Kinetic Modelling of Impurity Transport and Deposition in the HIDRA Scrape-off Layer during Lithium Evaporation

The extreme environment present inside future fusion reactors will require plasma-facing components (PFCs) that can handle high temperatures and damaging ions fluxes without failure or poisoning the plasma via sputtering of the PFC material. The use of liquid lithium as a PFC has drawn increased interest in recent years due to lithium’s low atomic number and the self-healing characteristics of flowing liquid metal divertors. Recent experimental campaigns on the Hybrid Illinois Device for Research Applications (HIDRA) as part of the domestic LMPFC program have identified and confirmed additional benefits of the use of lithium: helium retention behavior during in-operando evaporation of lithium into a helium plasma. Post-experimental observations revealed patterns characterized by ‘double streaks’ of lithium deposition on the vacuum-vessel wall.

In this work, we introduce a kinetic GPU-accelerated particle tracer and compare these experimental observations with simulated impurity ion transport in the scrape-off layer on a full high-resolution 3D mesh of HIDRA’s magnetic field. The magnetic field, including a true-to-life perturbative error field, is solved through direct application of the Biot-Savart law and particle motion is modeled via the Boris-Buneman algorithm and parallelized on a GPU through CUDA. Our magnetic field model is corroborated against measurements taken on HIDRA with Hall probes at multiple positions around the vacuum vessel. Additionally, plasma density profiles are modelled via flux surface parameters and are validated against Reciprocating Langmuir Probe (RLP) measurements taken with various coil configurations. Results of our simulations display the same ‘double streak’ patterns as observed on HIDRA. Once validated, this code will be used in support of current and future experimental campaigns by predicting density profiles and impurity flux at proposed PFC installation locations