Automatic ELM detection and study of statistical ELM properties by Li-BES on COMPASS

Recently, the Lithium Beam Emission Spectroscopy (Li-BES) system [1,2] on the COMPASS tokamak (R = 0.56 m, a = 0.23 m, I < 400 kA) has reached its full diagnostic capabilities. The beam is operated standardly at 60 keV and the strong lithium emission line at 670.8 nm is detected by an array of 16 avalanche photodiodes (APDs) with a spatial resolution of ~1 cm. It is used for routine automatic measurements in COMPASS discharges and serves as a standard tool for reconstruction of density profiles in the edge/SOL plasma region. High signal-to-noise ratio of the detected light signal along with a fast chopping technique (up to 250 kHz) for plasma background correction allows the reconstruction of density profiles with unique temporal resolution down to 2 s [3]. Here we report on a first study of statistical properties of type-III ELMs in Hmode COMPASS plasmas [4] on the basis of Li-BES signals. Analysis of the waiting time, duration and density ELM distributions and their radial variation in the edge/SOL plasma region was performed. One of the main aims of the presented work was also to develop a reliable tool for automatic ELM event detection in the Li-BES data. For this purpose, the generalized Sequential Probability Ratio Test (gSPRT) technique [5] was successfully implemented for the COMPASS LiBES signals.