Scientists trying to duplicate the fusing of plasmas that occurs in stars have to deal with what are referred to as sawtooth instabilities. These are up and down swings in the central pressure and temperature of the plasmas in plasma reactors. When these swings are drawn on a graph, they resemble the teeth of a saw blade. If the swings are big enough, the entire discharge of the plasmas can suddenly collapse. These swings were first seen in 1974 and, to date, have escaped being captured in a widely accepted theory that explains the experimental observations.
     Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) are proposing a new theory to explain the sawtooth swings in tokamak fusion reactors. Their theory was created through the use of high-fidelity computer simulations. These simulations appear to be consistent with observations made during experiments with tokamak fusion reactors. Understanding such fusion processes will be important to next generation fusion facilities such as the international ITER experiment underway in France.
     Fusion processes involve the integration of lighter atoms into heavier atoms. If we can come to understand and control these processes, we will have as much cheap and non-polluting energy as we can possibly use.
    The recent research at PPPL shows that when the pressure in the core of the donut of plasma in a tokamak reaches a certain threshold, other instabilities can be triggered that suddenly produce the pressure and temperature drops. These instabilities created jumbled magnetic fields in the core of the plasma that cause the collapse. Stephen Jardin is the lead author of a paper describing the findings of the PPPL team. The paper was published in the journal Physics of Plasmas. Jardin says, “Most tokamak discharges exhibit sawteeth and we’re trying to provide the theory of the physics behind them.”
     The new research deviates significantly from a popular theory that the cause of the pressure and temperature swings is an instability that leads to magnetic reconnection. This is the breaking apart and snapping together of the magnetic field lines in the plasma. Jardin says, “That theory has been around for over 40 years.”
    The new theory is motivated by previous research at PPPL that demonstrates how the instability that was assumed to lead to magnetic reconnection can actually stabilize the plasma. This is accomplished by the production of a localized voltage that prevents the current in the core of the plasma from peaking sufficiently to be subject of magnetic reconnection.
    This new explanation of the well-known phenomena suggests that even if the magnetic reconnection is suppressed, intense heat in the core of the plasma can excite local instabilities that act together to flatten the pressure and temperature during the sawtooth cycle. Simulations were produced by special codes that were developed by Jardin and PPPL physicist Nate Ferraro who is a coauthor of the PPPL paper that shows how this process works. The new instabilities can grow very quickly which is consistent with the collapse of heat found in experiments. These results cannot be explained by the old theory.
    The PPPL advanced model provides a new way to understand the sawtooth phenomena. In the future, the PPPL researches are going to explore how the model can be applied to other processes such as describing the evolution of “monster sawteeth” and using high powered Radio Frequency antennas to control such sawtooth swings. Jardin said, “We want to develop a simulation model of a whole tokamak plasma and this new theory of the sawteeth is an important part of that effort.”