Quiet periods in edge turbulence preceding the L-H transition in the National Spherical Torus Experiment

This paper describes the first observations in the National Spherical Torus Experiment (NSTX) [S. M. Kaye et al., Phys. Plasmas 8, 1977 (2001)] of "quiet periods" in the edge turbulence preceding the low-to-high (L-H) mode transition, as diagnosed by the gas puff imaging (GPI) diagnostic near the outer midplane separatrix. During these quiet periods the GPI D-alpha light emission pattern was transiently similar to that seen during H-mode, i.e., with a relatively small fraction of the GPI light emission located outside the separatrix. These quiet periods had a frequency of similar to 3 kHz for at least 30 ms before the L-H transition, and were correlated with changes in the direction of the local poloidal velocity. The GPI turbulence images were also analyzed to obtain an estimate for the dimensionless poloidal shearing S = (dV(p)/dr)(L-r/L-p)tau. The values of S were strongly modulated by the quiet periods but did not significantly vary during the similar to 30 ms preceding the L-H transition. Since neither the quiet periods nor the shear flow increased immediately preceding the L-H transition, neither of these appears to be the trigger for this transition, at least for these cases in NSTX. (C) 2010 American Institute of Physics. [doi:10.1063/1.3476276]