Conditionings for plasma facing walls of large helical device

Abstract

The first and second experimental campaigns in large helical device (LHD) were 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 poloidal direction. After each campaign, change of surface morphology, impurity depth profile and gas desorption of the samples were examined. In the first experimental campaign, the ECR discharge cleanings were employed. After the first experimental campaign, the surface was modified by the deposition 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 the port. In the second experimental campaign, the glow discharge cleanings were employed and the number of main discharge shots increased. After the second experimental campaign, no significant deposition took place except for 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 discharge. In addition, the amount of gas desorption was considerably reduced even 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.