Advances in understanding of high-Zmaterial erosion and re-deposition in low-Zwall environment in DIII-D

Abstract

Dedicated DIII-D experiments coupled with modeling reveal that the net erosion rate of high-Z materials, i.e. Mo and W, is strongly affected by carbon concentration in the plasma and the magnetic pre-sheath properties. Different methods such as electrical biasing and local gas injection have been investigated to control high-Z material erosion. The net erosion rate of high-Z materials is significantly reduced due to the high local re-deposition ratio. The ERO modeling shows that the local re-deposition ratio is mainly controlled by the electric field and plasma density within the magnetic pre-sheath. The net erosion can be significantly suppressed by reducing the sheath potential drop. A high carbon impurity concentration in the background plasma is also found to reduce the net erosion rate of high-Z materials. Both DIII-D experiments and modeling show that local 13 CH 4 injection can create a carbon coating on the metal surface. The profile of 13 C deposition provides quantitative information on radial transport due to E × B drift and the cross-field diffusion. The deuterium gas injection upstream of the W sample can reduce W net erosion rate by plasma perturbation. In H-mode plasmas, the measured inter-ELM W erosion rates at different radial locations are well reproduced by ERO modeling taking into account charge-state-resolved carbon ion flux in the background plasma calculated using the OEDGE code.

Modeling of ExB effects on tungsten re-deposition and transport in the DIII-D divertor

“…With no ExB transport (0% ExB), simulated W deposition largely exponentially decays with distance away from the W source, similar to what was observed in ERO simulations without ExB drifts [39]. However, with any level of ExB transport, a radially-separated W deposition peak appears, though the exact location and height of the peak depend on the ExB scale factor.…”

Evaluation of silicon carbide as a divertor armor material in DIII-D H-mode discharges

“…Because the SiC coatings used in this work had fairly rough surfaces (Table 2), an average ion impact angle of 45 degrees is assumed to be more reasonable. The 45 degree impact angle assumption has also been successfully used in other recent PMI model validation studies in DIII-D involving tungsten coatings with similar surface roughness [24] [25]. The sensitivitiy of the calculations to the chosen ion impact angle is discussed in Section 4.2.…”

“…Figure 10: Measured C chemical erosion yields from (a) a graphite DiMES sample and (b) a SiC-coated DiMES sample as a function of divertor electron temperature. For graphite, the predictions from the Roth scaling[25] are overlaid. For SiC, the predictions from the mixed-material erosion model are overlaid, along with the expected value for pure SiC (10% of the Roth scaling[15]).…”

Critically Evaluated Spectral Data for Singly Ionized Carbon (C ii)

“…For example, lines of C II are used for diagnostics of tokamak plasmas (Isler et al 1997;Menmuir et al 2014). It was recently demonstrated that the erosion of heavy metals (W or Mo) by tokamak plasmas can be abated by injection of methane and deuterium gases (Ding et al 2017); carbon lines are important for diagnostics of such plasmas. Spectral features of various atomic species including C II-V have been recorded in emission of tokamak plasmas in a wide range of wavelengths from visible to VUV (von Hellermann et al 2005;Graf et al 2011;Oishi et al 2014;McCarthy et al 2016).…”