Questions about the practical construction of a Linac

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Noah C Hoppis
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

Oh Yes the specs! the Diameter is 8", the RF supply is 4kV @ 1-20 MHz, the magnet has 8" pole faces @ .8 T. The first experiment will be measuring for a beam current of accelerated Protons, and the second is for the break down of Li by Protons into Alpha particles. The plan is to net get the magnet / magnet core (and in the latter case, then wind the coils), then purchase the parts for the acceleration cavity, mill the necessary parts in the summer / fall, get access to a TIG and weld the vacuum assembly together. Then final assembly and testing.
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Re: Questions about the practical construction of a Cyclotron

Post by Richard Hull »

Equations are cool things. They are SI, MKS and CGS independent in most cases where a system specific fudge factor has not been used. What is important is that every value you plug in must be in the same units system.

If you get a whacko result, go back and examine the unit system you used. Were all the variables you supplied in the same system?

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Noah C Hoppis
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

That's what I thought, the problem is the radius of the Dee is either 4" or ~10cm and it isn't ever balanced, but I assume it is cm and after using coulombs instead of elementary charge I got -0.5 turns of the beam (well at least its not in SI notation anymore!) the Lack of units is really screwy though, I still am betting it is some kind of mix of units!
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Re: Questions about the practical construction of a Cyclotron

Post by Carl Willis »

Elementary charge is a constant that has dimensions of charge in whatever unit system you choose. I recommend the MKS (SI) unit system. It's pretty much the standard.

So you have some specifications:

B = 0.8T (probably reasonable for a small cyclotron; this is already in MKS units)
m = 1.67E-27 kg (you mentioned your particles would be protons)
q = +1e = 1.602E-19 C (elementary charge)
V = 4kV assumed to be rms, so Vpeak = 1.414*4kV = 5700 V
R = (8"/2) = 4" = 0.102m

Emin isn't very influential in this situation, but I will assume it is 5 keV = 8E-16 joules (SI unit of energy) just for the hell of it.

Run the numbers. The first thing you find (if you didn't already have a handle on it) is that Emax is 5E-14 joules. Often this is reported in electronvolts instead; it's about 320 keV. Next you wanted to know approximate number of orbits:

(5E-14 - 8E-16)/(2*1.602E-19*5700) = 28 orbits

Emax is a good solid number you can take to the bank, as long as max radius and mag field strength are well-defined. The calculated number of orbits is just approximate and likely to be too small chiefly because of the transit time effect.

If you did this math using units of inches, slugs, foot-pounds, statcoulombs, etc., you would still get a right answer, strictly speaking. It would still be dimensionally correct. But you would have trouble contextualizing it. Best to stick with the same SI units that everyone else uses, and maybe convert to electronvolts for beam energy since that is in widespread use too.

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Re: Questions about the practical construction of a Cyclotron

Post by Rich Feldman »

Noah,
Not just because of units issues, I urge you to run your numbers using a spreadsheet calculator program. If you don't have one, get one and learn how to use it. It's a great way to document your units and formulas, and facilitates debugging and changing variables.

Here's a freshly whipped example. Blue cells are the only numerical inputs, and three of them are physical constants you can look up.
I have included some unit conversions, as always NEVER in the same cell (calculation step) as a physics formula. Cutting corners invites mistakes. Remember, cells don't cost much at all. In fact, I make a habit of using separate formula cells to convert between SI values (hertz, farad) and practical I/O units (megahertz, microfarad).

The example has some sloppiness and minor mistakes. I should have given a B value of 8894 gauss, to match the 13.56 MHz ISM band. As if your flux uniformity could justify four significant figures.

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Noah C Hoppis
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

So I should need about 1.0e-6torr, correct? Wow 320 keV is better than I was expecting! I assume the next step is to start contacting local universities in search of cheap / free stuff from there labs!
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Re: Questions about the practical construction of a Cyclotron

Post by Dennis P Brown »

The bottom of the 10^-6 torr should work fine; getting there with a diffusion pump will require a good cold trap.
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

That much I assumed; I am wondering how I should extract the beam of protons out of orbit to a target without destroying the beam quality. I think a magnet may be necessary maybe NdFeB's or ceramics, because I picture a hole in the wall of the chamber, and it would just chop the beam up right? any suggestions?
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Re: Questions about the practical construction of a Cyclotron

Post by Carl Willis »

You might want to pitch it as a collaborative effort, rather than just a scrounging effort.

