Energy efficient proton production?

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Quantum
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Energy efficient proton production?

Post by Quantum » Sat Jun 20, 2009 5:17 pm

Firstly, I hope I've managed to post this question in the correct forum now.

Secondly, It would seem that ALL the big synchrocyclotrons use RF/magnetron sources for proton beams, but even they only achieve about 13% efficiency producing protons from hydrogen.

Thirdly, while I appreciate that the energy of the protons produced (in electron volts) is relative to the energy used to produce them, their energies can be increased (accelerated) using magnetic/electrostatic fields.

Now, assuming the purpose of the exercise is to produce protons for proton/boron 11 fusion, the first criteria is to produce as many protons as possible using the least amount of energy possible.

Producing unwanted ions, and then 'filtering' them out wastes energy, but is probably unavoidable.

So the most energy efficient way to produce protons would appear to be to produce ions of low energy (electron volts), filter out the unwanted ions, and then accelarate the remaining protons to the required velocities.

Or, would producing low energy ions produce more H2 ions and less protons?

Would producing high energy ions result in more proton production?

I hope this question is 'specific enough'....Any suggestions will be appreciated.

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Chris Bradley
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Re: Energy efficient proton production?

Post by Chris Bradley » Sat Jun 20, 2009 5:43 pm

By electrostatic means, you can't just produce high energy protons straight-off. First you stick in ~13eV's worth of energy to get a proton, then it gets accelerated by whatever field or process you've devised to accelerate it. For p11-B, you really need to be talking 161keV to get it up to enough energy to hit the point where there is a little resonant peak for fusion.

So if you get only 1% 'yield' of H+ out of your process, as compared with 100%, then providing you 'filter' it with magnetic means before you accelerate it significantly the different between the average energy for each proton you've 'wasted' by having just a 1% ionisation efficiency is 13eV x 99, which is <1% increase in power input into the overall acceleration of the beam protons. Corollary; not worth worrying about. Just get a mixed-up bag of low energy ions by the cheapest and most abundant means you choose, but after that make sure it's only the protons you actually accelerate.

Quantum
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Re: Energy efficient proton production?

Post by Quantum » Sat Jun 20, 2009 7:57 pm

So I then presumably have to use a Penning trap to 'store' the H2 ions?

Or can I trap them with a cathode?

I imagine I can't trap them by 'sputtering', in the usual sense, due to the fact that the H2 ions would combine with electrons from the cathode, and I'd end up with hydrogen gas again, which would just contaminate the whole system.

And I can't re-circulate them back to behind the ionizer, where they could possibly be re-combined with electrons, and re-cycled as fuel, because his would create a path for un-ionized hudrogen molecules to leak into the system after the ionizer, again contaminating it.

Or have I missed something somewhere?

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Re: Energy efficient proton production?

Post by Quantum » Sat Jun 20, 2009 8:19 pm

Unless, of course, I have three ionizers......arranged so that any hydrogen molecules that try to leak past get ionized and 'pulled back' to where they can be filtered. If I can design this system, it should (eventually) convert all hydrogen fed into it into protons.

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Chris Bradley
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Re: Energy efficient proton production?

Post by Chris Bradley » Sat Jun 20, 2009 8:22 pm

Ash Small wrote:
> So I then presumably have to use a Penning trap to 'store' the H2 ions?
>
> Or can I trap them with a cathode?
>
> I imagine I can't trap them by 'sputtering', in the usual sense, due to the fact that the H2 ions would combine with electrons from the cathode, and I'd end up with hydrogen gas again, which would just contaminate the whole system.
>
> And I can't re-circulate them back to behind the ionizer, where they could possibly be re-combined with electrons, and re-cycled as fuel, because his would create a path for un-ionized hudrogen molecules to leak into the system after the ionizer, again contaminating it.
>
> Or have I missed something somewhere?

hmmm.... you're kinda missing a question, I think?!

*If* your objective is to ensure that you only generate protons AND you want to keep those without allowing them any chance to recombine, then, I guess, yeah you need an ion trap like a Penning trap. I'd have to ask "why" though - overall you'll not get the efficiency of just generating a stack of ions and picking out the ones you want.

If you're expecting to be accelerating protons to 100's keV then your losses in generating the actual slow, pre-accelerated ions in a discharge, whatever that is, are going to be trivial. Like I say, if you *want* to have THE most efficient way of generating protons PRIOR to their acceleration, well, I'd suggest it's not exactly a well-research area as there's not much point. If you want to do that *for the challenge* then I'll look forward to the results of your experiments, but I don't see why anyone would really [want to] know.

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Chris Bradley
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Re: Energy efficient proton production?

