Plasma doughnut currents made hollow, leading to greater efficiency for fusion

Doughnuts of plasma can be coaxed into configurations with hollow current rings, providing practical advantages over conventional “filled doughnut” shapes. Simulations suggest they will allow faster turn-on and greater efficiency of future nuclear fusion power plants.

Toroidal tokamaks, doughnut-shaped experimental fusion reactors, use a complex system of magnetic fields to hold a plasma together. Electrical currents flowing in the plasma itself are essential for making the internal magnetic fields needed for confinement. Plasma doughnuts normally carry large electrical currents throughout their volume but researchers expected the direction of the current could be changed back and forth.

However, in recent experiments at the Joint European Torus (JET) and JT-60U tokamaks in England and Japan, researchers tried to reverse the current and found, to their surprise, that the current doughnut became hollow.

Now computer simulations conducted by researchers at the DOE’s Princeton Plasma Physics Laboratory (PPPL) using supercomputers at the National Energy Research Supercomputer Center have explained this phenomenon. Instead of the electric current reversing direction, the plasma experiences magnetic reconnection (see Highlight 4) and the core becomes stabilized with zero current. As soon as a current tries to reverse in the center, it is pulled into the outer ring. (See images.) This new understanding should allow a more practical design of compact next-generation fusion experiments.

Contacts
Joshua Breslau, PPPL, 609-243-2677, jbreslau@pppl.gov