Empirical scaling of sawtooth period for onset of neoclassical tearing modes

Experimental observations from a range of tokamaks show that neoclassical tearing modes (NTMs) are triggered at lower plasma pressure when the sawtooth period is longer. A multi-machine database from nine tokamaks has been established in order to extrapolate the acceptable sawtooth period to avoid triggering NTMs in ITER. It is found that the governing physics is best compared between machines by normalizing the sawtooth period to the resistive diffusion time and using the normalized beta as a measure of performance and global stability. A multi-parameter power scaling is determined from regression analysis of the complete dataset and compared favourably with experimental data from a number of machines.