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 » Thu Mar 21, 2013 10:48 pm

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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Carl Willis
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Re: Questions about the practical construction of a Cyclotron

Post by Carl Willis » Thu Mar 21, 2013 11:05 pm

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 » Thu Mar 21, 2013 11:41 pm

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 » Thu Mar 21, 2013 11:42 pm

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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Rich Feldman
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Re: Questions about the practical construction of a Cyclotron

Post by Rich Feldman » Fri Mar 22, 2013 1:53 am

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.
All models are wrong; some models are useful. -- George Box

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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 » Fri Mar 22, 2013 5:15 am

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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Dennis P Brown
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Re: Questions about the practical construction of a Cyclotron

Post by Dennis P Brown » Fri Mar 22, 2013 12:37 pm

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

Post by Noah C Hoppis » Fri Mar 22, 2013 2:57 pm

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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Rich Feldman
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Re: Questions about the practical construction of a Cyclotron

Post by Rich Feldman » Fri Mar 22, 2013 5:48 pm

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?
All models are wrong; some models are useful. -- George Box

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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 » Fri Mar 22, 2013 7:18 pm

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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