Dynamical Evolution of Pedestal Parameters in ELMy H-mode in the National Spherical Torus Experiment
Authors: A. Diallo, R. Maingi, S. Kubota, A. Sontag, T. Osborne, M. Podesta, R. E. Bell, B.P. LeBlanc, J. Menard, and S. Sabbagh
Abstract:
Characterizations of the pedestal parameter dynamics throughout
the edge localized modes(ELM) cycles are performed on the National Spherical
Torus Experiment (NSTX, [M. Ono et al., Nucl. Fusion 40, 557 (2000)]). A clear
buildup of the pedestal height is observed between ELMs for three di erent plasma
currents, which tends to saturate prior to the onset of ELM at low and medium
plasma current. 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 is proportional to the square-root of the poloidal Β at the top
of the pedestal. Coherent density uctuations 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 pedestal parameters evolution during the ELM
cycle as well as the scaling with Ip of the pedestal pressure prior to the onset ELM
are found to be qualitatively consistent with the peeling ballooning theory.
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Submitted to: Nuclear Fusion (July 2011)
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