Dynamic behavior of theL-Htransition

The dynamic behavior of the L-H transition in the ASDEX-Upgrade tokamak is described. We focus on the periodic L-H-L transitions known as ``dithering H mode'' and show that there is an intrinsic time scale in this process which is characteristic for the L-H transition. We give a tentative explanation for the phenomenon using an extension of Itoh's model for the L-H transition and find that this simple bifurcation model can qualitatively explain many of the experimental observations. According to our model, the dithering cycles are a limit cycle oscillation due to a difference in the response of the system to a rise in either T or n. The implications for H-mode theories are discussed.