You need climate-controlled floorspace rated for the weight of the cyclotron, and provisions for its significant power and water requirements. You might benefit enormously from the expert and physical resources of a university physics department (including design mentoring, a competent machine shop, and possibly an existing radiation machines license, because unlike fusors, home cyclotrons have gotten people in trouble with their state radiation control departments).

I think your pole size and field strength are best left undetermined until a suitable core is available. Unfortunately, this is probably an item of opportunity, rather than something you can reasonably specify up-front. Even more than a shiny set of SolidWorks models, having a lead on a core tells possible collaborators that this is a serious project with a good chance of success.

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Re: Questions about the practical construction of a Cyclotron

Post by Carl Willis »

Do you really need to "extract" the beam? Why not just shove something into it, which is a far easier proposition for starters. A radial linear motion feedthrough with a target on it can be moved into the beam to intercept particles at various energies depending on radius. It is a valuable diagnostic, answering questions about where your beam is and isn't, and what its energy is. There are reasons to extract a beam into a beamline to a distant target, for example to get PMT-based detectors away from the magnet, or to debunch the RF on it. But why not just cross that bridge when you come to it?

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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

OK. I have already contacted the University of Washington's Accelerator & high energy physics department (when I called the front office guy said,"OK I know just the guy to call" if you can believe it!). I guess It may be almost impossible to construct this thing alone, Though I am not quite willing to relinquish this thing openly to the community yet, however the idea of donations / mentor-ship are still very attractive. It seems that in Washington state it is only defined as an accelerator if by 246-229-0010,"any machine capable of accelerating electrons, protons, deuterons, or other charged particles in a vacuum and of discharging the resultant particulate or other radiation into a medium at energies usually in excess of 1 MeV". does this mean if I stay under 1MeV I am legal?
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Re: Questions about the practical construction of a Cyclotron

Post by Carl Willis »

That's an easy question for the Washington State Health Department. It all depends on how the individuals running the joint interpret their administrative rules. If you call or email them and ask if a proton cyclotron with beam energy less than 1 MeV is a "particle accelerator" under the regulatory definition, they will tell you their thoughts on the subject. The wording of your state's rules seems to leave a lot to their discretion.

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Re: Questions about the practical construction of a Cyclotron

Post by JakeJHecla »

Talk to Mike Brennan: Mike.Brennan@doh.wa.gov, (360) 236-3253. He checked over the NWNC fusor.
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Noah C Hoppis
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

I still wonder how I am going to extract the beam so I can do anything with It, any suggestions? It is true I do plan to use a PMT/scintillator based detector much to the chagrin of others...
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Re: Questions about the practical construction of a Cyclotron

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Noah C Hoppis wrote:
> That much I assumed; I am wondering how I should extract the beam of protons out of orbit to a target without destroying the beam quality. I think a magnet may be necessary maybe NdFeB's or ceramics, because I picture a hole in the wall of the chamber, and it would just chop the beam up right? any suggestions?

Noah C Hoppis wrote:
> I still wonder how I am going to extract the beam so I can do anything with It, any suggestions? It is true I do plan to use a PMT/scintillator based detector much to the chagrin of others...

A few thoughts, grouped by category.

1. The problem has been solved many times before.
2. Search and ask questions on the amateur cyclotron forum.
3. Investigate known cyclotrons using the miracle of the Internet.
4. Consider metal foil windows, which can withstand atmospheric pressure and transmit most of the beam.

5. As Carl said, you can have a motion feedthrough to explore your proton beam within the chamber. A fluorescent screen might be handy, but maybe you can simply measure the DC current on a conductive target. Even in the presence of a strong RF field. Your experimental target for atom-smashing can go inside the vacuum, with resulting alpha particles or consequent scintillation photons passing through windows.

