NSTX-U research supporting the development of a steady-state Compact Fusion Power Plant*

Steady-state tokamak Compact Fusion Power Plant (CFPP) designs target enhanced thermal confinement (H$_{\mathrm{98y,2}}$\textgreater 1.5) and large bootstrap current fraction (f$_{\mathrm{BS}}\ge $ 0.5) concurrently with low disruptivity and suitable divertor power handling and exhaust. NSTX-U will advance the physics basis and technology solutions required for an Advanced Tokamak CFPP by producing scenarios at large non-inductive current fraction (f$_{\mathrm{NI}}=$ 60 -- 100{\%}) with strong boundary shaping ($\kappa $\textgreater 2.5, $\delta $\textgreater 0.7), f$_{\mathrm{BS}}=$ 60 -- 90{\%}, $\beta_{\mathrm{N}}=$ 4 -- 6 and $\beta_{\mathrm{T}}=$ 5 -- 25{\%} uniquely accessed at small aspect ratio (A\textless 2). The technical capabilities of NSTX-U are directed at exploring the unique transport and stability properties at high $\beta $ and the lowest collisionality ($\nu_{\mathrm{e}}$*\textless 0.1) of any spherical tokamak. This includes investigating if the strong favorable scaling of confinement with collisionality in regimes dominated by electron thermal transport persists at lower $\nu_{\mathrm{e}}$*. The compact nature of NSTX-U, coupled with high heating power leads to high power exhaust levels that enable the evaluation of integrated tests of reactor-relevant divertor solutions, such as liquid lithium PFCs, in order to qualify these potentially transformative solutions for a CFPP.