Transition to high mode induced by reduction of magnetic stress

The transition to the high mode (H mode) of plasma operation in tokamaks and stellarators can be triggered by reduction of the magnetic stress. The magnetic stress can be reduced directly, by stabilization of the magnetic perturbation, or by increasing the plasma density and ion temperature if all of the other parameters are fixed. This implies the existence of thresholds in density and ion temperature. The theory is applied to a class of H-mode phenomena observed experimentally.