Lithium Transport and Stability in Capillary Porous Structures Exposed to Tokamak Divertor Conditions

Lithium enclosed in an additively manufactured tungsten capillary porous structure (CPS) was exposed to the divertor strike point in the DIII-D tokamak, demonstrating effective Li containment, suppression of droplet formation at elevated temperatures, and no measurable core contamination.

This study addresses key challenges for liquid metal divertors: stability under high heat fluxes and control of Li migration. Two identical CPS samples, with 0.5 g of Li, were exposed to ELMy H-mode plasmas with 5–10 MW of NBI. Heat fluxes reached 2.0–3.0 MW/m². The only variable between the samples was the initial temperature: 23°C (low temperature) vs. 350°C achieved with active pre-heating (high temperature).

Charge exchange recombination spectroscopy (CER) and impurity measurements confirmed minimal Li content in the plasma core under both conditions. Fast camera diagnostics revealed that the low-temperature CPS case led to visible droplet ejection and an increased Zeff, indicating deeper lithium penetration into the plasma. In contrast, the high-temperature CPS completely suppressed droplet formation and resulted in a lower Zeff​ than the reference case. Thermocouple readings reached approximately 700 °C during plasma exposure, corresponding to expected evaporation rates of 2 × 10²⁰ #/cm²s. However, lithium evaporation measured by filterscopes at the CPS location did not scale with temperature and instead stabilized at 4 × 10¹⁸ #/cm²s, suggesting a suppression mechanism limiting lithium evaporation.

Evaporated Li dispersed rapidly toroidally, as confirmed by a tangential visible camera imaging, limiting localized Li buildup. Emission from the inner strike point revealed Li transport from the outer strike point to the inner strike point which was partially mitigated by deuterium puffing from the private flux region. Puffing had little effect on the heat flux to the target or Li emission levels at the CPS.

These results offer direct experimental evidence that CPS systems can provide stable Li evaporation with no negative impact on the plasma under divertor plasma conditions. The relatively small scale experiment in DIII-D is a significant step towards validation of liquid Li divertors as an alternative to solid divertors in future fusion reactors.