The Influence of Confinement Scaling on Spherical Tokamak Power Plant Design and Performance

Spherical tokamaks (STs) exhibit significant promise as the foundation for compact fusion power plants, offering reduced aspect ratios and enhanced plasma performance that can potentially lower capital costs compared to conventional tokamak designs. The key to achieving an optimal design lies in understanding the sensitivity of the fusion power plant to plasma energy confinement times. However, due to the intricate nature of transport physics and the scarcity of data on highly radiative plasmas required for power plants, extrapolating performance from existing machines introduces substantial uncertainties. In this study, we employed the world-leading fusion power plant systems code, PROCESS, to explore the effects of different energy confinement time scalings on scoping and determining the design of a 1-
$\text{GW}_{e}$
net electric ST power plant. By comparing various commonly used scalings, we highlight the design impact of employing ST scalings versus those typically applied to conventional aspect ratios, considering both size and performance aspects. Our findings demonstrate that when allowed to freely optimize the choice of confinement scaling has negligible impact on the optimally found design point and is instead driven highly by engineering constraints. In a highly constrained scenario, the conventional IPB98(y,2) scaling consistently shows conservative values across a range of ST plasma performance scenarios. We recommend its utilization for future large design space exploration studies as a low-risk choice due to its intermediary performance between the broad scope of ST scalings and also as a proxy for addressing complex transport considerations in configuring initial ST concept designs.