2013 Volume 8 Pages 1102168
In this article, experimental observations of limit cycle oscillations (LCO) that precede L-to-H transition are discussed. Issues are: (1) the existence of zonal flows, (2) spatio-temporal evolutions of turbulence intensity, and (3) periodic generations/decays of mean radial electric field and density. The role of Reynolds stress to accelerate the LCO flow is also addressed. The propagation of changes of the density gradient and turbulence amplitude into the core is commented. Varieties in experimental reports on these issues are explained, and possible origins of different interpretations are discussed. Problem definitions for the future research for resolution are presented.