Cyclotron seeds

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George Schmermund
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Cyclotron seeds

Post by George Schmermund »

There were a couple of posts a month or so ago about using permanent magnets as replacements for electromagnets. The moniker cyclotron was also mentioned. That sounded pretty cool and I got all excited about the idea. As usual, the fire under me was hot and I got propelled forward for a couple of days. Hot fires don't burn long when the fuel is limited (and damp with beer), so I became easily distracted by some other boondoggle. My magnet creation was put into a baggie and stashed unceremoniously onto a shelf in the nether regions of the lab.

Today was halfheartedly assigned, once again, to another futile attempt at some minimal organization that might result in regaining at least a square foot of bench space. By good fortune, my drudgery was inadvertently deflected by the discovery of the aforementioned baggie containing the mortal remains of the magnet folly. In a beer induced reverie I reflected upon the notes jotted down on a piece of paper left inside the baggie. The paper stated the dry facts about the measurements. What I consider to be more interesting is the basic concept I was working on to obtain a magnetic circuit. I don't claim to be the first to take this path, but here's how it played out.

There is something to be said about using laminated sheets instead of bulk metal for building a yoke. I decided that the transformer guys had it worked out already, so I followed their route. You can buy a used (or new) transformer in about any size imaginable, so I started there. As can be seen from the photo, my choice was a small one that was laying around the lab. It measures about 2" x 2 1/2" x 7/8". The magnets are about 1 1/2" x 3/4" x 5/8". They are a high grade NIB type left over from my planar magnetic loudspeaker fiasco. (If this was an audiophile forum I would expound endlessly about the sublime rapture of experiencing the confluence of time coherent cylindrical waves. Consider yourselves spared).

Anyhow, What I did was to pull the laminations apart and remove the coil which was wrapped around the center leg. This left an E shaped sheet. Well, lots of them actually. I placed each piece into a vice and leveled the inside back of the E with the top of the jaws of the vice. Using a wood chisel ( this stuff is really soft), it was a simple matter to remove the center leg of the E. I then reassembled the pieces and squared them up. Clamping them back into the vice I was able to file the chiseled surfaces back to smooth and flush. I now had a square yoke that was ready for mounting the magnets into. This was also an opportunity to tryout some JB WELD. I have tended to shy away from it until now because it is loaded with powered iron and can really screw up experiments that can't tolerate an epoxy that is magnetic. For this application the iron content would appear to be a big plus.

I pasted the stuff onto the top and bottom faces of the inside of the yoke and everything seemed to be under control. I figured I had applied about enough epoxy to allow for a nice bead to form around the magnets. When I slid the magnets into the yoke, the project suddenly took on a life of it's own. The epoxy started to climb up hill and down dale tracing the field lines, all the while forming hideous tendrils that I swear were trying to reach out and make a snatch at me. I knew that I'd have to work fast to regain control, as the stuff had already maneuvered its way into the gap between the magnet faces and was threatening to ruin the whole experiment. Arming myself with a Popsicle stick I was able to force the recalcitrant mass into submission. By the time it had become too thick to rally another attack, I was able to vigorously burnish the magnet faces with a tongue depressor and restore a uniform gap. When the epoxy finally seemed tame enough, I smoothed it out with my finger. I should have had gloves on. Next time I'll know my enemy better.

Having vanquished the insidious foe it was time for a wassail to my victory! I guess by now we now what that means....

After waiting overnight (to be sure that there was no pulse left in the JB Weld), I proceed with the measurements. Using my trusty F.W. Bell gaussmeter I was able to measure the magnets before and after insertion into the yoke. The internal gaussmeter calibration was compared to several RFL magnet calibration standards, I now had good confidence in the numbers I got with the NIBs. On the bench the NIB faces were close to identical at 4500 gauss. In the yoke, with a uniform 0.1" spacing between the faces, the gap measured a little over 10,000 gauss. That's 1 TESLA plus! The leakage around the outside surface of the yoke averaged about 34 gauss. This would indicated a good (unplanned) match of these magnets to the saturation limit of the yoke material.

Now, these numbers may seem expected or even trivial to some people, but I was damned impressed. At least one of my Am 241 sources is very close to the gap dimension and several other sources are not far removed from that size. This could lead to some interesting experiments inside a small vacuum chamber.

