Effect of carbon divertor plates on impurities, Z(eff) and density limit in Large Helical Device

LHD (Large Helical Device) has been operated during 2 years after initiatin g the first plasma on March of 1998. During the first year (Ist and 2nd exp erimental campaigns) the discharges were mainly carried out with a low magn etic field of B-t = 1.5 T. Before the 3rd experimental campaign begin in 19 99. carbon divertor plates were installed in the vacuum chamber at the end of the divertor legs. These were connected to the ergodic layer for the pur pose of heat removal and separation from the vacuum wall (SS316L). After th e installation metal impurities decreased and high influxes of carbon and o xygen were measured. During accumulated number of discharges the metal impu rities increased and returned finally to the former level. The re-depositio n of metals on the carbon plates was also observed. However, discharges wer e successful and the impurity level did not lower critically the plasma per formance. The behaviour of impurities, Z(eff) (effective charge) and densit y limit is discussed and results from spectroscopic measurements are presen ted.