by Staff Writers
Plainsboro NJ (SPX) Jul 25, 2017
Turbulence, the violently unruly disturbance of plasma, can prevent plasma from growing hot enough to fuel fusion reactions. Long a puzzling concern of researchers has been the impact on turbulence of atoms recycled from the walls of tokamaks that confine the plasma.
These atoms are neutral, meaning that they have no charge and are thus unaffected by the tokamak's magnetic field or plasma turbulence, unlike the electrons and ions - or atomic nuclei - in the plasma. Yet, experiments have suggested that the neutral atoms may be significantly enhancing the edge plasma turbulence, hence the theoretical interest in their effects.
In the first basic-physics attempt to study the atoms' impact, physicists at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have modeled how the recycled neutrals, which arise when hot plasma strikes a tokamak's walls, increase turbulence driven by what is called the "ion temperature gradient" (ITG). This gradient is present at the edge of a fusion plasma in tokamaks and represents the transition from the hot core of the plasma to the colder boundary adjacent to the surrounding material surfaces.
Extreme-scale computer code
The results, reported in the journal Nuclear Fusion in July, showed that neutral atoms enhance ITG turbulence in two ways:
+ First, they cool plasma in the pedestal, or transport barrier, at the edge of the plasma and thereby increase the ITG gradient.
+ Next, they reduce the sheared, or differing, rates of plasma rotation. Sheared rotation lessens turbulence and helps stabilize fusion plasmas.
Comparison with experiments
Researchers generally consider lower recycling, and hence fewer neutrals, as conducive to H-mode operation. This work may also lead to a better understanding of the plasma performance in ITER, the international fusion facility under construction in France, in which the neutral recycling may differ from that observed in existing tokamaks.
Plainsboro NJ (SPX) Jul 07, 2017
Two major issues confronting magnetic-confinement fusion energy are enabling the walls of devices that house fusion reactions to survive bombardment by energetic particles, and improving confinement of the plasma required for the reactions. At the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), researchers have found that coating tokamak walls with lithium-- a light ... read more
Princeton Plasma Physics Laboratory
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