MIT researchers discover an explanation to heat loss

[source: MIT news]

One of the biggest obstacles to making fusion power practical — and realizing its promise of virtually limitless and relatively clean energy — has been that computer models have been unable to predict how the hot, electrically charged gas inside a fusion reactor behaves under the intense heat and pressure required to make atoms stick together.

Now, researchers at MIT’s Plasma Science and Fusion Center, in collaboration with others at the University of California at San Diego, General Atomics, and the Princeton Plasma Physics Laboratory, say that they have found the key.

The new findings are detailed in a pair of papers published in the journals Nuclear Fusion and AIP Physics of Plasmas, by MIT research scientist Nathan Howard, doctoral student Juan Ruiz Ruiz, Cecil and Ida Green Associate Professor in Engineering Anne White, and 12 collaborators.

The expectation by physicists for more than a decade had been that turbulence associated with ions (atoms with an electric charge) was so much larger than turbulence caused by electrons — nearly two orders of magnitude smaller — that the latter would be completely smeared out by the much larger eddies. And even if the smaller eddies survived the larger-scale disruptions, the conventional thinking went, these electron-scale whirls would be so much smaller that their effects would be negligible.

The new findings show that this conventional wisdom was wrong on both counts. The two scales of turbulence do indeed coexist, the researchers found, and they interact with each other so strongly that it’s impossible to understand their effects without including both kinds in any simulations.

However, it requires prodigious amounts of computer time to run simulations that encompass such widely disparate scales, explains Howard, who is the lead author on the paper detailing these simulations. Accomplishing each simulation required 15 million hours of computation, carried out by 17,000 processors over a period of 37 days at the National Energy Research Scientific Computing Center — making this team the biggest user of that facility for the year. Using an ordinary MacBook Pro to run the full set of six simulations that the team carried out, Howard estimates, would have taken 3,000 years.

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