DEIXIS 2007 - 2008 THE DOE CSGF ANNUAL
Alumni profile:
It is easy to imagine Eric Held as a farmer. “I like being alone with my thoughts,” says the Utah State University physicist — one of the few people on campus who does not own a cell phone.
Held often uses the silence to contemplate fusion, which promises to produce clean, emission-free energy by binding hydrogen nuclei. Held’s simulations of magnetically confined plasmas may help scientists better understand this complex process.
This is a long way from Held’s original plans. His grandfather and uncles were farmers, and his father was director of the South Dakota Farm Bureau. He wanted to farm after high school graduation, and didn’t even apply to college until a few weeks before the fall semester. “My parents leaned on me to do it,” he recalls.
At South Dakota State University Held discovered the joy of unraveling physics and math problems. “I thought it would be cool to spend all day thinking about a problem,” he says. Such musings led him to the DOE CSGF while pursuing a doctoral degree in plasma physics at the University of Wisconsin.
Held honed his computer modeling skills and developed a new perspective on his work during his DOE CSGF practicum at Oak Ridge National Laboratory. “My advisor, Jean-Noel Leboeuf, was a bright, humble guy who thought carefully about what he said and did. I learned how to carry myself as a physicist by watching him,” Held says.
The DOE CSGF stipend gave Held the freedom to consider what problem he would choose for his doctoral thesis. He decided to model how heat escapes from magnetically confined plasma as it races around a donut-shaped torus, and he’s wrestled with the problem ever since.
“The torus is like a heating duct bent back on itself, and the magnetic field acts like insulation to prevent the heat from escaping through the walls,” Held explains. If too much heat escapes the magnetic field, the plasma loses energy and cannot sustain fusion.
Unfortunately, heat travels up to 10 billion times faster along the torus than it does when it escapes. Like a gear that has a ratio of 10 billion to one, it takes 10 billion simulation cycles of the “duct” gear to turn the “escaped heat” gear forward one tick. That extreme difference in scale makes the system very hard to model.
Held uses hybrid models to simplify the calculations. He starts with a relatively simple fluid model to describe the plasma’s density, flow, and temperature. He then adds elements of more complex kinetic models, which describe how individual particles interact with one another and with the electromagnetic fields around them.
“Imparting kinetic physics to fluid equations captures more of the real physics and enables us to do really big simulations,” Held says.
The approach has already achieved at least one payoff. “Heat normally flows from hot to cold, the way a radiator heats up a room,” Held explains. “But under some conditions in a magnetic field, heat will actually flow to even hotter areas.”
Will those results help create a clean fusion reactor? Perhaps, Held says. Regardless, they promise better insights into a process that has baffled researchers for two generations.
It’s nothing like farming, but it’s just as rewarding.
The Krell Institute
http://www.krellinst.org/