Lithium Droplet Transport in Tokamak Edge Plasmas

TL;DR

This paper presents a comprehensive lithium droplet transport and evaporation model integrated into the OpenEdge code, validated against analytical solutions, and applied to simulate droplet behavior in tokamak edge plasmas.

Contribution

The authors developed and validated a detailed lithium droplet model within OpenEdge, enabling self-consistent simulations of droplet transport, evaporation, and plasma interactions in tokamak edge conditions.

Findings

Droplet transport outcomes depend on initial size, velocity, and launch location.

Outer-divertor droplets mostly redeposit locally, inner-divertor droplets reach the wall.

Smaller droplets evaporate more before reaching the core, larger droplets redeposit nearby.

Abstract

A lithium droplet transport and evaporation model has been developed within the Direct Simulation Monte Carlo code OpenEdge. This model integrates gravity, collisional ion drag, orbital-motion-limited charging, energy-balance evaporation, and an anisotropic rocket recoil force using a Strang-split integrator. Validation against analytical drag-gravity solutions and independent RK45 evaporation integration demonstrates relative errors below 0.00001 for droplet radii of 1.5, 2.5, and 3.5 mm. Simulations of ensembles containing 100000 droplets, launched from inner and outer divertor surfaces in SOLPS-ITER plasma background for the CAT tokamak reactor concept, indicate that transport outcomes are determined by initial size, velocity, and launch location. Outer-divertor droplets predominantly redeposit locally, whereas inner-divertor droplets reach the low-field-side wall. Smaller droplets lose most of their mass to evaporation before reaching the core, while larger droplets retain their mass and redeposit on nearby tiles. Both one-way and iterative two-way coupling frameworks map the evaporated lithium onto the SOLPS-ITER mesh as volumetric sources, facilitating self-consistent evaluation of lithium droplet impacts on edge-plasma performance.