6. After the last accelerating pass, if your beam reaches the edge of your "uniform B field" it will curve less, which might be geometrically convenient.

7. Back to your first idea. If a strong local magnetic field is to divert the beam, it needn't employ permanent magnets. You can get similar field strength change (on the order of 1 tesla) with a steel structure that locally bypasses / shields the large electromagnet's field. For example, if the beam passes into the end of a mu-metal tube in the cyclotron plane, it will proceed without curving. I have not studied beam focusing issues.

8. This is purely speculative now. Your beam comes in bunches at the RF frequency. You could have a small but intense pulsed electromagnet, reaching full strength only at the phase that counts, perhaps reducing its power and cooling requirement.
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Noah C Hoppis
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

Sadly; the University fell through due to liabilities (Lawyers... why!?). It is a bit attractive to colide the protons and Li in the chamber, maybe a Li prism shape reflecting the alpha towards a view port? I am still open to free stuff... wink wink nudge nudge!
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Re: Questions about the practical construction of a Cyclotron

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I would not think with your low beam current that very many alpha particles would be emitted off a Li surface considering the wide angle they would cover but I guess a little could go a long way. More to the point, none of these will travel through any metal foil window that will withstand an atmosphere of pressure unless incredible thin which means they'd have to be very small in diameter.

Even protons near an MV would travel only a few microns through metal foil. A thin foil beryllium window is what would work best but those are rather hazardous materials and hard to safely use. Not sure they are easy to get, either.

You might be far better off to use a translation device to catch the beam. This device would then enable you to not just detect the beam but more your target materials into the beam. I do this for my ESLA rather easily using my own design - a steel bellows vacuum hose and a simple screw drive system (very, very inexpensive and easily works down to the bottom of 10^-6 torr.)

Building your magnet, I'd think, is your number one job; all else are just side jobs compared to that; again, the Sci. Am article did cover that to some degree and could offer insights. Your drawing didn't show any details of the magnet construction and the Sci. Am article does.
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

I did read the article and the magnet they use is a LV HA one like I am used to seeing. with that many turns I see no benefit to using a magnet like theirs. I think a magnet powered of rectified 120V with ~30A behind it for 600 turn 14 AWG coils should work just as well with less copper. I don't quite know though for a fact...
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Re: Questions about the practical construction of a Cyclotron

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Noah C Hoppis wrote:
> I did read the article and the magnet they use is a LV HA one like I am used to seeing. with that many turns I see no benefit to using a magnet like theirs. I think a magnet powered of rectified 120V with ~30A behind it for 600 turn 14 AWG coils should work just as well with less copper. I don't quite know though for a fact...
Noah old buddy,
Your electromagnet's gap length and field strength will dictate the ampere-turn requirement.
Run the numbers for some coil size, shape, and ampere-turns.
Learn that you can freely trade voltage against current, to match your power supply.
But the copper mass, power and cooling requirement, and efficiency are invariant.

[edit] Looks like you have been doing your homework (numbers, the easy part).
Unsolicited review: 18,000 ampere-turns is about right for 0.8 T and one-inch gap.
600 turns of 14 AWG copper, at mean diameter of 10" would be about 4 ohms at 20°C.

To get the DC current, remember to use the -average- rectified voltage instead of the RMS value. The coil's inductance will determine the ripple current.

How hot can the coil get, before 120 volts is not enough to reach your ampere-turn target?
Where are you going to put the cooling water?
How are you going to regulate the current?
What is your ripple current tolerance?
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Noah C Hoppis
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

Sadly not much of any tolerance! the whole thing will have a Capacitor to smooth ripple and the current will be limited by an inductive ballast from previous experiments. The 1 inch gap concerns me though, the gap between the pole faces is 2". I should mention that for every 5 turns there is one 1/4" copper coolant tube. As far as I remember I over engineered the thing to take 30 C before dropping below .8 Tesla.
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Re: Questions about the practical construction of a Linac

Post by John Futter »

