Dynamical evolution of pedestal parameters in ELMy H-mode in the National Spherical Torus Experiment

Characterizations of the pedestal parameter dynamics throughout the edge localized mode (ELM) cycles are performed on the National Spherical Torus Experiment (NSTX, (Ono et al 2000 Nucl. Fusion 40 557)). A clear buildup of the pedestal height between ELMs is observed for three different plasma currents. This buildup tends to saturate at low and medium plasma currents. Similarly, the pedestal width increases with no clear evidence of saturation during an ELM cycle. The maximum pedestal gradient increases as a function of plasma current, reaches a nominal value after the ELM crash, and remains constant until the end of the ELM cycle. The pedestal height just prior to the onset of ELM is shown to increase quadratically with plasma current. The pedestal width (Δ) scales as with the poloidal β at the top of the pedestal. Coherent density fluctuations strongly increasing at the plasma edge are observed to be maximum after the ELM crash and to decay during the rest of the ELM cycle. Finally, the evolution of the pedestal height and width during the ELM cycle as well as the scaling with I_p of the pedestal pressure prior to the onset ELM are found to be qualitatively consistent with the peeling-ballooning theory.