The impact of lithium conditioning on recycling and edge transport in NSTX, and plans for lithium use in NSTX-U*

Pre-discharge lithium (Li) wall conditioning via toroidally-separated overhead evaporators was used in NSTX to reduce wall recycling [Maingi FED 117 (2017) 150]. ELMs were suppressed, and stored energy reached the global stability limit. Interpretive analysis with SOLPS confirmed that the divertor recycling coefficient dropped from 0.99 to ~ 0.90. In the near-separatrix region where recycling dominated the fueling profile, the density gradient was reduced at nearly constant edge particle transport, but the steep gradient region was extended inwards to y_N of 0.7-0.8, as compared to y_N ~ 0.9-0.95 in the reference boronized discharges prior to Li use. Inside of y_N < 0.95, the T_e and T_i gradients both increased, leading to substantially reduced cross-field thermal diffusivity. Micro-stability analysis showed that this change in the density profile resulted in stabilization of edge micro-tearing modes that may have been responsible for electron thermal transport. Recent experiments on LTX-beta with global recycling coefficient dropped to < 0.5 with liquid Li PFCs confirm improved confinement and broad temperature profiles with peaked density profiles and neutral beam heating [Boyle NF 63 (2023) 056020]. To follow up on these promising results, NSTX-U is planning to use four toroidally-separated lithium evaporators, including two new upward-facing evaporators to condition the upper divertor. Future plans include the introduction of flowing Li components and/or hardware to test the Li vapor box concept.