Measurement of sheath potential and metal surface phenomena at the Alcator C plasma edge

Abstract The radial profile of sheath potential (φ w ) on tungsten surfaces has been measured in the boundary layer of Alcator C plasma using a retarding field energy analyzer. Simultaneous measurement of the electron temperature ( T e ) and ion temperature ( T i ) profiles was also made. The φ w , T e and T i profiles are monotonically decreasing functions of radius but are much steeper on the electron drift side of the probe. Plotted as a function of T i / T e the normalized sheath potential — eφ w / kT e decrease steeply on the electron drift side but increases weakly on the ion drift side. Such a difference in characteristics was not predicted theoretically. Unipolar arcing was found on the tungsten surface examined with a scanning electron microscope. Microscopic craters were formed due to arcing. The surface density of the craters is as high as 10 13 cm −2 . The density and size of the craters increases with sheath potential. No sheath potential and unipolar arcing were found on the SiC surface.