Progress in the development of ELM pace-making with non-axisymmetric magnetic perturbations in NSTX

The application of non-axisymmetric magnetic perturbations has been shown to destabilize edge-localized modes (ELMs) in the National Spherical Torus Experiment. This ELM-triggering effect is used to controllably introduce ELMs into lithium-enhanced ELM-free H-mode discharges, reducing the typical impurity accumulation while maintaining high energy confinement. Recent improvements to the triggering techniques are described. The perturbation waveform has been improved, with large amplitude, short duration perturbations allowing rapid, highly reliable triggering, with reduced braking of toroidal rotation. The rapid triggering allowed the ELM frequency to be increased to over 60 Hz, leading to a reduction in the average ELM size. Combined with improved gas fuelling, this method has resulted in periods of stationary global conditions, although plasma profiles do continue to evolve.