This work examines H-mode and I-mode access on the SPARC tokamak, combining new simulations of core L-mode power balance with the best empirical scalings available for L–H and L–I thresholds. Scalings applied to SPARC for the first time include a novel projection of I-mode access based on critical edge ion heat flux and a new scaling for H-mode access derived from a database of metal-wall tokamaks. Doing this informs the prospects of the device to access these regimes for a given set of operational parameters, or potentially to avoid them altogether. SPARC will seek to access high fusion energy gain Q in a compact (R = 1.85 m, a = 0.57 m) high field (reference T) device fueled by DT, with a performance level that is highly sensitive to the plasma edge temperature obtained, and thus to the confinement regime in which it operates. While an H-mode pedestal provides the highest feasible edge pressure and is the basis of a SPARC reference discharge with , the early operation of SPARC will seek to avoid H-mode formation, and instead opt for obtaining a more modest Q value with reduced edge pressure. L-mode-like discharges have been simulated in the reference SPARC shape at full field parameters and with a range of assumed input auxiliary power, plasma density and temperature at . Using the PORTALS transport solver, multi-channel flux-matched profiles were obtained for a variety of scenarios. Nonlinear CGYRO was used to explore several combinations of input parameters and this database has now been complemented with quasilinear TGLF for a wider exploration of the space. The outputs of these simulations include realistic net power transported through the edge, along with its distribution within the ion and the electron channels. A substantial window for L-mode operation is seen, based on the net power through the edge being generally below typical H-mode or I-mode power threshold projections. Access to H-mode and I-mode at full field operation of SPARC appears likely in DT plasmas, with a potentially larger access window if one assumes that scalings of critical ion heat flux threshold are the appropriate thresholds to consider.
This paper examines the access and avoidance of high confinement regimes (H-mode and I-mode) on the SPARC tokamak. It combines simulations of the core power balance with empirical scalings for L-H and L-I thresholds to inform the prospects of SPARC accessing these regimes. The goal is to achieve high fusion energy gain (Q) while potentially avoiding H-mode formation, which could reduce the edge pressure.