but it would resonate out with the RF coupler -- and then it would be more the diffusion through copper, which is nicely low -- he can seal the edges after all.

I'd missed the further thinking that since the Dees are so small, they might be easily self-supporting against atmospheric pressure..
That solves a bunch of problems -- like the big tank, cooling the magnets, and so on. Aha!

I didn't give George enough credit on this -- it's *really* cool. I suspect that what would kill this would be sputtering of metal from the Dees onto the gap on the PCB inside side. But that may not take a lot more work to deal with.

If the fiberglass doesn't pan out, you can get PCB blanks of 20 or 30 mil alumina and that will take it fine.
Why guess when you can know? Measure!

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Chris Bradley
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Re: Micro-cyclotron

Post by Chris Bradley » Wed Mar 17, 2010 9:50 pm

One of the issues I have found is the aspect of working on such small kit. Trying to get a high mag field with permanent magnets will always mean you are working in small spaces, and also can't work with regular tools.

I had to ask a friend in a precision machine shop to help out with some of the parts. It can get rather tricky as everything needs to be pretty accurate.

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Re: Micro-cyclotron

Post by richnormand » Wed Mar 17, 2010 10:03 pm

Eye candy...
Cant wait to see your results.
thanks for posting.

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Rich Feldman
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Re: Micro-cyclotron

Post by Rich Feldman » Thu Mar 18, 2010 3:02 am

Thanks for sharing.
Have you measured the magnetic field strength yet, and found it to be consistent with the geometry and purported N50 permanent magnet material?

This is an good place to use a plain old fluxmeter. Integrate the voltage pulse, as a sense coil of known area and number of turns is quickly moved from middle of gap to "away from magnet". One volt-second indicates a flux linkage change of one weber. You can numerically integrate a digital scope trace, or use simple op-amp circuit, or buy it in a box like:
http://www.maginst.com/specifications/9 ... xmeter.htm
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