Access to Edge Pedestals in the SPARC Tokamak

Access to a sufficiently large edge pedestal on the SPARC tokamak will be a key ingredient in the realization of high fusion gain on this compact (R=1.85m, a=0.57m) high field (reference B_T=12.2T) device. Building on available empirical results, projections of H-mode and I-mode access are made, with a goal of identifying potential scenarios of high confinement operation free of large edge localized modes (ELMs). Because L-I power thresholds scale less strongly with B_T than L-H thresholds, a significant window of I-mode access appears feasible in SPARC single null plasmas with B×∇B directed away from the active X-point. In order to refine these projections, we model the core plasma of SPARC L-modes to obtain heat flows through the edge, accounting for heating power deposition, ion-electron energy exchange, and core radiation losses. We find that L-modes with a 50:50 DT mix, and heated with up to 15MW of ICRF, typically have large ratios (~2x) of ion-to-electron heat flux at the edge. High ion power fractions may create more favorable conditions for H-mode and I-mode access than simple scalings predict.