The NSTX-U program on integration of attractive core operation with high heat flux exhaust*

The NSTX-U device is making good progress in an upgrade that will enable auxiliary heating power up to 15 MW, plasma current up to 2 MA, and toroidal field up to 1 T. At full performance, NSTX-U will be able to produce perpendicular heat fluxes above 90 MW/m² in certain equilibria [1] and assuming Eich/Goldston scaling of the scrape-off layer power decay length. This will make NSTX-U an excellent technology testbed for advanced plasma-facing components (PFCs) using liquid lithium (Li), and an excellent scientific testbed for optimized core performance with simultaneously high power exhaust. Three main thrusts for NSTX-U include core turbulence and transport, integrated scenario development, and innovative and conventional power and particle handling techniques. With an increasing emphasis on liquid lithium technology, NSTX-U should be able to access lower collisionality and potentially much higher energy confinement than with boron wall coatings [2]. To enable the technology and science testbed, NSTX-U will transition in the future from carbon PFCs to high-Z PFCs that will serve as a substrate for liquid Li. A design to flow hot He through the cooling lines behind the PFCs to heat Li above the melting point is being initiated. Designs of Li vapor box and fast flowing Li PFCs will be presented, along with status of the current NSTX-U upgrade. [1] E. Emdee and R.J. Goldston, Nucl. Fusion 63 (2023) 096003 [2] S.M. Kaye et al., Nucl. Fusion 53 (2013) 063005