Evaluation of the surface morphologies and erosion/deposition profiles on the LHD first-wall by using toroidal array probes

Toroidal profiles of the microscopic damage and erosion/deposition on the Large Helical Device (LHD) first-wall (SUS316L) were simultaneously evaluated. 10 pairs of SUS316L and Si specimens were mounted on 10 sets of special holders, and were located on the outer side of the first-wall surface in each 36 toroidal angle section (Nos. 1-10). For separate determination of the effects of glow discharge cleanings (GDCs) and main plasma discharges, two types of holders - "floating-potential" and "ground-potential" - were used in each toroidal section. The former was electrically insulated from the first wall; therefore, energetic ions could not be injected into the specimens during GDC. Hence, we could analyze two cases: with and without GDCs exposure. Sputtering erosion of the first-wall surfaces was mainly caused by GDCs and not main plasma discharges, and the erosion depths of each toroidal section were varied from 50 nm to 1 μm. Characteristics of the deposition layers and microscopic damages on the SUS316L matrix were different in each toroidal section.