Identification of molecular carbon sources in the JET divertor by means of emission spectroscopy

Strong light emission from the C2 Swan band and the CD Gerِ band has been observed during strike-point sweeps in the corner region of the inner MKII SRP divertor of JET. The light emission could be correlated to the decomposition of probably soft hydrocarbon layers at the corner entrance. The C2/CD photon-flux ratio indicates a preferred emission of C2 and thus of C2Dy particles. The layer has been removed within one H-mode discharge, where the strike point was fixed at the corner. In the consecutive discharges no enhanced light emission from C2 and CD could be observed and the C2/CD photon-flux ratio drops significantly. The spectroscopic observations are in line with measurements on the quartz microbalance (QMB). A similar layer in the outer divertor could not be identified from the spectroscopy.