Effect of carbon divertor plates on impurities, Zeff and density limit in large helical device

Abstract

LHD (Large Helical Device) has been operated during 2 years after initiating the first plasma on March of 1998. During the first year (1st and 2nd experimental campaigns) the discharges were mainly carried out with a low magnetic field of Bt = 1.5 T Before the 3rd experimental campaign begin in 1999, 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 purpose of heat removal and separation from the vacuum wall (SS316L). After the installation metal impurities decreased and high influxes of carbon and oxygen were measured. During accumulated number of discharges the metal impurities increased and returned finally to the former level. The re-deposition of metals on the carbon plates was also observed. However, discharges were successful and the impurity level did not lower critically the plasma performance. The behaviour of impurities, Zeff (effective charge) and density limit is discussed and r esults from spectroscopic measurements are presented.