3D extended magnetohydrodynamic simulations lead to new sawtooth phenomena theory

The sawtooth phenomena are related to an instability inside a tokamak where central pressure peaks and falls repeatedly. Previous theories on the sawtooth phenomena have attempted to attribute it to fast magnetic reconnection, anomalous electron viscosity, plasmoids, plasma compressibility, and more, but these studies were often limited to simulating only a single sawtooth event and remain incomplete.

Building on a pressure driven interchange model by one of the main theorists in the field, Jardin et al. propose a theory to explain the sawtooth phenomena that does not involve magnetic reconnection. Thanks to the National Energy Research Supercomputer Center, the authors performed many 3D extended magnetohydrodynamic simulations of sawtooth cycles in extremely high resolution.

Unlike in previous simulations, the authors found that pressure, rather than current, plays a critical role in the sawtooth cycle.

“We find that a dynamo voltage forms and acts to keep the central current density right at a marginal stability point,” said author Stephen Jardin. “As the pressure rises due to external heating, the configuration suddenly becomes unstable to many fine-scale modes, which cause the magnetic field to become stochastic in the center, causing the central pressure to suddenly drop.”

The authors intend on validating their model experimentally by predicting changes in sawtooth behavior as external heating, particle, and/or momentum sources are applied.

“Our goal is to develop a complete predictive model of the tokamak that can be used to control off-normal events and to optimize future tokamak power plants,” said Jardin. “Sawtooth phenomena not only affect the confinement properties of the device but potentially can interact with other instabilities to produce a sequence of events that leads to the termination of a discharge.”

Source: “A new explanation of the sawtooth phenomena in tokamaks,” by S. C. Jardin, I. Krebs, and N. Ferraro, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/1.5140968 .