Researchers at MIT have discovered a powerful tool for creating and manipulating coveted "internal transport barriers" which prevent unwanted heat leakage from magnetically confined fusion plasmas. At the Alcator C-Mod National Tokamak Facility, located at the MIT Plasma Science and Fusion Center, researchers are developing a technique known as "off-axis ion cyclotron radio frequency" (ICRF) heating. C-Mod is a tokamak, a doughnut-shaped device which uses magnetic fields to confine plasmas.

Normally, ions in these plasmas circle around the magnetic fields at different rates; the ions' resulting "cyclotron frequencies" vary according to their positions with respect to the tokamak's many fields. And for reasons not completely understood, the overall plasma rotates around the tokamak. In traditional techniques for heating the plasma with radio waves, researchers send in waves with a frequency that matches the cyclotron frequency of ions at the center of the plasma. However, MIT researchers studied the effects of moving the resonance location for the ICRF heating; in other words, they applied a radio frequency that matched the cyclotron frequency of ions at a location elsewhere in the plasma. When this resonance location was moved sufficiently far away from the center of the plasma, the overall rotation of the plasma was significantly slowed, or even reversed, and simultaneously with this change, a clear internal transport barrier developed, resulting in an extraordinary peaking of the plasma density, one that was at least two times greater than before.

Internal transport barriers have been created before, but they often require the introduction of neutral atom beams which could not be feasibly placed in the designs envisioned for commercial fusion power plants. This new approach of creating internal barriers could prove to be extremely important, as it is potentially attractive for power plant applications.

For further information, contact Catherine Fiore, 617-253-8440 fiores@psfc.mit.edu or John Rice, 617-253-5395 rice@psfc.mit.edu