Relationship between edge localized mode severity and electron transport in the National Spherical Torus Experiment

Abstract

In the National Spherical Torus Experiment [J. Menard et al., Nucl. Fusion 47, S645 (2007)], “giant” edge localized modes (ELMs) can occur resulting in a loss of plasma stored energy of up to 30%. These events are distinct from type I ELMs, whose energy loss is typically 4–10%, and they are accompanied by a cold pulse that causes a global decrease in the electron temperature profile. Estimates of the electron thermal transport during the cold pulses show a large enhancement over the underlying cross-field thermal diffusivity, χe, of up to several tens of m2∕s. Following the ELM, short-wavelength fluctuations increase in the plasma edge and core, corresponding to an increase in the electron temperature gradient from the propagating cold pulse. Fast electron temperature measurements indicate that the normalized electron temperature scale length, R∕LTe, reaches the threshold value for instability predicted by a fit to linear stability calculations. This is observed on time scales that match the growth of the high-k fluctuations in the plasma core, indicating that the enhanced χe and energy loss from the “giant” ELM appears to be related to critical gradient physics and the destabilization of electron temperature gradient modes.