Post by Chris Bradley » Sat Jun 20, 2009 8:44 pm

What, exactly, is the end result you're aiming at? If you're serious about trying some beam-target p-11B then you do realise that worrying about efficiencies at the proton-generating end is like trying to work out what colour to paint your home-build rocket to add the least weight so that you'll get as high as possible? (If you're anywhere near of dreaming that you'll get energy pay-back, then your target for your home-build rocket is no less than a Moon landing.)

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Re: Energy efficient proton production?

Post by John Futter » Sat Jun 20, 2009 8:50 pm

Ash
We use cold cathode penning type ion sources at work these run at around 2kV at 2mA giving up to 1.8mA in a faraday cup inserted downstream. By adjusting the axial magnetic field and voltage on the ion source a sweet spot is found for the gas species in use.
Hydrogen is neat as it only gives one +state of charge. Neutrals tend to stay within the ion source and only the +charge state ones are drawn out through the extraction electrode which is at a negative potential as compared to the cathodes of the ion source.

plenty of info on the web on ion sources
check out hot and cold cathode penninig ion source
also duoplasmatron ion source

and of course Andrews anode layer source and the Sesselman- Hendron ion source here on this site

Quantum
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Re: Energy efficient proton production?

Post by Quantum » Sat Jun 20, 2009 9:42 pm

I may be missing the point here, but, Chris, what happens to the ions that are filtered out...Where do they go, and what do they do if you don't 'trap' them. They would need o be removed somehow, surely? ...I appreciate that 're-cycling' them could prove to be less efficient than just removing them., but 'storing' an 'ever increasing number in a Penning trap will use up more and more energy, surely?

John, Will a 'cold cathode' trap hydrogen?...I can see how it would trap heavier ions, but won't the H2 ions just become gaseous hydrogen molecules again?....and eventually build up in concentration until they contaminate the whole system?

Or do I just run the vacuum pumps in overdrive?

While I don't wish to get ahead of myself yet (I'm dealing with the proton production here, not the problems associated with a Boron 11 target, and any alpha particles produced, but I do have experience of installing the Tungsten target at RAL, along with it's beryllium neutron moderators, I could post some photos I took if any-one is interested) I'm dealing with the problems associated with operating a 'sustained' proton beam here, as opposed to a system that can only be run for short periods of time, before it has to be shut down for maintenance..

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Carl Willis
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Re: Energy efficient proton production?

Post by Carl Willis » Sat Jun 20, 2009 10:03 pm

Hi Ash,

This is a well-written line of inquiry and is in the right place.

The key to good efficiency in proton or deuteron ion sources is to prevent recombination of ions that have been made. Huge losses in the proton fraction occur when metal surfaces are exposed to the plasma and when high-vapor-pressure materials outgas into the plasma. The company I work for lines parts of the copper discharge waveguide with HIPped boron nitride sheets, and that turns out to be quite important. Historically, the RF types of ion sources that are electrodeless and carried out in glass tubes have been pretty efficient, but they don't produce much current.

Another thing to think about is how much plasma volume is being excited. Except for the region of plasma that feels the extraction fields, you might as well be heating meatloaf. So one possible line of experimentation concerns how to keep the discharge appropriately restricted in size.

I have to disclose some prejudices. Every time p-B11 gets mentioned on this site I get ready for a lot of silliness. Absolutely, identically, NO amateur effort has taken place in which this reaction has been done. Lots of people hemm and haw about it, but it has been exclusively an armchair topic. Anyway, enough about my prejudices.

-Carl
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Quantum
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Re: Energy efficient proton production?

Post by Quantum » Sun Jun 21, 2009 12:30 am

Thankyou for your comments, Carl.

Surely if the hydrogen is introduced into the region that feels the extraction fields, and ionized upon entry, all will be extracted?

I've not yet done the maths, as there is no point until you can estimate proposed losses, but , if my memory serves me correctly, 6x(10^23) protons at 161keV=161kW (or something of that order). And 6x(10^23) Alpha particles at 3.76MeV+12x(10^23) Alpha paricles at 2.46MeV= 8.68MegaWatts, more than 50 times the original power, so break-even is 2% efficiency.

So, Working on the fact that fission reactors are only 40% efficient at converting energy to electricity, Any proposed fusion reaction has to be at least 5% efficient to reach break-even (as far as the fusion reaction itself is concerned)

Due to the limitations of RF ion sources, certainly the ones I've come across so far, I'm forming the opinion that only some form of high efficiency plasma torch will be up to the job. I built a prototype in 2002. (well, a fore-runner of a prototype, to be exact).

Even the plasma torch they've constructed to test proposed materials for ITER is only 5% efficient when burning 100% hydrogen, but I'm pretty certain I can improve on this. Also, plasma can be introduced directly into the region that feels the extraction field.

It looks like I could now be looking for some suitable ceramic electrodes.

Maybe a combined plasma torch/RF ion gun would be more efficient that either on it's own?.

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