Is the Hohlraum the Key to Fusion Energy?

[Robert Ott]

The Threshold to Fusion Energy
I'd like to start some conversation on Hohlraum design, particularly on the opportunity to construct a fullerene capsule as the ignition chamber for indirect-drive fusion.

Given more than two decades of evolution in hohlraum configuration* there are many variations on the original theme in existence, yet it is becoming increasing apparent that a radically different capsule, utilizing all we have learned in nanomaterials science, is needed.**

My ultimate goal is to run simulations of hohlraum ignition utilizing a fullerene fuel capsule of Bose-Einstein Condensate. To that end, input from the PF community would be infinitely helpful. After centuries in experimentation, the problem of achieving symmetric flux in the hohlraum is still under research. Li Xin et als as recently as 2016*** proposed a complex six-cylinder port hohlraum “...to provide high symmetry flux on capsule.” Theirs is a 1.2 mm radius ICF, and justification for its configuration is that “The six-cylinder-port hohlraum could be superior to the traditional cylindrical hohlraum and the octahedral hohlraum in both higher symmetry and lower backscattering…” adding that “...the hohlraum will add to the diversity of ICF approaches.” I would suggest that while the fusion industry may need a more diverse selection, our ideal would be one working configuration that takes advantage of the latest technologies i.e. nanomaterials.

Nanoscience is universally revolutionizing scientific approaches in almost every field of research, and in almost every field of endeavor we have yet to catch up with the grand implications of the science.

The hohlraum plays a crucial role in indirect-drive fusion energy. It has existed in various forms since the 90s, and is intrinsically a prime candidate for miniaturization. Nonetheless, the hohlraum of the 90s, with targets measured in millimeters, has evolved as of 2016 to a fabrication with a dimension of 1.2 mm in radius, a highly-complex construction with six cylinder ports.

The Problem
The axisymmetric outcomes of hohlraum capsule implosion seem to be inherent in their traditional forms.*** As a new alternative, a fullerene capsule with appropriate fuel filler may be expected to create a more consistently symmetric implosion, due to its highly-controlled structure, purity of carbon allotropes, and the limited space over which asymmetries might develop. Fullerene is composed exclusively of C-C bonds of varied orientation, so it would be reasonable to conclude that well-controlled and symmetric laser targeting would result in well-controlled and symmetric implosion. Simply stated, the proposed would eliminate any axisymmetric influence, providing a thoroughly symmetric capsule.

The axi symmetry of the traditional hohlraum can be seen in most available images (click here). The hollow carbon shells of fullerene proposed can instead be fabricated in perfect spherical form, and in various dimensions, from a few to many hundreds of carbon atoms. The shell capsule may be experimentally buttressed by multiple layers as experimentation progresses and the ideal capsule is developed, just as nanotubes can now be fabricated in double-wall (DWCNT) or multi-wall (MWCNT). Capsule size may be varied to accommodate the ideal mass of fuel. Experimental use of C60 or C70 can be tested to gauge response to targeting.

Proposed Solution
A nanofactor fuel capsule, in a “buckyball” configuration, could be manufactured and quality-tested right now using off-the-shelf technology. By extension, the investigation I propose is to infuse an appropriate Bose-Einstein Condensate seed material into the capsule. This proposed hohlraum target would start with commonly applied physics at such facilities as NIF, but would require rethinking at every level due to temperature requirements and scale.

Any direction or constructive comment from the community of Physics Forum would be greatly appreciated.

