Synthetic diamond: excellent hard x-ray reflector

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

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DeltaV
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Synthetic diamond: excellent hard x-ray reflector

Postby DeltaV » Tue Aug 23, 2011 5:58 pm

For normal, not just grazing incidence, reflection.

Bragg reflectivity of X-rays: At the limit of the possible
Researchers utilizing high-brightness x-rays at the U.S. Department of Energy Office of Science’s Advanced Photon Source at Argonne National Laboratory have demonstrated that synthetic, nearly defect-free diamond crystals can reflect more than 99% of hard x-ray photons backward in Bragg diffraction, with a remarkably small variation in the magnitude of reflectivity across the sample.

Theoretical analysis presented in the Nature Photonics paper establishes that diamond has the highest (higher than any other crystal) Bragg reflectivity due to the uniquely small ratio of the extinction length in Bragg diffraction to the absorption length. Even in backscattering, where the reflectivity is lowest, it was thought to be more than 99%, and this has now been demonstrated.

Systematic observations of the more than 99% reflectivity show that high-reflectivity mirror limitations in the regime of hard x-rays have been eliminated.

Is something similar possible for shielding against gamma rays (which may or may not be emitted by a p-11B fusion process)?

Image

quixote
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Postby quixote » Tue Aug 23, 2011 9:24 pm

Maybe this is a stupid question, but would it be possible to recover losses from Bremsstrahlung radiation using sheets of these synthetic (presumably cvd) diamonds? I'm thinking of something like the device being enclosed in a box (maybe the vacuum chamber?) the inside surface of which is covered in these diamonds, reflecting the X-rays back inside.

hanelyp
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Postby hanelyp » Wed Aug 24, 2011 1:35 am

Reflecting the x-rays back is of little use for lowering losses unless the plasma absorbs the x-ray and the plasma is thermal. Neither applies to the polywell. Only the latter would apply for a tokomak. In both devices the plasma is thin enough for light of visible or shorter wavelength to pass through with little absorption.

quixote
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Postby quixote » Wed Aug 24, 2011 1:50 am

darn.

Thanks for clearing that up for me, hanelyp.

DeltaV
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Postby DeltaV » Thu Aug 25, 2011 3:57 am

There must be some applicability of CVD diamond x-ray reflectors to LPP's work.
http://focusfusion.org/index.php/site/a ... easibility

quixote
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Postby quixote » Thu Aug 25, 2011 8:40 am

I was thinking the same thing, but not only for the imaging spin off. It seems like their reactor design might benefit from being able to direct the x-rays being emitted to their golden onion.

ladajo
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Postby ladajo » Thu Aug 25, 2011 1:33 pm

Either way, but thi sdoes open the door significantly (although other issues need to be worked) for X-ray FELs. And THAT would be COOL and USEFUL.

93143
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Postby 93143 » Fri Aug 26, 2011 8:23 am

You'd need a lot less shielding (neglecting the gamma rays and neutrons). Just put the diamond layers (you'd need more than one) on the outside of the vacuum vessel, and the x-rays will attenuate passing through the metal. (Assuming this works with the bremsstrahlung spectrum you'd get from a Polywell; the paper's idea of "hard" X-rays seems to be 23.7 keV...) Neutron exposure might screw with the mirrors' effectiveness, so they'd need to be outside a pretty solid neutron shield too...

I very much doubt this will work with gammas, though, so if those gammas really do show up they will still dominate the shielding requirements.

However, a hard X-ray mirror is still a very impressive development, and as ladajo notes is still useful for building death rays... sorry, X-ray lasers...

ladajo
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Postby ladajo » Fri Aug 26, 2011 1:37 pm

Well, I admittently wasn't thinking the word DeathRay, when I posted that, but maybe I did have visions of ships, aircraft and missiles vaporizing into clouds of pretty colors... :D

But seriously, this will more than likely add another significant dimension to potential FEL applications, which are many.


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