Tokamak sawtooth

The safety factor profile shortly before and shortly after a sawtooth relaxation in a numerical resistive MHD simulation. After the relaxation, and the q profile has a broader, more square-like shape.
Magnetic reconnection during a numerical resistive MHD simulation of a sawtooth relaxation. The arrows showing the direction of the flow are overlaid on top of a plot of the toroidal current density. The size of the arrows corresponds to the magnitude of the flow velocity.

A sawtooth is a relaxation that is commonly observed in the core of tokamak plasmas, first reported in 1974.[1] The relaxations occur quasi-periodically and cause a sudden drop in the temperature and density in the center of the plasma. A soft-xray pinhole camera pointed toward the plasma core during sawtooth activity will produce a sawtooth-like signal. Sawteeth effectively limit the amplitude of the central current density. The Kadomtsev model of sawteeth is a classic example of magnetic reconnection. Other repeated relaxation oscillations occurring in tokamaks include the edge localized mode (ELM) which effectively limits the pressure gradient at the plasma edge and the fishbone instability which effectively limits the density and pressure of fast particles.

  1. ^ von Goeler, S.; Stodiek, W.; Sauthoff, N. (1974-11-11). "Studies of Internal Disruptions and m=1 Oscillations in Tokamak Discharges with Soft—X-Ray Tecniques". Physical Review Letters. 33 (20). American Physical Society (APS): 1201–1203. Bibcode:1974PhRvL..33.1201V. doi:10.1103/physrevlett.33.1201. ISSN 0031-9007.