Robert Ott PMP
Professor of Physics

* “Three-Dimensional HYDRA Simulations of National Ignition Facility Targets” Marinak, M. M., G. D. Kerbel, N. A. Gentile, O. Jones, D. Munro, S. Pollaine, T. R. Dittrich, and S. W. Haan. "Three-dimensional HYDRA Simulations of National Ignition Facility Targets." Physics of Plasmas 8.5 (2001): 2275-280. Print.
** This Post is extracted from http://tinyurl.com/ThresholdToFusion-ottr
*** “A new ignition hohlraum design for indirect-drive inertial confinement fusion”
Li Xin, Wu Changshu, Dai Zhensheng, Zheng Wudi, Gu Jianfa, Gu Peijun, Zou Shiyang, Liu Jie,Zhu Shaoping (Submitted on 2 Jun 2016)

 

[etudiant]

A buckyball fuel capsule is certainly appealing from a symmetry perspective, but is only about a nanometer wide, so pretty small.
Presumably the thought is a larger fullerene capsule, but I'm unaware of any method for making even a 10 nanometer buckyball.
Getting all the laser beams properly balanced, focused and centered must be a major challenge even for a 1mm target, no idea how to do that for a micrometer sized object.

 

[Robert Ott]

A buckyball fuel capsule is certainly appealing from a symmetry perspective, but is only about a nanometer wide, so pretty small.
Presumably the thought is a larger fullerene capsule, but I'm unaware of any method for making even a 10 nanometer buckyball.
Getting all the laser beams properly balanced, focused and centered must be a major challenge even for a 1mm target, no idea how to do that for a micrometer sized object.

Thanks for the comment. NIF is rapidly refining the science of targeting, though certainly there are current limits. Buckyball size is also a question as you note, though the sizes of buckyballs range from those containing 20 carbon atoms to those containing more than 100 carbon atoms. Of course the nanotube configuration would have greater flexibility in terms of size and may in fact be preferable in the end.
I will try to fill in the blanks in response to the questions you raised, and see what might be in the literature. Again, thanks for your interest.
Best: Robert

 

[Gigaz]

I know a few people who make endohedral fullerenes for research on single molecular mangets, and from what I heard the yield of filled fullerenes is really small. I think they burn carbon soot mixed with whatever should end up in the fullerene, and they get a tiny product yield. You can probably say that simulations are simulations and to fill the actual bucky balls is someone elses problem. I'm not a chemist. I would still consult a chemist working in the field and ask how much hydrogen you can realistically put into a fullerene in a real world situation, and how much effort it would be.

 

[Robert Ott]

I know a few people who make endohedral fullerenes for research on single molecular mangets, and from what I heard the yield of filled fullerenes is really small. I think they burn carbon soot mixed with whatever should end up in the fullerene, and they get a tiny product yield. You can probably say that simulations are simulations and to fill the actual bucky balls is someone elses problem. I'm not a chemist. I would still consult a chemist working in the field and ask how much hydrogen you can realistically put into a fullerene in a real world situation, and how much effort it would be.

Thanks Gigaz: I believe I would indeed be well-advised to get the input from a chemistry professional. The yield I'm looking for is just enough for ignition in the fusion reactor, and the chamber is intended to provide the enclosure for implosion. The exact configuration of the nanocontainer has yet to be determined, and I wonder still if a nano-tube might be a more flexible container in terms of range of size and number of potential overlays.
As you note, there would certainly be a great deal of effort involved if one wishes to pursue a container of Bose-Einstein Condensate (temperature not the least). What intrigues me about this fill is that you have atoms that start at the same quantum state, which would develop a parallel instability as heat and implosive pressure are applied.

 

[berkeman]

Thanks Gigaz: I believe I would indeed be well-advised to get the input from a chemistry professional. The yield I'm looking for is just enough for ignition in the fusion reactor, and the chamber is intended to provide the enclosure for implosion. The exact configuration of the nanocontainer has yet to be determined, and I wonder still if a nano-tube might be a more flexible container in terms of range of size and number of potential overlays.
As you note, there would certainly be a great deal of effort involved if one wishes to pursue a container of Bose-Einstein Condensate (temperature not the least). What intrigues me about this fill is that you have atoms that start at the same quantum state, which would develop a parallel instability as heat and implosive pressure are applied.

Can you say more about this part?

 

[Robert Ott]

Hi Berkeman: In response to Gigaz' suggestion, I started a private conversation with Astronuc, who might have the answers needed.

