Synchronization of L-mode to H-mode transitions on the sawtooth cycle in Ohmic TCV plasmas

Abstract

L-mode to H-mode transitions (LH transitions) are obtained in a wide range of plasma parameters in TCV Ohmic discharges. LH transitions either occur directly, indicated by a single abrupt drop in Dα emission, or present an intermediate ‘dithering’ phase before the H-mode state is attained, in which Dα emission shows rapid excursions between L-mode and H-mode levels. The type of transition, with or without a dithering phase, depends on the plasma current and plasma triangularity. It is less sensitive to plasma density and divertor geometry. The relative direction of the ion ∇ B drift also plays a role in the characteristics of the LH transition.The time delay between an LH transition and the preceding sawtooth crash has been measured in every TCV H-mode discharge. Most LH transitions are found to occur in the first 30% of the sawtooth cycle, i.e. in the first 2 ms after a sawtooth crash. Moreover, LH transitions often occur soon after a large crash of a ‘double sawtooth’. The distribution function of these time delays clearly indicates a synchronization of these two processes. However, the heat pulse released during a sawtooth crash has a transit time from the q = 1 surface to the plasma edge, which is much shorter than the time delay between the sawtooth crash and the LH transition.