Speaker
Description
The island divertor concept implemented at Wendelstein 7-X (W7-X) is one of the most extensively investigated solutions for power and particle exhaust in future quasi-isodynamic stellarator power plants. In the latest experimental campaign (OP2.3), W7-X demonstrated successful feedback control of radiative detachment via impurity seeding, based on real-time bolometric measurements of the total radiated power. This capability marks an important step toward reactor-relevant power exhaust scenarios, where strong impurity radiation in the Scrape-Off Layer (SOL) is required to protect plasma-facing components. At the same time, future fusion reactors must minimize the impurity content in the core plasma to sustain the fusion reaction. The impurity enrichment factor η = cimp,SOL/cimp,Core quantifies the divertors ability to retain impurities and maintain high divertor concentrations without degrading core performance. This work presents the experimental methods for impurity enrichment studies at W7-X, illustrated with representative discharges for helium, neon and argon.
We measure the impurity concentration in the residual gas with a novel Time of Flight (ToF) mass spectrometer in the Diagnostic Residual Gas Analyzer (DRGA). This measurement relies on the long-lasting, steady state conditions (≈20s) provided by the feedback system to equilibrate the neutral gas components at the DRGA with the pumping gap several meters away.
We complement the residual gas analysis by divertor spectroscopy measurements, which grant direct insight into local plasma parameters of the SOL including impurity concentrations. We investigated line ratio spectroscopy NeII in the divertor plasmas and found transport processes of neutrals to increase the radiance of the spectral lines and complicate quantitative concentration estimates. However, the line ratio spectroscopy remains sensitive to electron density. These values derived from the impurity line ratios as well as from Stark broadening analysis both indicate W7-X operating at significantly lower electron densities in the SOL (ne = 1–5×10¹⁹ m⁻³) compared to tokamaks such as AUG and JET with densities typically exceeding ne = 10²⁰ m⁻³.
To assess the enrichment factor, we measure the impurity concentration in the core plasma via Charge Exchange Recombination Spectroscopy (CXRS) during diagnostic NBI blips.
A first evaluation of neon yields an enrichment of η = cimp,SOL/cimp,Core ≈ 1, largely independent of upstream density and total radiation fraction. The same methodology is now being extended to helium and argon to provide a comprehensive comparison of impurity enrichment across species.