The hot ionized gas called a plasma is confined in a bagel-shaped tokamak by a strong magnetic field, part of which is generated by a strong electric current flowing through the plasma. Periodically, a sawtooth instability occurs. It causes the central plasma temperature to abruptly drop and then recover in a sawtooth pattern. The instability limits how much current can be concentrated in the center of the plasma. However, there are types of tokamak plasmas for which a previously unknown mechanism, called magnetic flux pumping, limits the current in the plasma center so that it stays just below the sawtooth threshold. Scientists were puzzled by how this self-regulating mechanism works. The results of highly complex numerical simulations now suggest a possible answer.