Experimental verification of ion impact angle distribution at divertor surfaces using micro-engineered targets on DiMES at DIII-D

Abstract

• The first detailed experimental verification of polar ion impact angle distribution (IAD). • Specially fabricated micro-trenches were exposed to L-mode D plasmas at DIII-D DiMES. • Calculated polar and azimuthal IADs were combined with a Monte-Carlo MPR code. • MPR revealed the areal erosion yield on the trench floor due to D sputtering. • Good agreement between MPR erosion and EDS net C deposition profiles was seen. • Such agreement strongly verifies the IADs, and the modeling of the Chodura sheath. We report the first detailed experimental verification of the polar deuterium ion impact angle distribution (IAD) on the DIII-D divertor surface in L-mode plasmas using micro-engineered trenches in samples mounted on the DiMES probe. These trenches were fabricated via focused ion beam (FIB) milling of a silicon surface partially coated with aluminum. The sample surfaces were exposed to eight repeat L-mode deuterium discharges (30 s total exposure time). The samples were examined by scanning electron microscopy (SEM), which revealed changes on the trench floor due to material deposition and evidence for shadowing of the incident deuterium ions by the trench walls. The areal distribution of carbon and aluminum deposition was measured by energy-dispersive X-ray spectroscopy (EDS). One-dimensional profiles of this deposition are in agreement with net erosion profiles calculated from a Monte Carlo micro-patterning and roughness (MPR) code for ion sputtering using as input the polar and azimuthal deuterium IADs reported previously (Chrobak et al., Nucl. Fusion 58, 106019 (2018)). The deposition profiles verified the characteristic shape of the polar IADs, which have a broad maximum from 79° to 86°, over the experimental range of 68°–83°, where 0° is the surface normal direction.