Tracking turbulence hints
at fusion’s mysterious mode
Posted September 11, 2009
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In this poloidal cross-section tokamak simulation,turbulence is generated in two places: at the edge and at the center. The center turbulence is confined as it rotates quickly, but the edge turbulence propagates inward, colliding with the center turbulence and filling the entire volume. Researchers believe this is a key factor in forming H mode. |
For almost 30 years, scientists have been unsure how the tail wags the dog in fusion energy reactors.
That is, they’ve struggled to explain why conditions on the edge of a superheated plasma cloud have such big impact on events in the cloud’s core.
Now researchers working under Department of Energy (DOE) programs have significant clues, thanks to simulations on one of the world’s most powerful computers.
A team of investigators led by C.S. Chang of New York University’s Courant Institute of Mathematical Sciences devised and ran the models. The simulations are among the first to portray high-confinement mode, or H-mode, a phenomenon that’s critical to fusion’s viability as an abundant, clean power source.
Now the group is working on even more accurate models. It’s backed with 20 million computer processor hours from the Department of Energy’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. What the team learns could have implications for ITER, the largest magnetic fusion experiment to date. ITER, under construction in France, is designed to be the first fusion reactor to produce 10 times more energy than it consumes.
Fusion harnesses physics that fuel the sun. In magnetic fusion, one of the technology’s most promising forms, hydrogen isotopes are heated and electrified to temperatures hotter than the sun’s interior in a torus – a ring-shaped vacuum chamber called a tokamak. Powerful magnetic fields contain the plasma in the torus’ center, keeping it from touching the walls.
The high temperature and pressure strip electrons from the hydrogen atoms. The resulting ions spin through the circular cavity, fusing and releasing energy.
To help ITER and the next fusion reactors succeed, scientists must understand what drives H-mode. Without it, heating the plasma core to temperatures necessary for economical fusion energy will be nearly impossible.
German fusion scientists discovered H-mode in the early 1980s. They found that with enough core heating the plasma jumps into a different state – hot and turbulent in the middle, with a thin outer layer that’s cooler and almost free of turbulence. The increase in core temperature and density dramatically increases the chances for fusion.
“Right at the edge, the plasma temperature gradient forms a pedestal,” Chang says – an instantaneous jump in the curve tracking temperature from the plasma edge to the interior. “They found out that when plasma gets above some critical power, that’s what it wants to do. We call it self-organization – plasma self-organizes into that state.”
H-mode opened horizons for fusion, but scientists still are unsure how it works. “Without understanding that, we just extrapolate that ITER will behave the same way,” Chang says. “Our mission is to understand that by large-scale simulation.”
