 It had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix.Devices like the z-pinch and stellarator had attempted this, but demonstrated serious instabilities.It was the development of the concept now known as the safety factor (labelled "q" in mathematical notation) that guided tokamak development; by arranging the reactor so this critical factor "q" was always greater than 1, the tokamaks strongly suppressed the instabilities which plagued earlier designs.