Fast flows prevent buildup of impurities on edge of tokamak plasmas
Impurities in the plasmas in fusion tokamaks can reduce performance. These impurities are created by interactions between the hot plasma and the metal tokamak walls. These walls are often armored with tungsten. This material resists heat, but degrades over time, releasing impurities into the plasma. Simulations predict how these impurities behave, but they are difficult to measure directly because many impurities only emit weak levels of radiation. The experiment detailed here used a collector probe to sample the plasma. It determined that detrimental impurities accumulate in the region just outside the plasma edge only when the tokamak magnetic fields rotate around the donut-shaped tokamak in a counter-clockwise direction. This is the opposite direction that the plasma current moves. Clockwise rotation causes fast flows that remove the impurities.
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