“…In toroidal magnetic fusion devices, transport can be enhanced by current and pressure driven magnetic instabilities, 2 and particularly by chains of magnetic islands. 7 In most magnetic fusion devices pressure limits are usually expressed in terms of the obtainable plasma  ͑=nT/ B 2 ͒ for which a range of scalings have been obtained by considering the deleterious effect of both current and pressure-driven modes ͑see, e.g., Troyon in Ref. The ideal shift and tilt modes are stabilized by a closefitting copper shell, 4 and more recently, current-profile control has allowed the fluctuations from ideal current-driven modes to be suppressed, 5 but islands sometimes remain with finite amplitude, in agreement with fieldline calculations for equilibria in the presence of field errors.…”