I have since acquired a larger transformer and plan to expand upon the work using some of the many identical magnets I still have left. I'm becoming increasingly fascinated with the idea of making the world's smallest working cyclotron. I have no idea what the record is, but I did read that a group of Swiss scientist, after years of research, discovered the fact that many things can be reduced in size by simply making them smaller.
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Doug Coulter
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Re: Cyclotron seeds

Post by Doug Coulter »

Actually, that's a bit better than I got with my magnets and yoke. Transformer steel is good stuff, but not needed in this application unless you're going for an AC magnetic field -- the steel is divided into laminations to avoid eddy currents in the "shorted turn" in that case. But -- if it works, and yours obviously does, why not? I was trying for a bit larger gap, so I only got about 8k gauss in mine, which is less than the online magnet/reluctance calculator said for 12k gauss magnets, so I'm glad I checked.

And there it sits. I made some Dees out of Ti for it, and got hung up with how to insulate them from the magnet etc, and at the gauss lower than I wanted, not getting the energy I'd wanted, so I shelved it for awhile. My yoke is machined and welded 1008 iron, pretty good for this, Cu plated so it doesn't rust and outgas in the tank. Shown are the shims needed to handle those magnets almost safely, and the jack screws on the top so you can take it apart -- once together it wants to stay that way.

Here's a pic of the thing with an NMR coil in it I used to check the field before I got a "real" magnetometer. The idea here was going to be beam on target D fusion, with D implanted in the Ti Dees, so no extraction needed, then later H on Li fusion, with an Li layer flashed on the dees as the target, again for simplicity.

Soon as I get that round 'tuit I'll fire this puppy up. Doing plain old Farnsworth just now.
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Todd Massure
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Re: Cyclotron seeds

Post by Todd Massure »

An interesting and entertaining experiment to read about. I don't think I would have forseen that problem with the JB Weld either. How big's the next one slated to be?
George Schmermund
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Re: Cyclotron seeds

Post by George Schmermund »

Doug - The theme of my report was to illustrate that I didn't have to use the lathe, mill, drill presses, grinders, welders or any of the other fabrication machinery that I currently maintain a roof over. The entire project was done on the back patio with the most primitive of hand tools and with good results. The method I described should open the door for anyone who wants to try making a simple and effective magnetic structure, but lacks access to or the skills needed to use machine tools for fabrication.
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Chris Bradley
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Re: Cyclotron seeds

Post by Chris Bradley »

I'm happy to see other folks pondering how to make some form of amateur kit utilising magnetic fields, as it is the tune I'm trying to play as well.

Of course, on the whole you're working with a limited amount of 'magnetic energy' stored in the magnets which can fill up a sum total of volume according to B^2=E.2u, so you end up loosing field strength very rapidly by a squared function of increased separation. Without any pole pieces to focus the flux you're also never going to exceed the max surface flux of the magnets you're using, though in the case of neo that is up around 1.2 to 1.3T.

Your effort to develop something that can be made easily is not lost on me. My efforts with just plain mild steel sheets as an inside-out 'yoke' has paid dividends, but suffers a little magnetic energy loss;

viewtopic.php?f=15&t=7189#p51095

I've now managed to configure three sets of magnets at each corner and with increasing mag energy by 50% and reducing the total flux return area, I appear to be getting around 0.2T in this 40mm x 130mm diameter volume.

If it helps demonstrate the back-of-envelope calc for such things; the BH (magnetic energy density) is around 200kJ/m^3 (or more) for neo, so my 24 magnets total, each of 7.6cm^3, have a total 'BH energy' of 36J, so in a volume of 530cm^3 I could hope for a field max of SQRT[(36.2u)/5.3e-4]= 0.4T. So in my case I'm actually loosing a half of the field elsewhere (leakage outside) [which is to be largely expected seeing as the field of any given magnet goes 'inwards' to the centre as much as it goes 'out' through air!] according to the bald numbers of the theory, but nonetheless it's a sufficiently useful volume in which to progress a few experiments that, along with the 0.2T achieved, I am hoping for some interesting results!
George Schmermund
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Re: Cyclotron seeds

Post by George Schmermund »

Chris - I'm just getting started. I might head towards a hybrid using an electromagnet with pole pieces and the NIBs to prime the pump, so to speak.
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George Schmermund
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Re: Cyclotron seeds

Post by George Schmermund »

