Perturbative studies of electron transport in NSTX H-modes

The study of electron transport is important for an understanding of present machines and predicting performance in future machines, such as ITER. NSTX provides a unique test bed for probing electron transport due to its dominant role in the overall power balance [1]. Global T_e profile crashes of 10-25% amplitude are observed following large Type I ELMs in some beam heated H-mode NSTX discharges. While the SXR imaging indicates that the ELM itself is causing only a peripheral MHD perturbation, the propagation of the cold pulse initiated by the ELM is unusually fast (ms time scale) and can extend to the core of the plasma. The perturbed electron thermal diffusivity is estimated to be in the hundred m²/s range in the outer plasma, decreasing towards the centre. Motivated by these observations, we produced also controlled perturbations at the plasma edge by injecting small low-Z pellets into beam heated H-mode plasmas, and compared the ELM and pellet induced cold pulse using two-colour SXR imaging. In plasmas which exhibit large Type I ELMs the pellet perturbation has a similarly large effect on the global T_e profile, with a stiff behaviour of the core profile. In contrast, the particle diffusivity estimated from the decay of pellet injected impurity is in the m²/s range. The around two order of magnitude difference suggests magnetic effects in the perturbed NSTX electron transport.