Numerical investigation of plasma edge transport and limiter heat fluxes in Wendelstein 7-X startup plasmas with EMC3-EIRENE

Abstract

The results of a first systematic assessment of plasma edge transport processes for the limiter startup configuration at Wendelstein 7-X are presented. This includes an investigation of transport from intrinsic and externally injected impurities and their impact on the power balance and limiter heat fluxes. The fully 3-D coupled plasma fluid and kinetic neutral transport Monte Carlo code EMC3-EIRENE is used.The analysis of the magnetic topology shows that the poloidally and toroidally localized limiters cause a 3-D helical scrape-off layer (SOL) consisting of magnetic flux tubes of three different connection lengths L C . The transport in the helical SOL is governed by L C as topological scale length for the parallel plasma loss channel to the limiters. A clear modulation of the plasma pressure with L C is seen. The helical flux tube topology results in counter streaming sonic plasma flows.The heterogeneous SOL plasma structure yields an uneven limiter heat load distribution with localized peaking. Assuming spatially constant anomalous transport coefficients, increasing plasma density yields a reduction of the maximum peak heat loads from 12MWm −2 to 7.5MWm −2 and a broadening of the deposited heat fluxes.The impact of impurities on the limiter heat loads is studied by assuming intrinsic carbon impurities eroded from the limiter surfaces with a gross chemical sputtering yield of 2%. The resulting radiative losses account for less than 10% of the input power in the power balance with marginal impact on the limiter heat loads. It is shown that a significant mitigation of peak heat loads, 40-50%, can be achieved with controlled impurity seeding with nitrogen and neon, which is a method of particular interest for the later island divertor phase.

Limiter observations during W7-X first plasmas

“…As predicted [1,2,15], the highest points or 'watershed' of the curved limiters between the two heating stripes did not intercept much energy. This is because at the peak of the limiter curved surface, the field lines are nearly tangential, whereas further down on the side of the curved limiter, the field lines intercept the limiter at a large angle.…”

Reconstruction of recycling flux from synthetic camera images, evaluated for the Wendelstein 7-X startup limiter

“…due to mismatching plasma conditions related to transport assumptions). Analysis of the heat load patterns has shown reasonable agreement with predictions by EMC3-EIRENE [28,29], which suggest that particle loads onto the limiter should exhibit a similar pattern. Of considerable concern are perturbations from impurities which can contaminate the experimental observations because of the red-shifted transmission curve of the filter.…”

“…These flux bundles have a connection length of 1, 1.2 and 2.2 toroidal turns (see e.g. figure 2 in reference [28]), i.e. they connect back to the same limiter after one turn, or connect to the next limiter after one or two turns.…”

Feasibility of line-ratio spectroscopy on helium and neon as edge diagnostic tool for Wendelstein 7-X

“…Their adjustment is based on EMC3-EIRENE simulation of the He line emissions. 2 This optical setup leads to a spatial resolution of 1 cm and an observable radial range of 8 cm in the scrape-off layer. The width of each LOS at the nozzle position is 1 cm.…”