Modifications of impurity transport and divertor sources by lithium wall conditioning in the National Spherical Torus Experiment

Abstract

In the National Spherical Torus Experiment (NSTX), lithium coatings are evaporated on graphite plasma facing components (PFCs) for wall conditioning. In lithium-conditioned H-mode discharges, carbon accumulation is observed with core concentrations <10%, leading to a lack of density control, while lithium ions have concentrations <0.1%. In this thesis, modifications of carbon and lithium divertor sources as well as scrape-off layer (SOL) and core transport due to lithium conditioning are studied. Spectroscopic impurity influxes (measured by filtered cameras) and 2D multi-fluid edge transport simulations via the UEDGE code are employed to study divertor impurity sources and SOL transport, respectively. Core transport of carbon and lithium is analyzed using the impurity transport code MIST and the neoclassical transport codes NEO and NCLASS.

A reduction of the carbon sputtering yield in the lower divertor is observed with lithium evaporation. However, weaker divertor impurity retention resulting from reduced recycling (inferred from UEDGE simulations) and the possible importance of wall sources can counteract this reduction in divertor carbon influxes. The suppression of edge-localized-modes (ELMs) is the primary cause of the increased carbon inventories in lithium-conditioned discharges, leading to lack of density control. Deviations from neoclassical predictions for carbon transport are observed at the pedestal top in lithium-conditioned discharges, indicating the presence of anomalous outward convection.

While the lithium sputtering yield from lithium-coated graphite in the divertor is consistent with physical and temperature-enhanced sputtering, a strong reduction in ionized lithium influxes is observed, possibly due to prompt re-deposition. The different poloidal source distribution and the stronger divertor retention for lithium (inferred from UEDGE simulations) contribute to a lower edge lithium source with respect to carbon. The latter is due to the shorter lithium ionization mean-free-path and weaker classical parallel forces. The high neoclassical lithium diffusivity, due to collisions with carbon ions, is partially responsible for the low lithium core densities. Furthermore, the higher neoclassical levels make lithium transport closer to neoclassical expectations.

The NSTX-Upgrade baseline scenario includes lithium coatings on graphite PFCs for deuterium particle control. While the remarkably good lithium divertor retention makes it an attractive choice, the suppression of ELMs leads to carbon accumulation and therefore to the need to develop impurity mitigation techniques.