Deposition of carbon inside gaps of castellated tungsten blocks with intrinsic misalignment

Abstract

When PFCs have castellated structure, co-deposition of fuel inside gaps between castellated blocks is an important issue. Four different shapes of castellated tungsten blocks were fabricated to study corresponding issues in KSTAR: Conventional “basic” rectangular shape, single chamfer leading edge, double-chamfer and rounded edge, with two different poloidal gap distances of 0.5mm and 1.0mm. These tungsten blocks were exposed plasma of L- and H-mode discharges during a whole campaign in 2014. The blocks were taken out from the vacuum vessel after the campaign. Gap deposition was analyzed by Electron Probe X-ray Micro Analyzer (EPMA) to obtain carbon surface density (atoms/cm2), and by Raman spectroscopy to identify chemical bonding structure of carbon deposits in gaps. The carbon surface density in toroidal and poloidal gaps was in a range from 0.5 × 1015 atom/cm2 up to 6.7 × 1015 atom/cm2. At the gap entrance, contribution of ions is 6.0–6.7 × 1015 atom/cm2, decreased down to 1.0 × 1015 atom/cm2 at a depth of 0.5mm, and remains constant afterwards. The contribution of charge exchange neutral is about 3.0 × 1015atom/cm2 at the gap entrance, and then gradually decreases as a function of distance from the entrance. Deposition in 1.0mm wide gaps show much larger deposition patterns and that particles have reached much deeper inside the gap. Raman spectra show that the intensity ratio I(D)/I(G) decreases from top to the depth of 5mm indicating the increase of hydrogen contents. Keywords: Plasma-facing components, Tungsten, Castellation, Gap geometry, Deposition, Retention