Conditionings for plasma facing walls of large helical device

The first and second experimental campaigns in large helical device (LHD) w ere carried out from March to December 1998. Before the each campaign, the material probes were placed at the inner wall of vacuum vessel along the po loidal direction. After each campaign, change of surface morphology, impuri ty depth profile and gas desorption of the samples were examined. In the fi rst experimental campaign, the ECR discharge cleanings were employed. After the first experimental campaign, the surface was modified by the depositio n of sub-micron particles, and the concentrations such as oxygen and carbon were still high. Impurity gas desorption was also large in the sample at t he port. In the second experimental campaign, the glow discharge cleanings were employed and the number of main discharge shots increased. After the s econd experimental campaign, no significant deposition took place except fo r the position close to the divertor leg, and the oxygen impurity level was reduced. In every sample, the helium was retained by the helium glow disch arge. In addition, the amount of gas desorption was considerably reduced ev en in the sample at the port. In the second experimental campaign, the wall conditionings largely progressed by using the glow discharge cleanings and the increase of main discharge shots with a high heating power. (C) 2001 E lsevier Science B.V. All rights reserved.