X-Ray Reflection and Absorption

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

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topher
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Joined: Tue Feb 07, 2017 6:51 am

X-Ray Reflection and Absorption

Postby topher » Mon Feb 13, 2017 2:22 am

Hi everyone - this is my first post. Been excited about the polywell for years, finally decided I should start trying to contribute to the discussion instead of just waiting for it to happen. I was thinking about x-ray losses and looking for information about reflecting the x-rays back into the plasma, but haven't been able to find much (in general or regarding the polywell specifically). My idea is to coat the interior of the vacuum chamber with iridium, which is effective at reflecting x-rays as well as resistant to high temperatures and corrosion/oxidation. I know that there are conditions required for re-absorption of the x-rays (like the plasma being sufficiently dense) but overall this approach seems simple and straightforward. So, what I'm asking is if this won't work, why not? In general terms, what factors am I missing? And if anyone knows of some good sources/material covering absorption of x-rays by plasma that I can read, I'd love the direction. Thanks for reading!

paperburn1
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Re: X-Ray Reflection and Absorption

Postby paperburn1 » Wed Feb 15, 2017 2:49 am

Not my field of expertise but X-rays can only be reflected off smooth metallic surfaces at very shallow angles---grazing incidence. Such reflections are particularly efficient for metals with high density, such as gold, platinum or iridium. but mots x-rays would likely be at higher angles of incidence and therefore just be absorbed IMHO.
I am not a nuclear physicist, but play one on the internet.

topher
Posts: 2
Joined: Tue Feb 07, 2017 6:51 am

Re: X-Ray Reflection and Absorption

Postby topher » Wed Feb 15, 2017 6:35 am

Thank you, I knew I must have been missing something obvious. You're right, I found that the critical angle is less than generally less than 1 degree, even going from a vacuum to something dense like iridium. Yikes. I had assumed that a smooth, nearly spherical chamber would reflect the x-rays back toward the center, but I see that won't work, as the x-rays would be emanating from the center and perpendicular to the surface (and even those that were reflected would still not make it back to the core.)

Another possibility is to absorb the x-rays and convert it their energy into electricity... from what I can find this is less than 2% efficient, though:

http://www.tandfonline.com/doi/pdf/10.1 ... 11.9711683

But if that could be improved, then at least some of the energy lost to x-rays could be harnessed and cycled back to improve efficiency.

quixote
Posts: 128
Joined: Fri Feb 05, 2010 8:44 pm

Re: X-Ray Reflection and Absorption

Postby quixote » Fri Feb 17, 2017 8:48 pm

You might be interested in this old thread.
http://www.talk-polywell.org/bb/viewtopic.php?f=4&t=3271

ohiovr
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Re: X-Ray Reflection and Absorption

Postby ohiovr » Mon Mar 06, 2017 3:30 pm

It is a toughie, I thought of the same thing just recently. Even if you could bend the Xrays back in on itself, you would probably lose too much energy from losses at the reflectors. To make matters worse, the X-rays would probably pass right through the ignited gas or plasma.

D Tibbets
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Re: X-Ray Reflection and Absorption

Postby D Tibbets » Sun Apr 23, 2017 4:31 am

X-ray energy recovery or reflection is difficult (other than as heat). Two considerations. The first is the heavy metal U238 tamper or shell of some boosted atomic bombs and early hydrogen bombs. This helps to trap X-ray energy so that the plasma stays hotter longer so that significant fusion of D-T (lithium derived) can occur.I'm not certain if the X-rays are reflected or if it is more the heating of the uranium shell with subsequent x-ray emission- sort of recycling the X-rays, while also having enough thermal absorbing mass and toughness to contain the fission plasma longer and also providing secondary plutonium from U238 + Neutron reactions .

The second consideration is harvesting the X-ray energy outside the reaction space through a multiple layered photovoltaic process. Eric Learner of Lawrenceville Plasma Physics received a patent for a scheme to do this. I don't know if one has ever been built or what the efficiency compared to a thermal steam plant would be.

[EDIT}

https://en.wikipedia.org/wiki/Direct_energy_conversion

"X-ray photoelectric converter

A significant amount of the energy released by fusion reactions is composed of electromagnetic radiations, essentially X-rays due to Bremsstrahlung. Those X-rays can not be converted into electric power with the various electrostatic and magnetic direct energy converters listed above, and their energy is lost.

Whereas more classical thermal conversion has been considered with the use of a radiation/boiler/energy exchanger where the X-ray energy is absorbed by a working fluid at temperatures of several thousand degrees,[24] more recent research done by companies developing nuclear aneutronic fusion reactors, like Lawrenceville Plasma Physics (LPP) with the Dense Plasma Focus, and Tri Alpha Energy, Inc. with the Colliding Beam Fusion Reactor (CBFR), plan to harness the photoelectric and Auger effects to recover energy carried by X-rays and other high-energy photons. Those photoelectric converters are composed of X-ray absorber and electron collector sheets nested concentrically in an onion-like array. Indeed, since X-rays can go through far greater thickness of material than electrons can, many layers are needed to absorb most of the X-rays. LPP announces an overall efficiency of 81% for the photoelectric conversion scheme.[25][26]"

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


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