particle beam?

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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kunkmiester
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particle beam?

Postby kunkmiester » Mon Jul 13, 2009 2:23 am

The mention of particle beams in another thread sparked a different idea.

I'd imagine the normal method of creating a particle beam is to dump loads of power into an accelerator, which the polywell would make more practical.

Some of what I'm hearing indicates that the alphas come out on the cusps. This provides a limited surface area to harvest them--I'm thinking that with a polywell, you're getting 2+MeV alpha particles out without the need to accelerate them up to speed. Would it be reasonable to harvest these for use in a beam? Alphas from PB11 wouldn't be terribly practical in atmosphere if I recall some basic physics properly, but I'm sure the system could probably be used to run a different reaction, producing the desired particles.

Would something like that work?
Evil is evil, no matter how small

D Tibbets
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Postby D Tibbets » Thu Jul 16, 2009 1:46 am

The problem with particle beams is that the atmosphere quickly absorbes them, along with defocusing , and distorting them. Alpha and beta beams would loose 99.9 % or more of thier power in probably just a few10's of meters. Gama rays and neutrons would do better, but a few kilometers of atmosphere is alot for them to plow through. Gamma rays will lose ~ 99.9% of the dose in only ~2000 ft. of air. You can cheat and first heat the air in a column that you are going to shoot the beam through, so that most of the atmosphere in the way is removed, though this would also have range limits. Also, it is very difficult, if not impossible, to make a directed beam with neutrons.

http://adsabs.harvard.edu/abs/1980Natur.284..219B

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

kunkmiester
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Postby kunkmiester » Thu Jul 16, 2009 2:25 am

That's what I figured for the particle systems, I'm surprised it takes that little for gamma rays. They have a reputation, probably not entirely deserved.
Evil is evil, no matter how small

D Tibbets
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Joined: Thu Jun 26, 2008 6:52 am

Postby D Tibbets » Fri Jul 17, 2009 5:29 am

kunkmiester wrote:That's what I figured for the particle systems, I'm surprised it takes that little for gamma rays. They have a reputation, probably not entirely deserved.


Well, this number is for the gamma ray energies expected in nuclear bombs. Higher energy gamma rays would penitrate further.
Back in the 1980's when I was worried that I might need to dig a hole I accumulated soms information about fallout and since I never throw anything away I still have it.
To reduce exposure to gamma rays from an atomic bomb and it's fallout products by a factor of 1000 requires:
6.7 inches of steel
23 inches of concrete
36 inches of packed earth
50 inches of water
90 inches of wood
2000 feet of air

I ignored the neutron doses, either through ignorance or the conclusion that if you were close enough for the neutrons from the explosion to get you , you would already be dead from the blast (unless it was a neutron bomb, in which case the lethel neutron dose extends a modest distance further than the other lethel effects) . Neutrons from the fallout products would be trivial. And danger from low penitrating alphas and betas could be easily shielded against with a mask and clothing, so long as you could keep them out of your food and water supply. So gamma radiation was by far the major consideration fo shielding needs.

As a side issue, the decay rates of the fallout products would decrease the dose by a facter of ten for each 7 x multiple of the time elapsed. ie- after 7 seconds the dose rate would have fallen by 10 x. After 49 seconds the dose would have fallen by 100 X, etc. till after ~ 2 weeks the dose in a heavily contaminated area would have droped enough that you could leave your shelter for brief periods of time and not suffer radiation sickness or death (this ignores lower dose effects like increased rates of delayed leukemia and other cancers).
Presumably this bomb fallout dynamic matches the radiation from fission plant decay products. That is why the spent fuel rods are stored for several weeks to months before you can safely approach them for short times with modest protection. I understand that the radioactive transmuted elements in the structure of neutron producing fusion reactors decay much faster, so are much less problamatical for long term storage. But, approaching the fusion reactor within the first few hours after shutdown may be more dangerous than approaching an equivalent spent fuel rod. The same would apply to climbing into the fission reactor core after the spent fuel rods were removed.

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


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