Towards the Implementation of a Lithium Vapor Box Divertor on Near-Term Pulsed-Tokamak Experiments

The lithium vapor box divertor is an attractive concept for mitigating divertor heat flux while minimizing disturbance to the main plasma [1]. The central element for a near-term device such as NSTX-U is a lithium evaporator composed of a heated molybdenum plate coated with Capillary Porous System (CPS) material. This toroidally segmented plate faces the divertor leg, near the strike point, from the private flux region side. The CPS can be fed lithium from a reservoir inside the torus which would be refilled when the fields are off. We explore electromagnetic approaches to measure the level of liquid lithium in the reservoir during refilling. An investigation of the temperature response of the molybdenum plate is also performed to minimize the fraction of lithium evaporated into the vessel before and after the pulse. Taking into consideration the strong evaporative cooling of the plate, evaporation before and after a 5-second plasma pulse can be limited to values comparable to that evaporated during the pulse. Finally, we utilize ANSYS to calculate the heat response of the carbon divertor tiles in NSTX-U, given the heat flux calculated by SOLPS with the lithium vapor box, and find acceptable results. These (and other) aspects of a possible future lithium vapor box divertor design are crucial to model and understand, as we look towards constructing such a system for future experiments.

[1] E.D. Emdee and R.J. Goldston, Nuclear Fusion (2023), DOI: 10.1088/1741-4326/ace6be.