Deuterium retention in pre-lithiated samples and Li-D co-deposits in the DIII-D tokamak

TL;DR

This study compares deuterium retention in pre-lithiated samples and Li-D co-deposits in the DIII-D tokamak, revealing that co-deposits primarily influence fuel retention at temperatures below lithium's melting point, with implications for fusion reactor divertor design.

Contribution

First direct comparison of Li-D co-deposits and pre-deposited Li films in a tokamak, highlighting the dominant role of co-deposits in fuel retention below lithium's melting point.

Findings

Deuterium retention is independent of Li layer thickness below lithium's melting point.

Li-D co-deposits are the main factor for fuel retention at these temperatures.

Both Li coatings show lower erosion than sputtering predictions.

Abstract

Divertor designs involving liquid lithium have been proposed as an alternative to solid designs and wall conditioning techniques. However, Li affinity with tritium poses a risk for the fuel cycle. This study investigates deuterium retention in pre-lithiated samples and Li-D co-deposits in the DIII-D tokamak, making for the first time a direct comparison between Li-D co-deposits and pre-deposited Li films. Samples were exposed to H-mode plasmas in the far scrape-off layer (SOL), and Li powder was injected in-situ with the impurity powder dropper to study the uniformity of Li coatings, and the dependence of fuel retention on Li thickness. The results show that at temperatures below the melting point of lithium, deuterium retention is independent of the thickness of pre-deposited Li layers, with Li-D co-deposits being the primary factor for fuel retention. Both pre-deposited and in-situ deposited Li showed lower erosion than predicted by sputtering yield calculations. These results suggest that fuel retention in fusion reactors using lithium in the divertor will likely be dominated by co-deposits rather than in the divertor itself. If one desires to use Li to achieve flatter temperature profiles, operando Li injection is advantageous over pre-deposited Li films, at least at temperatures below the melting point of lithium.