Hello Astronuc:
I'm developing a theory proposing the use of nanostructures as a hohlraum for fusion ignition, specifically use of fullerene as a microcontainer for Bose-Einstein Condensate in inertial confinement ignition. The theory is based on the proposition that the Condensate, with multiple atoms at the same quantum state, will react to laser impact with a most parallel or similar intramolecular instabilities. Attaining symmetric flux has always been a primary hurdle in fusion ignition.

My question is: how much BEC might be contained in a fullerene capsule, and how much net gain for fusion ignition might be achieved within a capsule of multiple layers. Of course since we are seeking internal compression in the capsule, a "bucky onion" form would be ideal, and I understand that onions of up to 70 layers have been known. Of course the Nanotube might be an alternate, but perfect symmetry could be more difficult in the linear configuration.

Your perspective would be much appreciated.
Best: R
​

 

[TeethWhitener]

I did my graduate research on endohedral fullerenes. It's very difficult to discern what exactly you have in mind.

My ultimate goal is to run simulations of hohlraum ignition utilizing a fullerene fuel capsule of Bose-Einstein Condensate.

Do you want a single fullerene to encapsulate a BEC? The fullerene would have to be huge. As of right now, most giant fullerenes top out at less than 10nm in diameter. C₆₀'s van der Waals radius is only about a nanometer. In addition, the larger the fullerene gets, the more easily deformable its structure is. Kind of like how a piece of paper gets more difficult to deform the smaller it gets. There's also the nontrivial matter of synthesizing fullerenes larger than a few hundred atoms with any sort of yield. As far as I know, there has not been much progress in this area in the past 20 years. Also, you have to consider: how are you going to get the BEC material into the fullerene? Are you going to open a hole chemically in the fullerene? shoot the material into it with an energetic beam? The interior of the fullerene is a lot smaller than people anticipate. For reference, a few groups have succeeded in getting 2 H₂ molecules into C₇₀ using something like 3000 atmospheres of pressure (this was a typical pressure when we were attempting to put a single Xe atom into C₆₀).

This is all assuming your goal is to have a single fullerene containing BEC be your target. If you have multiple fullerenes, you lose the symmetry that a single fullerene would nominally have.

Finally, I seem to recall that the reason hohlraums are gold is because you need a high Z material to effectively re-radiate x-rays and compress the fuel efficiently.

 

[Robert Ott]

Your researches would be my ideal advance research; thanks for the insights in your comments.

For reference, a few groups have succeeded in getting 2 H2 molecules into C70 using something like 3000 atmospheres of pressure (this was a typical pressure when we were attempting to put a single Xe atom into C60).

I'd love to look over any write-ups on this work if you have citations. Right now I'm seeking to determine how much fuel material might be encapsulated under cryogenic conditions. If it requires extreme pressure, so much the better, since high pressure and temperature are needed for ignition.

In other conversations I'm trying to get an estimate of the energy gain that might be expected from the amount of BEC that might be enclosed within a reasonably large fullerene capsule.

re. the latter part of your comments

This is all assuming your goal is to have a single fullerene containing BEC be your target. If you have multiple fullerenes, you lose the symmetry that a single fullerene would nominally have.

Finally, I seem to recall that the reason hohlraums are gold is because you need a high Z material to effectively re-radiate x-rays and compress the fuel efficiently.

I envision a single fullerene of multiple layers ("onion"). I would not expect to accomplish re-radiation of x-rays using hi-Z material, to accomplish compression. The theory is that by starting with bosons of low and equivalent quantum (or "ground") state, the ignition has its greatest chance of maintaining symmetry. Perhaps you can illuminate on the behavior of fullerene as the capsule itself "ignites". How would the carbon-based fullerene itself behave under sudden assault by laser array, and what compression might it be expected to apply on its enclosed fuel?

Finally, I noted recently Brookhaven's Center for Functional Nanomaterials work on "Trapping Argon Atoms in Two-Dimensional Arrays of Tiny Cages" https://www.bnl.gov/cfn/research/highlights/news.php?a=212323 which might be of interest to you
Again, thanks so much for your reply. Your knowledge of the fullerene material is invaluable.
Best: R