L-H transitions and ELMs on COMPASS-D

The phenomena occurring at transition from L-mode to H-mode confinement regimes in tokamaks are often regarded as different from those that produce type-III Edge Localized Modes (ELMs) once the H-mode is established. However, in recent theories revival of the previous concept of an ELM as a L-H auto-oscillation can be seen. Here, the L-H transition is associated with the suppression of MHD-fluctuations by a process of spontaneous generation of laminar flow from turbulence. The resistive interchange instability is considered as a possible candidate for turbulence. In this context, it is of interest to determine whether we can distinguish between LH-ELMs and MHD-ELMs. In experiments described below, data from ohmically heated H-modes in deuterium single-null divertor plasmas on COMPASS-D (R = 0.56 m, a = 0.164 m, b/a = 1.6, Ip = 150-180 kA, BT = 0.93-1.1 T) are examined. We used two poloidal arrays each of 24 Mirnov coils placed on the inside wall of vacuum vessel at toroidal angles Φ = 0o and 180o. A protective shield limits the frequency response to about 160 kHz. (author) 7 refs., 4 figs