Figure 1. Sample images of edge turbulence in NSTX taken at 100,000 frames/sec. The plasma is highly turbulent in the Low-Confinement Mode (L-Mode), but relatively quiescent in the High-Confinement Mode (H-Mode).
Figure 2. Changing the optical interference filter used for the gas-puff imaging diagnostic on NSTX are, from the left, Stewart Zweben of PPPL, graduate student Amy Keesee of West Virginia University, and Ricardo Maqueda of Los Alamos National Laboratory.
Researchers from the U. S. Department of Energy's Princeton Plasma Physics Laboratory, the Los Alamos National Laboratory, and the Massachusetts Institute of Technology have captured high-resolution images of instabilities that cause heat to leak rapidly from the plasma edge of the National Spherical Torus Experiment (NSTX) at Princeton and the Alcator C-Mod tokamak at MIT. Advanced imaging cameras developed by Princeton Scientific Instruments, Inc., under a U.S. Department of Energy Small Business Innovative Research Project, were used to freeze plasma action at a rate of up to 1 million frames per second.
NSTX and Alcator C-Mod are experimental devices used to study the magnetic confinement of plasmas -- the hot ionized gases that serve as fuel for the production of fusion energy. In NSTX, instabilities emerge from the plasma edge at hundreds of meters per second. They appear as long filaments many meters in length, but only several centimeters thick. The filaments carry plasma energy and particles to the nearby vacuum vessel wall, causing a release of wall surface atoms, which in turn cool the plasma edge. When such instabilities are absent or much reduced, the plasma edge forms a barrier against heat loss, resulting in a steep rise in pressure at the plasma edge. The higher edge pressure in turn raises the core plasma temperature and density.
Images of the light emitted from a cloud of neutral helium atoms puffed into the plasma edge are shown in Figure 1. The intensity of the light in the image on the right indicates that instabilities can fluctuate wildly in space and time. The left image shows light emitted when the instabilities are suppressed. These are the highest resolution images of the motion of edge turbulence in fusion research plasmas ever obtained.
Images of edge turbulence reveal a fascinating variety of shapes and motions, often taking the appearances of flickering flames, swaying aurora, or explosive flares on the surface of the Sun (for more examples, visit http://w3.pppl.gov/~szweben/psi/).
Plasma physicists are studying the complexity and looking at the relationship between these plasmas and those found in space and on the sun. Progress will enable fusion researchers to determine the conditions under which the instabilities can be reduced or completely avoided for long durations.
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