Two-dimensional MHD simulations are carried out which demonstrate that the cross-field thermal conduction near the magnetic axis controls the structure of sawteeth in tokamaks. In simulations of discharges with good central confinement, skin currents form at a finite radius causing the safety factor $q$ to fall below 1 away from the magnetic axis. Both normal and compound sawteeth are produced which have many features consistent with experimental observations.

• Received 13 January 1986

DOI:https://doi.org/10.1103/PhysRevLett.56.2477