Dissociation of Boranes by High Energy Alphas

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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DeltaV
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Joined: Mon Oct 12, 2009 5:05 am

Dissociation of Boranes by High Energy Alphas

Postby DeltaV » Fri Jul 26, 2013 5:01 pm

Way outside my fields, but I have to ask:

What happens to boranes (compounds of boron and hydrogen, there are many) when subjected to a flux of high-energy alpha particles?

This was probably discussed here years ago...

hanelyp
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Joined: Fri Oct 26, 2007 8:50 pm

Re: Dissociation of Boranes by High Energy Alphas

Postby hanelyp » Fri Jul 26, 2013 8:31 pm

One nuclear reaction between boron11 and high energy alphas produces neutrons. An advantage for the polywell leaking fusion products so much faster than fuel.
The daylight is uncomfortably bright for eyes so long in the dark.

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

Re: Dissociation of Boranes by High Energy Alphas

Postby D Tibbets » Sat Jul 27, 2013 5:58 am

The relative inertness of the alphas in the Polywell is significant - their Coulomb collisions are few before loss. By extension any fusion reactions would also be even fewer.

As for various boranes and their breakdown into a plasma of free protons and Boron 11 ions, the environment in the Polywell may be challenging. But this is related in the fuel plasma parameters- thousands to several hundred thousand eV . The boron compounds may have problems with vacuum deposition on surfaces, etc. and handling of borane gasses may be challenging, but again this is unrelated to the fusion processes.

Using an operating fuel ion energy of ~ 200 KeV, and with the alphas at perhaps ~3 MeV the difference is ~ 15X and Coulomb collisionality would be ~ 100 times less (1/1.75th power). I don't know what the cross section is for B11- He4 fusion reactions is, but I suspect it is well below the primary fusion cross sections and especially the Coulomb cross sections.

Also a factor is the density of the alphas in the reactor and their dwell time. If ~ 10^20 alphas are produced per second, and dwell time (confinement time) is ~ 10 ms, this would produce a steady state number of ~ 10^18 and this is spread over perhaps 20 cubic meters so the density would be ~ 5*10^16 /M^3.

Compare this to the claimed fuel density of ~ 10^22 /M^3. This is a density difference of ~ 100,000 and would result is a fusion rate difference of 10 billion. This is further multiplied by the difference in the fusion cross sections of the reactants at relevant KE. I'm guessing that the B11-He4 reaction at a few million eV is several orders of magnitude less than the rate for p-B11 at 200 KeV so the numbers would be further separated.

I suspect the gamma production from an alternate P-B11 pathway and the neutrons from deuterium contamination of the hydrogen fuel could both be more significant concerns, but...

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


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