University of Wisconsin-Madison College of Engineering Annual Report 2003
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Engineering Physics

Dennis Whyte

Following particle paths in magnetic fusion experiments

The interior wall of large magnetic-fusion experiments can take a beating: As it circulates, plasma — an ionized gas that fuels fusion energy production — attacks the reactor wall.

If too many wall particles slough off and join the plasma, they poison the fusion reaction, says Assistant Professor Dennis Whyte (pictured). Conversely, a vigorous plasma could eat through the interior reactor wall. And most critically, some of the radioactive tritium generated through fusion fuel can embed in the reactor walls, creating safety issues. "We need to answer those questions now before we start building larger and larger devices where these problems become more and more acute," he says.

Using the tokamak at the DIII-D National Fusion Facility, San Diego, Whyte and his collaborators will insert carbon-13-doped methane in one reactor location, then discharge the plasma. Following that first experimental phase, he will use his ion beam to nondestructively examine sections of the reactor wall and map where the carbon-13 and other particles deposited in the wall. With that data, he hopes to develop models to explain how turbulent transport carries these particles around.

Whyte is collaborating with researchers from General Atomics, the University of Toronto, the University of California-San Diego and Sandia National Laboratories.

The project is funded with a three-year, $450,000 grant from the Department of Energy and a nine-month, $35,000 grant from General Atomics.

Studying small-scale friction for MEMS success

At the nanoscale, surfaces have topographies that look like craggy mountain ranges. Friction, adhesion and bonding between surfaces can be such strong factors that micromachines won't work.

Building on projects with both Sandia and Argonne National Laboratories, Assistant Professor Robert Carpick and Professor Michael Plesha have received a three-year, $525,000 Department of Energy Nanoscale Science Engineering and Technology grant to study these surfaces.

With their group — PhD students Erin Flater and Can Bora and research associate Anirudha Sumant — they are examining the topographies of silicon and diamond-film surfaces, based on variables such as pressure, sliding speed, type of material and environmental conditions. So far, they've learned that chemically modifying the diamond surface will affect adhesion. And with data from the silicon surface images, they are developing models that will help MEMS fabricators predict the friction forces when surfaces contact in a device. "Ultimately, our goal is to be able to tell them, 'If you want a micromachine that's going to last, this is how you should design your surface,'" says Carpick.

The results could open a whole new market for micromachines such as motors, pumps, actuators, communications devices and miniature mechanical combination locks with sliding parts.

New boundaries: Experiments verify ion behavior in plasmas

How ions are lost from confined plasmas can affect everything from semiconductor processing to current extraction in fusion devices, yet for more than a half century, theories and models of their behavior were untested. However, recently Irving Langmuir Professor Noah Hershkowitz and graduate student Lutfi Oksuz conducted experiments that verified theory critical to basic understanding of plasmas. The duo determined that a weakly collisional plasma's potential profile consists of a presheath, where ions accelerate, a transition region of the sheath in which most plasma electrons are reflected, and an electron-free region of the sheath, where ions pick up velocity.

For semiconductors, this knowledge means better understanding of how plasmas can be used for etching. "The presheath ion-scattering provides the ultimate limitation on how vertical structures can be etched," says Hershkowitz. And in a fusion device, loss of plasma at a limiter or diverter determines both the heating and particle recirculation at that point and provides basic limits as to how much current researchers can extract, what density level they can achieve, how long materials last, and more.

Building on the results, Hershkowitz, Visiting Professor Greg Severn and graduate students Eunsuk Ko and Xu Wang investigated a plasma made up of equal amounts of argon and helium ions using laser-induced fluorescence. They learned that the heavier argon ions fell out faster at the presheath-sheath boundary than they would have if they had been the only ion species in the plasma. The groups conducted their research through the college's Center for Plasma Aided Manufacturing with funding from the Department of Energy.

 





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Date last modified: 03-Oct-2003
Date created: 03-Oct-2003