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EPISODE: The Engineering Physics Department Newsletter

 

Fall / Winter 2006-2007
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University of Wisconsin Energy Institute engages stakeholders in creative solutions

Designing ways to help ITER operate safely

Learning why fusion plasmas sometimes act unpredictably

GOOD HEAVENS: Space telescopes may have steadier view

EP alum honored on Engineers' Day

A "hot" idea for insulating tiny batteries

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Carl Sovinec

Carl Sovinec
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Learning why fusion plasmas sometimes act unpredictably

Decorative initial cap Using a numerical simulation tool called NIMROD, a group of plasma physics researchers is making strides toward understanding why the magnetic “thermos” that insulates plasma in toroidal fusion experiments sometimes springs a leak.

Fusion research seeks to generate energy by heating fuels like deuterium and tritium to a very high temperature. The intense heat causes atoms to ionize, form a plasma, and ultimately fuse to release energy. These electrically charged particles are guided by a magnetic field. If a reactor based on the concept of magnetic confinement can sustain the plasma at sufficiently high pressure, it will produce energy in abundance.

However, the plasma doesn’t always cooperate, says Associate Professor Carl Sovinec. “As we add more particles and more energy, the plasma can organize itself into collective motions that distort the magnetic field and poke holes in it,” he says.

Sovinec recently became head of NIMROD, a computer code development project sponsored by the U.S. Department of Energy (DOE) Office of Fusion Energy Sciences. In this collaborative effort, researchers from across the country are studying how to model this set of distorting motions, commonly known as magnetohydrodynamic, or MHD, instabilities.

Within the last two years, the group has focused on an instability called an edge-localized mode, or ELM, which releases some of the plasma’s stored internal energy in short bursts. ELMs manifest near the boundary between closed and open magnetic-field regions in tokamak-style experiments.

Carl Sovinec

Deformed pressure surfaces and magnetic field line trajectories in a numerical simulation of the nonlinear MHD evolution of shot 86144 in the DIII-D tokamak at General Atomics.
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Inside the boundary, where the field lines are closed, there are large net plasma flows—but the profile drops quickly near the open-field region. This sudden change in flow leads to a transport barrier, which, says Sovinec, is good news for fusion. “Ultimately, the inside is hotter and you’re closer to getting a reactor-grade plasma,” he says. But the associated pressure and charge-current gradients also tend to drive MHD instabilities, including ELMs.

In the past few years, researchers have significantly increased our understanding of the linear properties of ELMs, says Sovinec. However, NIMROD researchers are among the first to conduct global nonlinear calculations, which yield a more comprehensive description of ELM events—in this case, ELMs in a discharge of the DIII-D tokamak at General Atomics in San Diego.

Capitalizing on recent advances in NIMROD’s modeling capabilities, the team used a two-fluid model that enables the plasma electron dynamics to evolve separately from the ion behavior. “It makes the simulations more difficult because not only are there new effects to account for, but it also extends the range of temporal scales you need to follow,” says Sovinec.

The expanded model, which includes electron and ion behavior, reproduces an important stabilization mechanism in the linear spectrum. “What we find, then, is nonlinear coupling among the modes that are still unstable,” he says. “It’s similar to waves beating together and generating higher and lower harmonics. And those harmonics group the wiggles from these ELMs into a helical band that goes around the torus. So instead of having ripples that cover the whole outboard side, we’re seeing perturbations that group together in just one helical region.”

The findings are significant to fusion researchers because certain classes of ELM events would damage components of the planned ITER (International Thermonuclear Experimental Reactor) experiment, the first magnetic-confinement configuration that will produce a self-sustaining plasma. Improved understanding of ELMs will lead to techniques for controlling them and, as a result, avoiding reactor damage.

In addition, the results point to a time when the NIMROD team can incorporate ELM models into more comprehensive tokamak simulations. “We would like to be able to do the modeling more efficiently,” says Sovinec. “In doing this exercise, we’ve really stretched the algorithm into places where we haven’t been before.”

NIMROD logo

The recent calculation required 15 12-hour segments on the IBM SP5 computer at the National Energy Research Scientific Computing Center, using 344 processors rated at 7.6 gigaflops each.

Currently comprising researchers from UW-Madison, Utah State University, the University of Colorado at Boulder, the University of Washington, the University of Oklahoma at Tulsa, and X Corporation, the NIMROD team has existed for nearly a decade. In the future, says Sovinec, its challenges not only will include developing more accurate plasma models, but also developing new strategies for moving into the petaflop generation of parallel computing, which will enable them to run calculations on tens of thousands of processors.

Sovinec says the NIMROD project is a good vehicle for training future computational physicists. At UW-Madison, he advises or co-advises six PhD-level students and two postdoctoral researchers who use the NIMROD code for everything from conducting simulations of on-campus plasma experiments to applying it to astrophysical situations. “They see how all the different aspects, ranging from analytical plasma physics to the mathematics of numerical analysis, to getting an efficient implementation of the algorithm, to running numerical experiments, come together when modeling a particular situation,” he says.

 


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Date last modified: Friday, 22-Dec-2006 11:49:00 CDT
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