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 above) 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.
Joshua Breslau, PPPL, 609-243-2677, jbreslau@pppl.gov
Extended summary of work and images: http://www.aps.org/meet/DPP02/baps/press/breslau.pdf
[FI1.005] Simulation Studies of the Role of Reconnection in the "Current Hole" Experiments in JET Abstract: http://www.aps.org/meet/DPP02/baps/abs/S610005.html