Noah
It is funny thing with magnets that field strength is proportional to pole gap and power in watts in.


the formulas uses ampere turns but in the end because of the copper resistance this equates to watts or in this case kilowatts. 220 volts at 30 amps is about right ie 6.6 kW for a magnet of 0.8T and 75-100mm gap.
Most modern magnets are wound with copper tape with figures such as 260Amps @30 volts with the coils capped with aluminum plates with a serpentine watercooling channel to keep it cool.
Thicker coils mean less insulation in the coil space which aids getting more turns on and better cooling as more copper means better heat spread.
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Re: Questions about the practical construction of a Linac

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Oops, incorrect post!
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Re: Questions about the practical construction of a Linac

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Noah C Hoppis wrote:
> Sadly not much of any tolerance! the whole thing will have a Capacitor to smooth ripple
How can you choose a capacitor without a quantitative ripple current requirement? I have doubts about the practicality of that solution. With no capacitor, your current draw from the mains will be a resistance-limited steady current (average rectified voltage divided by R), rapidly reversing direction twice each cycle.

Noah C Hoppis wrote:
> and the current will be limited by an inductive ballast from previous experiments.
I know you want to set a record for low cost cyclotrons.
How much (number, please) is your magnetic field strength allowed to vary as the coolant inlet temperature changes, and as the mains voltage wanders from second to second and minute to minute, for reasons you can't control? A DC power supply might be easier to control.

Noah C Hoppis wrote:
> The 1 inch gap concerns me though, the gap between the pole faces is 2".
Can we see your calculations that suggested 18,000 ampere turns?

Noah C Hoppis wrote:
> I should mention that for every 5 turns there is one 1/4" copper coolant tube. As far as I remember I over engineered the thing to take 30 C before dropping below .8 Tesla.
Good plan, but be sure to compute the thermal drop between copper hot spots and the coolant at outlet.
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Noah C Hoppis
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Re: Questions about the practical construction of a Cyclotron

Post by Noah C Hoppis »

The cap was to avoid the lump from the rectification process, and a DC Power supply *would* be nice, and so would a new RF amp, and a new oscilloscope... (the point is for now, I am picking and choosing my battles and the tolerances in the acceleration cavity seemed to at he time take precedence. Though I have no objection to spending others money... (ah sponsorship)). I think a new DC supply might be feasible but I just don't see where it's going to come from. This is also why I have trouble looking directly at the ~12in magnets on ebay without noticing the $4000+ price tag. I also think it is interesting to hear that the newer coils are flat ribbon. would that at all be plausible for a 8 in magnet? Does anyone know of any recently dismantled / decommissioned accelerator facilities in washington state or the surrounding states?
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Re: Questions about the practical construction of a Linac

Post by Rich Feldman »

Noah,
I'm about to send you a direct email & hope to start a dialog that won't pile more stuff onto this unwieldy forum thread. Offer is to be an electromagnet coach. If you make a good electromagnet & then lose interest, I bet you could sell it for more than the cost of materials you put into it.

Practical hint #1, to preserve flexibility as to power supplies. We see your coil is in two sections. It's easy to switch them between series or parallel. In fact, if you make them as 4 identical windings, you get 3 choices of operating voltage. Might facilitate conversion from an initial design (e.g. to run from rectified 120 VAC) to a DC regulated system using a simple switch-mode buck regulator, with no new coils on cores.

Practical hint #2, to acquire big iron at low cost. Does your metropolitan area have a used-metal dealer? Often associated with a places that buy scrap metal. Is there a machine shop that makes big parts and has a stock of leftovers? Poke around and look for pieces of steel that would fit your plan.

Used or new, parts with smaller cross section might be easier to find, and to finish at a home or school shop. For example, a rectangular bar with 8 x 3 inch cross section could be made by stacking 1/2 x 3 inch bars, or 1 x 1 inch bars. An 8" diameter pole piece could be made by stacking circles cut from thick plates. This has nothing to do with lamination for AC applications. Steel costs, new or used, are generally by the pound, so there's no big cost penalty.
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