Physics of stimulated and spontaneous L-H transitions

Miki, Kazuhiro; Diamond, P. H.*; Hahn, S.-H.*; Xiao, W. W.*; Grcan, . D.*; Tynan, G. R.*

A central issue in H-mode physics is to achieve control, not only understanding. Work in control has focused mainly on fueling by pellet injection at the near edge. We here introduce a developed one-dimensional mesoscale model, to report on the L-H transition stimulated by pellet injection. For highly heated plasmas, transition occurs spontaneously; when heat flux increases, a strong zonal flow is sufficiently excited, leading to I-phase oscillations or a single burst. When the zonal flow is excited, turbulence is reduced, allowing an ion pressure gradient to steepen. Then, mean flow shear increases to lock in the H-mode. On the other hand, studies reveal that L-H transition can be triggered by particle injection into a subcritical state. We also observe that zonal flows do not play a critical role in stimulated transitions.