Mystery Of Plasma Turbulence Solved By Multi-Scale Simulations

Posted: Mar 8 2016, 1:33am CST | by , Updated: Mar 8 2016, 10:10pm CST, in News | Latest Science News

Mystery of Plasma Turbulence Solved by Multi-Scale Simulations
Credit: Nathan Howard, MIT

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The findings will help create better nuclear reactor designs and could pave the way for alternate energy sources.

Fusion energy requires intense heating for its production, but sometimes fusion experiments fail before energy is generated. The reason is the turbulence that circulates inside a fusion reactor and cause superhot, electrically charged gas plasma to lose much of its heat.

Scientists have been working to pinpoint the causes and the ways to control plasma turbulence for a long time. But since fusion reactors are extremely complex and expensive, they have been unable to conduct any practical inside them; rather, they have turned to supercomputers to replicate the conditions and to help create better reactor designs.

However, computer models have often been unable to predict exactly how turbulence will behave inside the reactor. In fact, there have been huge differences between predictions and experimental results in fusion experiments when the contribution of turbulence in losing heat in plasma is studied.

Now, researchers from MIT’s Plasma Science and Fusion Reactor in collaboration with colleagues at University of California have found a solution to this inconsistency in results. They performed high resolution, multi-scale simulations where they resolved multiple turbulence instabilities. Then, they ran a series of these simulations on NERSC’s facility and found that interactions between turbulence at tiniest scale (that of electrons) and at larger scale (that of ions) should be taken into account for the mismatch between theoretical predictions and experimental observations of the heat loss.

“For a very long time, the predictions from leading theories have been unable to explain how much heat loss is coming from electrons in fusion plasma,” said lead researcher Nathan Howard. “You apply your best theories, but they have underpredicted the amount of heat loss coming from the electrons.”

“In this particular work, we have shown that using the coupled model – where you can capture both the large-scale and small-scale turbulence simultaneously – you can actually reproduce the experimental electron heat losses, in part because there appear to be strong interactions between the large-scale and small-scale turbulence that weren’t well understood previously.”

For the simulations, researchers used data from experiments conducted at MIT's small fusion plant tokamak in 2012 and it took 36 hours to run one entire simulation on NERSC’s processors. If the full set of six simulations had been carried out on ordinary MacBook Pro, it would take about 3,000 years to complete the task.

Latest research suggests that two scales of turbulence can indeed coexist. Most of the previous studies mostly focused on large scale turbulence but researchers found that when do these two scales interact with each other strongly, it’s impossible to predict the total heat loss accurately unless simulations have not been used simultaneously. The tiny swirls created by electrons were found more visible during the experiments.

“This is the first time a lot of these very big hypothesis have been confirmed and shown to be operative in relevant plasma conditions,” said co-author Chris Holland. “The challenge we are working on right now is figuring out how to get the complicated simulations, which require significant memory size and bandwidth, to work efficiently and scale well on these platforms so we can continue to study even more complex scenarios.”

“It we can find a way to use the new generation of platforms and make these simulations more routine, then it becomes a really exciting tool.”

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