Todd - The photo shows the scale of the next attempt. This new yoke should allow me to have a gap wide enough to get more than just my gaussmeter probe inside. The goal for me is to be able to make a measurable beam of accelerated protons. At the moment I'm not concerned about high energy, but just a measurable beam. Not knowing any better I'll probably try to use a stripped down and slightly modified nuvistor triode for the ion source. This approach should keep things fairly miniaturized. If all goes even slightly well I'll be able to easily connect it to the little bench top vacuum system that I described construction details of some time ago. It looks as though my VDG accelerator has some compitition. I'll let them duel it out!
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Doug Coulter
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Re: Cyclotron seeds

Post by Doug Coulter »

George,
I am there with that -- doing it on the cheap and simple, it's a mantra here, and if I indicated otherwise, that's "my bad". I do have all that stuff, and in fact got it specifically to do this work with, so I used it.

But I also have a decent demo fusor in a wine jug with a rebuilt compressor as the pump, and a pirani gage that costs under a buck to prove that can be done (and show how).

I very much do like the idea of having both a permanent magnet and some windings for tuning things, best of both worlds if done right -- anything fast-changing, use the wire, for DC magnetism use the permanent guy. So, if you're doing fast changing things, the laminations are the way to go, that is, if you didn't short them all together -- the insulation is kind of fragile.

I understand, however, that "back in the day" when all tuning was done with the magnets, it was because mere physicists couldn't figure out how to tune an RF amplifier as easily. That should be less of a problem now -- you can always hire a mere engineer who would find that to be child's play.

Or, if you're richer than I, buy a broadband linear amp and synthesized waveform generator to do that sort of thing even with non sine waveforms. But hey, I bought a new car for less than the price of a used AR or ENI amp, so I'll just go for a drive when I think like that. Or build my own broadband linear amp.
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Todd Massure
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Re: Cyclotron seeds

Post by Todd Massure »

George, I'll have to get the details of your new experiments over a couple of beers sometime soon...

I was originally planning to use a DC magnet with the pole pieces integral with the chamber - to decrease the gap - but after Chris Bradley's post of his "inside out" configuration I may have to "borrow" that design because if I were to use round plates I could get it to fit neatly into the SS vacuum chamber that I have (that George S. gave me incidentally) Hope you don't mind to much Chris...I give you all the credit for that innovation.
The inside diameter of the chamber is 11" so a design using 10" outer yoke plates and 4" to 6" pole pieces would allow for quite a few magnets arranged around the edges.

The permanent magnet approach is looking much more attractive given the elimination of all the problems of DC ripple, cooling issues and maintaining a stable field strength. Also the cost of the neo magnets seems to have dropped quite a bit since I first started looking at the different approaches. The B field could be fine tuned with the addition or subtraction of smaller magnets which seems easier than a complete disassembly to add or subtract shims in the case of a permanent magnet design. For me it seems much better to fine tune the B field and use RF that is set at one of the designated "industrial frequencies".

George, maybe it would work out to do a joint effort at some point.
George Schmermund
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Re: Cyclotron seeds

Post by George Schmermund »

Todd - That's possible. I am a registered medical marijuana patient. I like the beer idea, too!
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Todd Massure
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Re: Cyclotron seeds

Post by Todd Massure »

Ha ha "joint effort" ....I get it. Took me a minute.
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Chris Bradley
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Re: Cyclotron seeds

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Todd Massure wrote:
> after Chris Bradley's post of his "inside out" configuration I may have to "borrow" that design because if I were to use round plates I could get it to fit neatly into the SS vacuum chamber that I have (that George S. gave me incidentally) Hope you don't mind to much Chris.
On the contrary - any help I can give, just let me know.

One thing I must mention before you make too many plans - the thickness of the plates is important and you should experiment with your set up and size first. I found that less than 5mm and the plates saturated around the corners and lost field unnecessarily. Above 6mm and it seemed to get lost above and below the plates. 5mm of mild steel seemed optimum in my tests. It is more than just outright permeability or anything like that - I got a couple of pieces of mu metal from a specialist firm and sandwiched one each, of 1mm, between two steel plates then made the assembly up. I was thinking that it would really help and stop stray fields, but instead it pulled the internal field strength down considerably! It was too easy then for the flux to escape outside the assembly at the edges. Maxwell SV confirms this behaviour. So, super high permeability doesn't help in my inside-out yoke arrangement and I recommend 5mm of mild steel first as your baseline to improve on.
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Re: Cyclotron seeds

