A letter to Newbie about the Polywell

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mattman
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A letter to Newbie about the Polywell

Postby mattman » Fri Jan 18, 2013 7:36 pm


Tyler Jordan
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Re: A letter to Newbie about the Polywell

Postby Tyler Jordan » Mon Aug 10, 2015 9:25 am

I know this is an old thread, but only one post in it ... so fair game?

I've posted some wild ideas on talk-polywell from time to time - just to let people know, I'm the 'admin' at protonboron.com ... so, don't think I'm designing reactors or anything critical over there! :lol: I'm just their village idiot ... who reads about things and tries my best to contribute something interesting.


So ...

after reading a lot of papers that I mostly didn't understand, I'm sceptical of the approach of reflecting x-rays back into the machine. Still we will most likely have an abundance of x-rays to deal with and finding some proposition to solve the problem might be useful.

If the vacuum chamber were made of beryllium which from what I've read is x-ray transparent to a large extent, then we could use a tough phosphor (term used loosely to apply to any type of phosphor that could conceivably handle hard x-rays) and then embedded in this phosphor some high efficiency photovoltaics. Perhaps layers of phosphor and photovoltaic. I've seen heavy duty photovoltaics being used experimentally before on large parabolic dishes that concentrate sunlight. In fact there is an experimental power station here in Australia that uses such a system. I'm not certain what frequency range these cells absorb, but they are very heat resistant.

I grant that normal photovoltaics aren't very efficient and certainly doing a conversion with some sort of phosphor is going to make it worse, however these photovoltaics are, I believe, much more efficient - over 40% @25C - which would imply a hell of a lot of cooling. .

Additionally, unlike solar, this would be always on! - unlike the sun and in a controlled environment, not outdoors.

Maybe this is just another crazy idea, I'll leave it to the geniuses here to be the judge.

cheers


EDIT: just read that Beryllium is a neutron multiplier with input neutrons over about 1.9 MeV - that doesn't seem like a good thing. So wondering what the energies would be from the neutron producing side-reaction in p-B11 fusion? Maybe this is an issue, maybe not, will keep researching ...

EDIT: Posted question on physics stack exchange regarding neutron energy levels in side-reactions in proton-boron fusion.

EDIT, just started looking at the cost of Beryllium - I did realize it would be expensive, but whoa! Thinking that making a large vacuum chamber out of this stuff would significantly add to the build cost of the reactor.

So, what other strong materials would be mostly transparent to hard x-rays?
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D Tibbets
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Re: A letter to Newbie about the Polywell

Postby D Tibbets » Tue Aug 11, 2015 1:39 pm

P-B11 neutrons are mostly a non issue. The flux of neutrons may be less than 1 /10,000,000 that of deuterium- deuterium fusion. If neutron damage limits a D-D fusion reactor to 1 day of operation before it had to be rebuilt, then with P-B11 and everything else being equal, the reactor would last ~ 10 million days or 30,000 years.

Photo voltaics for x-ray conversion may indeed be a solution. Eric Learner of LPP has a patent for such a device. I think the key is that there are multiple photovotaic layers that eventually absorb the highly penetrating X-rays. I don't know the durability or efficiency of the idea. Also, I don't think it has actually been built. The X-ray problem of cooling the plasma (Bremsstruhlung) can be addressed by decreasing Bremsstruhlung - there are two mechanisms that may apply to the Polywell, reflecting and reabsorbing the X-rays as in the title of this thread, or recovering the energy lost through the X-rays- conversion to electricity and recycling back into the reactor. The last, if it works at high efficiency would be a real game changer.

X- ray reflection in hydrogen bombs is practical but very destructive. Only the inertia of the heavy uranium shell/ tamper allows survival for long enough times. Reabsorption is another issue. With the plasma at densities of perhaps 10^28 high Z ions per cubic meter in a bomb, a large portion can be reabsorbed. With densities of 10^22 of low Z ions the absorption path may be many millions of times longer.

PS: If not directly converted to electricity, the X-rays end up as heat in the shell / coils of the reactor. This can be converted by a steam cycle at perhaps 30% efficiency. Or perhaps at even ~ 45 % efficiency if a supercritical CO2 steam cycle is used. For photovoltaics to be useful they would have to convert at efficiencies well above this, or perhaps supplement the thermal conversion. The high capacity cooling will be needed anyway so it is not like you are avoiding a cooling plant. The question is if you can avoid an additional steam/ turbine plant (expensive) and if the conversion efficiency by whatever means is enough to overcome the losses.

Dan Tibbets
To error is human... and I'm very human.


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