Physicists see solution to critical barrier to fusion
Posted: 24 April 2012 02:12 AM   [ Ignore ]
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FTA…
Physicists have discovered a possible solution to a mystery that has long baffled researchers working to harness fusion. If confirmed by experiment, the finding could help scientists eliminate a major impediment to the development of fusion as a clean and abundant source of energy for producing electric power.

http://phys.org/news/2012-04-physicists-solution-critical-barrier-fusion.html

Sounds a lot like they’re describing the instabilities that DPF intends to take advantage of.  Might be an interesting read…

Pat

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Posted: 24 April 2012 07:05 AM   [ Ignore ]   [ # 1 ]
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zeroed in on tiny, bubble-like islands that appear in hot, charged gases—or plasmas

It is very unlikely they are talking about plasmoids. It is still yet to be defined under what conditions plasmoids do form, but the conditions are much different and the results as well - a slow expanding region, unlike the plasmoid collapse like effect.

IMHO it is a good idea to leave the plasma for plasma physicists to analyse, or we will thread in to the twilight area of pseudo-science. On the other hand I like to speculate myself.

On unrelated notice: I wonder whether plasmoids can be modelled like a bursting bubble. Yes, they have plasma contents, but maybe the electro-magnetic field collapse can ignore its pressure when modelling, because its more powerful.

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Posted: 24 April 2012 11:48 AM   [ Ignore ]   [ # 2 ]
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Yes, is the “corkscrew-shaped phenomenon called snakes” the same type of thing as the “kinking telephone cord”? 

FYI, discussion of this also on Fusion Energy League site.

It’s neat how on this site the news is filtered through a DPF lens.  Feel free to make some of those remarks over on FEL.

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Posted: 24 April 2012 01:25 PM   [ Ignore ]   [ # 3 ]
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Talk about filtering—-Winston Bostick, a pioneer of DPF and the person who introduced me to the device,  pointed out the existence of vortex filaments in tokamaks 40 years ago. Those are the “cork screw” objects. His observations were dismissed for various reasons but mainly because the scientists involed were filtering their inputs to fit pre-concieved notions. Be intersting to see the technical papers this article is based on.

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Posted: 24 April 2012 01:56 PM   [ Ignore ]   [ # 4 ]
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Yep.  Pesky filters.

Separately, papers on plasma islands were beginning to surface in scientific circles. French physicist Paul-Henri Rebut described radiation-driven islands in a mid-1980s conference paper, but not in a periodical. German physicist Wolfgang Suttrop speculated a decade later that the islands were associated with the density limit. “The paper he wrote was actually the trigger for our idea, but he didn’t relate the islands directly to the Greenwald limit,” said Gates, who had worked with Suttrop on a tokamak experiment at the Max Planck Institute for Plasma Physics in Garching, Germany, in 1996 before joining PPPL the following year.

Why hadn’t researchers pieced together a similar theory of the density-limit puzzle before? The answer, says Gates, lies in how ideas percolate through the scientific community. “The radiation-driven islands idea never got a lot of press,” he says. “People thought of them as curiosities. The way we disseminate information is through publications, and this idea had a weak initial push.”

But a lot of this story is also about how individual scientists meet and talk face to face or email and call each other personally.  Gets back to relationships.

In early 2011, the topic of plasma islands had mostly receded from Gates’ mind. But a talk by Delgado-Aparicio about the possibility of such islands erupting in the plasmas contained within the Alcator C-Mod tokamak reignited his interest. Delgado-Aparicio spoke of corkscrew-shaped phenomena called snakes that had first been been observed by PPPL scientists in the 1980s and initially reported by German physicist Arthur Weller.

Intrigued by the talk, Gates urged Delgado-Aparicio to read the papers on islands by Rebut and Suttrop. An email from Delgado-Aparicio landed in Gates’ in-box some eight months later. In it was a paper that described the behavior of snakes in a way that fit nicely with the C-Mod data. “I said, ‘Wow! He’s made a lot of progress,’” Gates remembers. “I said, ‘You should come down and talk about this.’”

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Posted: 24 April 2012 03:32 PM   [ Ignore ]   [ # 5 ]
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The article seems heavy on PR trying to convince the reader of the important contributions the US programs at PPPL and MIT are making to fusion plasma research (presumably to bolster their requests for dwindling DOE funding) , but does little to explain how with this new ‘discovery’ we will be able to design better tokamaks.

In a quick skim read through the PRL paper http://prl.aps.org/abstract/PRL/v108/i16/e165004 they take a simple cylindrical model with a chain of small magnetic islands around it, and work out the power balance from the ohmic heating within the island, the external heating flowing around it and the radiation emitted via bremsstralung & line emission from impurities.    They find the scaling is roughly in line with the Greenwald limit on plasma density, which is proportional to the total plasma current.

They go on to mention the relationship between neoclassical tearing modes briefly at the end,

    Additionally, radiation driven islands should be exacerbated in plasmas with high noninductive current fractions, since only the Ohmic current participates in heating
the interior of the island. This may explain the common practice of using ‘‘preventative electron cyclotron resonant heating’’ to avoid the onset of neoclassical tearing modes. 
In fact, this phenomenon may partially explain the difficulty in finding a reliable predictor for the onset of neoclassical tearing modes because the radiation driven terms
are not considered in neoclassical island threshold analysis

But they completely fail to mention turbulence, which as far as I’m aware, is the primary way heat leaks into and out of island structures, as small eddies in the plasma flow on the scale of a few times the ion larmor radius (few mm) cause small perturbations in the electric & magnetic fields and let ions & electrons on the outside to collide with those ‘trapped’ on the inside & vice-versa.

The paper does suggest an explanation for several long standing problems, such as how the density limit changes when switching from a deuterium plasma to a helium one, and should be testable with some of the advanced diagnostics being fitted to several tokamaks.

 

 

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