Post by Todd Massure »

Chris, when you say "saturation around the corners" do you mean the top and bottom edges of the plate?
I saw that you were using square plates before so do you mean the square corners of the square plate?
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Chris Bradley
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Re: Cyclotron seeds

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Todd Massure wrote:
> Chris, when you say "saturation around the corners" do you mean the top and bottom edges of the plate?
> I saw that you were using square plates before so do you mean the square corners of the square plate?
I'm just implying that where the magnets are in contact with the plates, I think the plates saturate (with my particular magnets) because you've got the contact area of the magnet feeding into a 5mm cross-section. Further out that thickness of plate can cope (because the flux has distributed itself belw saturation) but right next to the magnet I have a hunch it is saturating. As such, if that is also co-incident with the edge, then you'll get more losses 'out' of the yoke.

In my case, yes, the bunch of magnets at the corners means the flux in the steel is very high right there and the biggest losses occur around (outside) those corners. But this is all generally speculation, with a little simulation and calculation thrown in. Ultimately, I'm not exactly sure this is why the field is optimum at some given thickness in my experimental evaluations, I'm just warning that there may be an optimum to be experimented for. As I say, I recommend you plan to experiment before fixing your design.
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Re: Cyclotron seeds

Post by Todd Massure »

Thanks for the background info from your experiments. It's my understanding that the magnet circuit will be most efficient when the field is generated closest to the gap which is why most electromagnets for cyclotron purposes etc. place the magnets right above and below the pole pieces as George has them.
The "inside out" configuration definitely puts the magnets far from the pole pieces ,at least in terms of the circuit, so that could be part of the problem of the losses. Interesting point about the steel being over saturated at the area of contact. Perhaps a different material would help. I was looking at using iron discs - there are some available from McMaster Carr, but I'm not sure if it's the right kind of iron so it might be better to use a low carbon steel.
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Chris Bradley
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Re: Cyclotron seeds

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Todd Massure wrote:
> Thanks for the background info from your experiments. It's my understanding that the magnet circuit will be most efficient when the field is generated closest to the gap which is why most electromagnets for cyclotron purposes etc. place the magnets right above and below the pole pieces as George has them.
I'm not rightly sure about that. As far as I see magnetic theory, a magnet is like a battery and the yoke is like the conductor. Doesn't matter where the battery goes... the issue is the flux in the yoke and the space in the gap that this magnetic energy has to fill. As per my calc, once you have that BH energy, it will be distributed somewhere. The orthodox yoke arrangement may well be better for other reasons, though, e.g. that the flux from the magnet's surface is less prone to jump across back into the yoke than it would if the magnets were in the middle of the yoke on the other side so that they, themselves, were sideways-on to the gap. Not sure about that, though, still not sure it would make much difference if the cross-section of the magnets and that of the yoke were the same because you'd expect a constant flux throughout. But I don't think it is strictly for the reason that the magnet itself is closer to the gap. Indeed, there is good reason for wanting to avoid this - in a vacuum with fast ions milling around, you don't really want them slamming into temperature-sensitive magnets. Ultimately, though, the success or otherwise will be in the actual setup you go for, so please do check your arrangement is satisfactory for your purposes.
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Re: Cyclotron seeds

Post by Kade »

Hi,I am very new to the group and am very impresssed and am learning a lot of usefull things with the activities and experiences discussed here.
Chris Bradley said:
I'm not rightly sure about that. As far as I see magnetic theory, a magnet is like a battery and the yoke is like the conductor. Doesn't matter where the battery goes... the issue is the flux in the yoke and the space in the gap that this magnetic energy has to fill.
Chris you are correct but the air circuit is still active even when a yoke is present so for many yoke configurations, there is also an "air yoke" through which leakage flux flows in parallel with the yoke flux and directly across the magnet through the air (or Vaccuum) as well. So Todd is correct in terms of maximum flux utility being obtained when the magnets are closest to the gap. The other consideration that Chris pointed out about potential particle damage to the magnet material is true also but may be avoided by incorporating a slice of iron or perhaps hi Cobalt steel as pole faces on the magnet face. Not sure how Cobalt would survive but a 50% mix provides about 2 Tesla flux density capability (good if you want to focus at the gap).
Best Regards: Kevin
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